[Federal Register: August 22, 2005 (Volume 70, Number 161)]
[Proposed Rules]
[Page 49013-49065]
From the Federal Register Online via GPO Access [wais.access.gpo.gov]
[DOCID:fr22au05-41]
[[Page 49013]]
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Part II
Environmental Protection Agency
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40 CFR Part 197
Public Health and Environmental Radiation Protection Standards for
Yucca Mountain, Nevada; Proposed Rule
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ENVIRONMENTAL PROTECTION AGENCY
40 CFR Part 197
[OAR-2005-0083; FRL-7952-1]
RIN 2060-AN15
Public Health and Environmental Radiation Protection Standards
for Yucca Mountain, NV
AGENCY: Environmental Protection Agency (EPA).
ACTION: Proposed rule.
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SUMMARY: We, the Environmental Protection Agency (EPA), are proposing
to revise certain of our public health and safety standards for
radioactive material stored or disposed of in the potential repository
at Yucca Mountain, Nevada. Section 801(a) of the Energy Policy Act of
1992 (EnPA, Pub. L. 102-486) directed us to develop these standards.
These standards (the 2001 standards) were originally promulgated on
June 13, 2001 (66 FR 32074). Section 801 of the EnPA also required us
to contract with the National Academy of Sciences (NAS) to conduct a
study to provide findings and recommendations on reasonable standards
for protection of the public health and safety. The health and safety
standards promulgated by EPA are to be ``based upon and consistent
with'' the findings and recommendations of NAS. On August 1, 1995, NAS
released its report (the NAS Report), titled ``Technical Bases for
Yucca Mountain Standards.'' In promulgating our standards, we
considered the NAS Report as the EnPA directs.
On July 9, 2004, in response to a legal challenge by the State of
Nevada and the Natural Resources Defense Council, the U.S. Court of
Appeals for the District of Columbia Circuit vacated portions of our
standards that addressed the period of time for which compliance must
be demonstrated. The Court ruled that the time frame for regulatory
compliance was not ``based upon and consistent with'' the findings and
recommendations of the NAS and remanded those portions of the standards
to us for revision. These remanded provisions are the subject of
today's action.
Today's proposal incorporates multiple compliance criteria
applicable at different times for protection of individuals and in
circumstances involving human intrusion into the repository. Compliance
will be judged against a standard of 150 microsievert per year (15
millirem per year) committed effective dose equivalent at times up to
10,000 years after disposal and against a standard of 3.5 millisievert
per year (350 millirem per year) committed effective dose equivalent at
times after 10,000 years and up to 1 million years after disposal.
Today's proposal also includes several supporting provisions affecting
DOE's performance projections. DOE will measure the median of the
distribution of doses against the dose standard beyond 10,000 years,
will calculate doses using updated scientific factors, and will
incorporate specific direction on analyzing features, events, and
processes that may affect performance.
Section 801(b) of the EnPA requires the Nuclear Regulatory
Commission (NRC) to modify its technical requirements for licensing of
the Yucca Mountain repository to be consistent with the standards
promulgated by EPA. NRC did incorporate EPA's Yucca Mountain standards
into its licensing regulations and the regulatory time frame provision
of these was similarly struck down by the Court of Appeals. Once
revised regulatory time frame components of our standards have been
promulgated, NRC must revise its licensing regulations to be consistent
with our revised standards. The Department of Energy (DOE) plans to
submit a license application providing a compliance demonstration. The
NRC will determine whether DOE has demonstrated compliance with NRC's
licensing regulations, which must be consistent with our standards,
prior to granting or denying the necessary licenses to dispose of
radioactive material in Yucca Mountain.
DATES: Comments must be received on or before October 21, 2005.
ADDRESSES: Submit your comments, identified by Docket ID No. OAR-2005-
0083, by one of the following methods:
1. Electronically. If you submit an electronic comment as
prescribed below, EPA recommends that you include your name, mailing
address, and an e-mail address or other contact information in the body
of your comment. Also include this contact information on the outside
of any disk or CD-ROM you submit, and in any cover letter accompanying
the disk or CD-ROM. This ensures that you can be identified as the
submitter of the comment and allows EPA to contact you in case we
cannot read your comment due to technical difficulties or we need
further information on the substance of your comment. EPA's policy is
that we will not edit your comment, and any identifying or contact
information provided in the body of a comment will be included as part
of the comment that is placed in the official public docket, and made
available in EPA's electronic public docket. If EPA cannot read your
comment due to technical difficulties and cannot contact you for
clarification, we may not be able to consider your comment.
i. Federal eRulemaking Portal: http://www.regulations.gov. Follow
the on-line instructions for submitting comments.
ii Agency Web site: EPA's preferred method for receiving comments
is via its website, EDOCKET. EDOCKET is an ``anonymous access'' system,
which means EPA will not know your identity, e-mail address, or other
contact information unless you provide it in the body of your comment.
Go directly to EDOCKET at http://www.epa.gov/edocket, or, from the EPA
Internet Home Page (http://www.epa.gov), select ``Information Sources'' (in
the left column), then ``Dockets,'' then ``EPA Dockets'' (in the first
paragraph). For either route, then click on ``Quick Search'' (in the
left column). In the search window, type in the docket identification
number OAR-2005-0083. Please be patient, the search could take about 30
seconds. This will bring you to the ``Docket Search Results'' page. At
that point, click on OAR-2005-0083. From the resulting page, you may
submit a comment by clicking on the balloon icon in the ``Submit
Comment'' column and following the subsequent directions.
iii. E-mail: Comments may be sent by electronic mail (e-mail) to
a-and-r-docket@epa.gov, Attention Docket ID No. OAR-2005-0083. In
contrast to EPA's electronic public docket, EPA's e-mail system is not
an ``anonymous access'' system. If you send an e-mail comment directly
to the Docket without going through EPA's electronic public docket,
EPA's e-mail system automatically captures your e-mail address. E-mail
addresses that are automatically captured by EPA's e-mail system are
included as part of the comment that is placed in the official public
docket, and made available in EPA's electronic public docket.
2. Surface Mail. Send your comments to: EPA Docket Center (EPA/DC),
Air and Radiation Docket, Environmental Protection Agency, EPA West,
Mail Code 6102T, 1200 Pennsylvania Avenue, NW., Washington, DC 20460.
Attention Docket ID No. OAR-2005-0083.
3. Hand Delivery or Courier. Deliver your comments to: Air and
Radiation Docket, EPA Docket Center, (EPA/DC) EPA West, Room B102, 1301
Constitution Ave., NW., Washington, DC, Attention Docket ID No. OAR-
2005-0083. Such deliveries are only
[[Page 49015]]
accepted during the Docket Center's normal hours of operation.
4. Facsimile. Fax your comments to: 202-566-1741, Attention Docket
ID. No. OAR-2005-0083.
Instructions for submitting information to EDOCKET: Direct your
comments and information to Docket ID No. OAR-2005-0083. It is
important to note that EPA's policy is that public comments, whether
submitted electronically or in paper, will be made available for public
viewing in EPA's electronic public docket as EPA receives them and
without change, unless the comment contains copyrighted material, CBI,
or other information whose disclosure is restricted by statute. When
EPA identifies a comment containing copyrighted material, EPA will
provide a reference to that material in the version of the comment that
is placed in EPA's electronic public docket. The entire printed
comment, including the copyrighted material, will be available in the
public docket.
Certain types of information will not be placed in EDOCKET.
Information claimed as CBI and other information whose disclosure is
restricted by statute, which is not included in the official public
docket, will not be available for public viewing in EPA's electronic
public docket. EPA's policy is that copyrighted material will not be
placed in EPA's electronic public docket but will be available only in
printed, paper form in the official public docket. To the extent
feasible, publicly available docket materials will be made available in
EPA's electronic public docket. When a document is selected from the
index list in EPA Dockets, the system will identify whether the
document is available for viewing in EPA's electronic public docket.
Although not all docket materials may be available electronically, you
may still access any of the publicly available docket materials through
the docket facility. EPA intends to work towards providing electronic
access to all of the publicly available docket materials through EPA's
electronic public docket.
The EPA EDOCKET and the federal regulations.gov websites are
``anonymous access'' systems, which means EPA will not know your
identity or contact information unless you provide it in the body of
your comment. If you send an e-mail comment directly to EPA without
going through EDOCKET or regulations.gov, your e-mail address will be
automatically captured and included as part of the comment that is
placed in the public docket and made available on the Internet. If you
submit an electronic comment, EPA recommends that you include your name
and other contact information in the body of your comment and with any
disk or CD-ROM you submit. If EPA cannot read your comment due to
technical difficulties and cannot contact you for clarification, EPA
may not be able to consider your comment. Electronic files should avoid
the use of special characters, any form of encryption, and be free of
any defects or viruses.
Public comments submitted on computer disks that are mailed or
delivered to the docket will be transferred to EPA's electronic public
docket. Public comments that are mailed or delivered to the docket will
be scanned and placed in EPA's electronic public docket. Where
practical, physical objects will be photographed, and the photograph
will be placed in EPA's electronic public docket along with a brief
description written by the docket staff.
For additional information about EPA's electronic public docket
visit EPA Dockets online or see 67 FR 38102, May 31, 2002.
Docket: The official docket is the collection of materials that is
available for public viewing at the Air and Radiation Docket in the EPA
Docket Center (EPA/DC), EPA West, Room B102, 1301 Constitution Ave.,
NW., Washington, DC. The EPA Docket Center Public Reading Room is open
from 8:30 a.m. to 4:30 p.m., Monday through Friday, excluding legal
holidays. The telephone number for the Public Reading Room is 202-566-
1744. The telephone number for the Air and Radiation Docket is 202-566-
1742. As provided in EPA's regulations at 40 CFR part 2, and in
accordance with normal EPA docket procedures, if copies of any docket
materials are requested, a reasonable fee may be charged.
All documents in the docket are listed in the EDOCKET index at
http://www.epa.gov/edocket. Although listed in the index, some
information is not publicly available since it will not be placed in
EDOCKET. That is, although a part of the official docket, EDOCKET does
not include Confidential Business Information (CBI) or other
information whose disclosure is restricted by statute. Information
claimed as CBI and other information whose disclosure is restricted by
statute, which is not included in the official public docket, will not
be available for public viewing in EPA's EDOCKET. In addition, EPA
policy is that copyrighted material will not be placed in EPA's
EDOCKET, but will be available only in printed, paper form in the
official public docket. To the extent feasible, publicly available
docket materials will be made available in EPA's EDOCKET. When a
document is selected from the index list in EDOCKET, the system will
identify whether the document is available for viewing. Although not
all docket materials may be available electronically, you may still
access any of the publicly available docket materials through the
docket facility. EPA intends to work towards providing electronic
access to all of the publicly available docket materials through EPA's
electronic public docket.
FOR FURTHER INFORMATION CONTACT: Ray Clark, Office of Radiation and
Indoor Air, Radiation Protection Division (6608J), U.S. Environmental
Protection Agency, 1200 Pennsylvania Ave., NW., Washington, DC 20460-
0001; telephone number: 202-343-9601; fax number: 202-343-2305; e-mail
address: clark.ray@epa.gov.
SUPPLEMENTARY INFORMATION:
I. General Information
A. Does This Action Apply to Me?
The DOE is the only entity regulated by these standards. Our
standards affect NRC only because, under Section 801(b) of the EnPA, 42
U.S.C. 10141 n., NRC must modify its licensing requirements, as
necessary, to make them consistent with our final standards. Before it
may accept waste at the Yucca Mountain site, DOE must obtain a license
from NRC. DOE will be subject to NRC's modified regulations, which NRC
will implement through its licensing proceedings.
B. What Should I Consider as I Prepare My Comments for EPA?
1. Submitting CBI. If you submit CBI, clearly mark the part or all
of the information that you claim to be CBI. For CBI information on a
disk or CD-ROM that you mail to EPA, mark the outside of the disk or
CD-ROM as CBI and then identify electronically within the disk or CD-
ROM the specific information that is claimed as CBI. In addition to one
complete version of the comment that includes information claimed as
CBI, a copy of the comment that does not contain the information
claimed as CBI must be submitted for inclusion in the public docket.
Information so marked will not be disclosed except in accordance with
procedures set forth in 40 CFR part 2.
2. Tips for Preparing Your Comments. You may find the following
suggestions helpful for preparing your comments:
1. Explain your views as clearly as possible.
2. Describe any assumptions that you used.
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3. Provide any technical information and/or data you used that
support your views.
4. If you estimate potential burden or costs, explain how you
arrived at your estimate.
5. Provide specific examples to illustrate your concerns.
6. Offer alternatives.
7. Make sure to submit your comments by the comment period deadline
identified.
8. Respond to specific questions from the Agency.
9. To ensure proper receipt by EPA, identify the appropriate docket
identification number in the subject line on the first page of your
response.
C. How Can I View Items in the Docket?
1. Information Files. EPA is working with the Lied Library at the
University of Nevada-Las Vegas (http://www.library.unlv.edu/about/hours.html#desks
) and the Amargosa Valley, Nevada public library
(http://www.amargosavalley.com/Library.html) to provide information
files on this rulemaking. These files are not legal dockets, however
every effort will be made to put the same material in them as in the
official public docket in Washington, DC. The Lied Library information
file is at the Research and Information Desk, Government Publications
Section (702-895-2200). Hours vary based upon the academic calendar, so
we suggest that you call ahead to be certain that the library will be
open at the time you wish to visit (for a recorded message, call 702-
895-2255). The other information file is in the Public Library in
Amargosa Valley, Nevada (phone 775-372-5340). As of the date of
publication, the hours are Monday, Wednesday, and Friday (9 a.m.-5
p.m.); Tuesday and Thursday (9 a.m.-7 p.m.); and Saturday (9 a.m.-1
p.m.). The library is closed on Sunday. These hours can change, so we
suggest that you call ahead to be certain when the library will be
open.
2. Electronic Access. An electronic version of the public docket is
available through EPA's electronic public docket and comment system,
EPA Dockets (EDOCKET). You may use EDOCKET to submit or view comments,
access the index listing of the contents of the official public docket,
and to access those documents in the public docket that are available
electronically. To access the docket either go directly to http://www.epa.gov/edocket/
or, from the EPA Internet Home Page (www.epa.gov),
select ``Information Sources'' (in the left column), then ``Dockets,''
then ``EPA Dockets'' (in the first paragraph). For either route, then
click on ``Quick Search'' (in the left column). In the search window,
type in the docket identification number OAR-2005-0083. Please be
patient, the search could take about 30 seconds. This will bring you to
the ``Docket Search Results'' page. At that point, click on OAR-2005-
0083. From the resulting page, you may access the docket contents
(e.g., OAR-2005-0083-0002) by clicking on the icon in the ``Rendition''
column.
D. Can I Access Information by Telephone or Via the Internet?
Yes. You may call our toll-free information line (800-331-9477) 24
hours per day. By calling this number, you may listen to a brief update
describing our rulemaking activities for Yucca Mountain, leave a
message requesting that we add your name and address to the Yucca
Mountain mailing list, or request that an EPA staff person return your
call. In addition, we have established an electronic listserv through
which you can receive electronic updates of activities related to this
rulemaking. To subscribe to the listserv, go to https://lists.epa.gov/read/all_forums.
In the alphabetical list, locate ``yucca-updates''
and select ``subscribe'' at the far right of the screen. You will be
asked to provide your e-mail address and choose a password. You also
can find information and documents relevant to this rulemaking on the
World Wide Web at http://www.epa.gov/radiation/yucca. We also recommend
that you examine the preamble and regulatory language for the earlier
proposed and final rules, which appeared in the Federal Register on
August 27, 1999 (64 FR 46976) and June 13, 2001 (66 FR 32074),
respectively.
E. What Documents Are Referenced in Today's Proposal?
We refer to a number of documents that provide supporting
information for our Yucca Mountain standards. All documents relied upon
by EPA in regulatory decisionmaking may be found in our docket (OAR-
2005-0083). Other documents, e.g., statutes, regulations, proposed
rules, are readily available from public sources. The documents below
are referenced most frequently in today's proposal.
Item No. (OAR-2005-0083-xxxx)
0044 ``Safety Indicators in Different Time Frames for the Safety
Assessment of Underground Radioactive Waste Repositories,''
International Atomic Energy Agency
TECDOC-767, 1994
0045 ``Regulatory Decision Making in the Presence of Uncertainty in
the Context of Disposal of Long Lived Radioactive Wastes,''
International Atomic Energy Agency
TECDOC-975, 1997
0046 ``The Handling of Timescales in Assessing Post-Closure Safety:
Lessons Learnt from the April 2002 Workshop in Paris, France,'' Nuclear
Energy Agency (Organisation for Economic Co-operation and Development),
2004
0051 ``Geological Disposal of Radioactive Waste,'' International
Atomic Energy Agency Draft Safety Requirements (DS154), April 2005
0061 ``Principles and Standards for Disposal of Long-Lived
Radioactive Wastes,'' Neil Chapman and Charles McCombie, Elsevier
Press, 2003
0062 ``An International Peer Review of the Yucca Mountain Project
TSPA-SR,'' Joint Report by the OECD Nuclear Energy Agency and the
International Atomic Energy Agency, OECD, 2002
0076 Technical Bases for Yucca Mountain Standards (the NAS Report),
National Research Council, National Academy Press, 1995
0077 ``Assessment of Variations in Radiation Exposure in the United
States,'' EPA Technical Support Document, July 2005
0085 ``Assumptions, Conservatisms, and Uncertainties in Yucca
Mountain Performance Assessments,'' EPA Technical Support Document,
July 2005
0086 DOE Final Environmental Impact Statement, DOE/EIS-0250,
February 2002
Acronyms and Abbreviations
We use many acronyms and abbreviations in this document. These
include:
BID--background information document
CED--committed effective dose
CEDE--committed effective dose equivalent
DOE--U.S. Department of Energy
DOE/VA--DOE's Viability Assessment
EIS--Environmental Impact Statement
EnPA--Energy Policy Act of 1992
EPA--U.S. Environmental Protection Agency
FEIS--Final Environmental Impact Statement
FEPs--features, events, and processes
FR--Federal Register
GCD--greater confinement disposal
HLW--high-level radioactive waste
HSK--Swiss Federal Nuclear Safety Inspectorate
IAEA--International Atomic Energy Agency
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ICRP--International Commission on Radiological Protection
KASAM--Swedish National Council for Nuclear Waste
LLW--low-level radioactive waste
MCL--maximum contaminant level
MTHM--metric tons of heavy metal
NAPA--National Academy of Public Administration
NAS--National Academy of Sciences
NEA--Nuclear Energy Agency
NEI--Nuclear Energy Institute
NRC--U.S. Nuclear Regulatory Commission
NRDC--Natural Resources Defense Council
NTS--Nevada Test Site
NTTAA--National Technology Transfer and Advancement Act
NWPA--Nuclear Waste Policy Act of 1982
NWPAA--Nuclear Waste Policy Amendments Act of 1987
OECD--Organization for Economic Cooperation and Development
OMB--Office of Management and Budget
RMEI--reasonably maximally exposed individual
SSI--Swedish Radiation Protection Authority
SNF--spent nuclear fuel
SR--Site recommendation
TRU--transuranic
TSPA--Total System Performance Assessment
UK--United Kingdom
UMRA--Unfunded Mandates Reform Act of 1995
U.S.C.--United States Code
WIPP LWA--Waste Isolation Pilot Plant Land Withdrawal Act of 1992
Outline of Today's Action
I. What is the History of Today's Action?
A. Promulgation of 40 CFR part 197 in 2001
1. What are the Elements of EPA's 2001 Standards?
a. What is the Standard for Storage of the Waste? (Subpart A,
Sec. Sec. 197.1 through 197.5)
b. What Are the Standards for Disposal? (Subpart B, Sec. Sec.
197.11 through 197.36)
i. What is the Standard for Protection of Individuals?
(Sec. Sec. 197.20 through 197.21)
aa. Who Represents the Exposed Population?
bb. How Far Into the Future Must Performance be Assessed?
ii. What is the Standard for Human Intrusion? (Sec. Sec. 197.25
through 197.26)
iii. What are the Standards to Protect Ground Water? (Sec. Sec.
197.30 through 197.31)
c. What is ``Reasonable Expectation''? (Sec. 197.14)
B. Legal Challenges to 40 CFR part 197
1. Challenges by the State of Nevada and Natural Resources
Defense Council
2. Challenge by the Nuclear Energy Institute
C. Ruling by the U.S. Court of Appeals for the District of
Columbia Circuit
1. What Did the Court of Appeals Rule on the Issue of Compliance
Period?
a. What Were NAS's Findings (``Conclusions'') and
Recommendations on the Issue of Compliance Period?
2. What Did the Court of Appeals Rule on Other Issues Related to
EPA's Standards?
II. How Will EPA Address the Decision by the Court of Appeals?
A. How Will Elements of the Disposal Standards be Affected?
1. Individual-Protection Standard
2. Human-Intrusion Standard
3. Ground-Water Protection Standards
4. Reasonable Expectation
5. Effects of Uncertainty
B. How Does the Application of ``Reasonable Expectation''
Influence Today's Proposal?
C. How Is EPA Proposing to Revise the Individual-Protection
Standard (Sec. 197.20) to Address Peak Dose?
1. Multiple Dose Standards Applicable to Different Compliance
Periods
2. What Other Options Did EPA Consider?
a. Maintain the 10,000-year Standard Alone Without Addressing
Peak Dose
b. Dose Standard To Apply at Peak Dose Alone
c. Peak Dose Standard Varying Over Time
d. Standard Expressed as a Dose Target, Rather Than Limit
e. Standard Expressed as a Statistical Distribution
3. What Dose Level is EPA Proposing for Peak Dose?
4. What Other Peak Dose Levels Did EPA Consider?
a. Maintain the 15 mrem/yr Standard at Peak Dose
b. 100 mrem/yr Standard at Peak Dose
c. Peak Dose Standard Based on Regional Background Radiation
Levels
5. How Will NRC Judge Compliance?
6. How Will DOE Calculate the Dose?
D. How Will Today's Proposal Affect the Way DOE Conducts
Performance Assessments?
1. Performance Assessments Up To 10,000 Years After Disposal
2. Performance Assessments for Periods Longer Than 10,000 Years
After Disposal
a. Consideration of Likely, Unlikely, and Very Unlikely FEPs
b. Consideration of Seismic FEPs
c. Consideration of Igneous (Volcanic) FEPs
d. Consideration of Climatological FEPs
E. How Is EPA Proposing To Revise the Human-Intrusion Standard
(Sec. 197.25) To Address Peak Dose?
F. Summary of Today's Proposal by Section
III. Statutory and Executive Order Reviews
A. Executive Order 12866: Regulatory Planning and Review
B. Paperwork Reduction Act
C. Regulatory Flexibility Act
D. Unfunded Mandates Reform Act
E. Executive Order 13132: Federalism
F. Executive Order 13175: Consultation and Coordination with
Indian Tribal Governments
G. Executive Order 13045: Protection of Children from
Environmental Health & Safety Risks
H. Executive Order 13211: Actions that Significantly Affect
Energy Supply, Distribution, or Use
I. National Technology Transfer and Advancement Act
I. What Is the History of Today's Action?
Radioactive wastes result from the use of nuclear fuel and other
radioactive materials. Today, we are proposing to revise certain
standards pertaining to spent nuclear fuel (SNF), high-level
radioactive waste (HLW), and other radioactive waste (we refer to these
items collectively as ``radioactive materials'' or ``waste'') that may
be stored or disposed of in the Yucca Mountain repository. (When we
discuss storage or disposal in this document in reference to Yucca
Mountain, we note that no decision has been made regarding the
acceptability of Yucca Mountain for storage or disposal as of the date
of this publication. To save space and to avoid excessive repetition,
we will not describe Yucca Mountain as a ``potential'' repository;
however, we intend this meaning to apply.) Pursuant to Section 801(a)
of the Energy Policy Act of 1992 (EnPA, Pub. L. 102-486), these
standards apply only to facilities at Yucca Mountain.
Once nuclear reactions have consumed a certain percentage of the
uranium or other fissionable material in nuclear reactor fuel, the fuel
no longer is useful for its intended purpose. It then is known as
``spent'' nuclear fuel (SNF). It is possible to recover specific
radionuclides from SNF through ``reprocessing,'' which is a process
that dissolves the SNF, thus separating the radionuclides from one
another. Radionuclides not recovered through reprocessing become part
of the acidic liquid wastes that the Department of Energy (DOE) plans
to convert into various types of solid materials. High-level waste
(HLW) is the highly radioactive liquid or solid wastes that result from
reprocessing SNF. The SNF that does not undergo reprocessing prior to
disposal remains inside the fuel assembly and becomes the final waste
form.
In the U.S., SNF and HLW have been produced since the 1940s, mainly
as a result of commercial power production and defense activities.
Since the inception of the nuclear age, the proper disposal of these
wastes has been the responsibility of the Federal government. The
Nuclear Waste Policy Act of 1982 (NWPA, 42 U.S.C. Chapter 108)
formalizes the current Federal
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program for the disposal of SNF and HLW by:
(1) Making DOE responsible for siting, building, and operating an
underground geologic repository for the disposal of SNF and HLW;
(2) Directing us to set generally applicable environmental
radiation protection standards based on authority established under
other laws \1\; and
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\1\ These laws include the Atomic Energy Act of 1954, as amended
(42 U.S.C. 2011-2296) and Reorganization Plan No. 3 of 1970 (5
U.S.C. Appendix 1).
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(3) Requiring the Nuclear Regulatory Commission (NRC) to implement
our standards by revising its licensing requirements for SNF and HLW
repositories to be consistent with our standards.
This general division of responsibilities continues for the Yucca
Mountain repository. Thus, today we are proposing to establish or
revise specific aspects of our public health protection standards at 40
CFR part 197 (which are, pursuant to EnPA Section 801(a), applicable
only to Yucca Mountain, rather than generally applicable). The NRC will
issue implementing regulations for these standards. The DOE plans to
submit a license application to NRC. The NRC then will determine
whether DOE has met NRC's regulations and whether to grant or deny a
license for Yucca Mountain.
In 1985, we established generic standards for the management,
storage, and disposal of SNF, HLW, and transuranic (TRU) radioactive
waste (see 40 CFR part 191, 50 FR 38066, September 19, 1985), which
were intended to apply to any facilities utilized for the storage or
disposal of these wastes, including Yucca Mountain. In 1987, the U.S.
Court of Appeals for the First Circuit remanded the disposal standards
in 40 CFR part 191 (NRDC v. EPA, 824 F.2d 1258 (1st Cir. 1987)). As
discussed below, we later amended and reissued these standards to
address issues that the court raised. Also in 1987, the Nuclear Waste
Policy Amendments Act (NWPAA, Pub. L. 100-203) amended the NWPA by,
among other actions, selecting Yucca Mountain, Nevada, as the only
potential site that DOE should characterize for a long-term geologic
repository. In October 1992, the Waste Isolation Pilot Plant Land
Withdrawal Act (WIPP LWA, Pub. L. 102-579) and the EnPA became law.
These statutes changed our obligations concerning radiation standards
for the Yucca Mountain candidate repository. The WIPP LWA:
(1) Reinstated the 40 CFR part 191 disposal standards, except those
portions that were the specific subject of the remand by the First
Circuit;
(2) Required us to issue standards to replace the portion of the
challenged standards remanded by the court; and
(3) Exempted the Yucca Mountain site from the 40 CFR part 191
disposal standards.
We issued the amended 40 CFR part 191 disposal standards, which
addressed the judicial remand, on December 20, 1993 (58 FR 66398). The
EnPA, enacted in 1992, set forth our responsibilities as they relate to
Yucca Mountain. In the EnPA, Congress directed us to set public health
and safety radiation standards for Yucca Mountain. Specifically,
section 801(a)(1) of the EnPA directed us to ``promulgate, by rule,
public health and safety standards for the protection of the public
from releases from radioactive materials stored or disposed of in the
repository at the Yucca Mountain site.'' Section 801(a)(2) directed us
to contract with the National Academy of Sciences (NAS) to conduct a
study to provide us with its findings and recommendations on reasonable
standards for protection of public health and safety from releases from
the Yucca Mountain disposal system. Moreover, it provided that our
standards shall be the only such standards applicable to the Yucca
Mountain site and are to be based upon and consistent with NAS's
findings and recommendations. On August 1, 1995, NAS released its
report, ``Technical Bases for Yucca Mountain Standards'' (the NAS
Report) (Docket No. OAR-2005-0083-0076).
A. Promulgation of 40 CFR Part 197 in 2001
Following the direction in the EnPA, we developed standards
specifically applicable to releases from radioactive material stored or
disposed of in the Yucca Mountain repository. In doing so, we gave
special weight to both the NAS Report and our generic standards in 40
CFR part 191, and also considered other relevant information,
precedents, and analyses.
We evaluated 40 CFR part 191 because those standards were developed
to apply to any site selected for storage and disposal of SNF and HLW,
and would have applied to Yucca Mountain had Congress not directed
otherwise. Thus, we believed that 40 CFR part 191 already included the
major components of standards needed for any specific site, such as
Yucca Mountain. However, we recognized that all the components would
not necessarily be directly transferable to the situation at Yucca
Mountain, and that some modification might be necessary. We also
considered that some components of the generic standards would not be
carried into site-specific standards, simply because not all of the
conditions found among all sites are present at each site. See 66 FR
32076-32078, June 13, 2001 (Docket No. OAR-2005-0083-0042), for a more
detailed discussion of the role of 40 CFR part 191 in developing 40 CFR
part 197.
We also considered the findings and recommendations of the NAS in
developing standards for Yucca Mountain. In some cases, provisions of
40 CFR part 191 were already consistent with NAS's analysis (e.g.,
level of protection for the individual). In other cases, we used the
NAS Report to modify or draw out parts of 40 CFR part 191 to apply more
directly to Yucca Mountain (e.g., the stylized drilling scenario for
human intrusion). See the NAS Report for a complete description of
findings and recommendations.
Because our standards are intended to apply specifically to the
Yucca Mountain disposal system, in a number of areas we tailored our
approach to consider the characteristics of the site and the local
populations. Yucca Mountain is in southwestern Nevada approximately 100
miles northwest of Las Vegas. The eastern part of the site is on the
Nevada Test Site (NTS). The northwestern part of the site is on the
Nellis Air Force Range. The southwestern part of the site is on Bureau
of Land Management land. The area has a desert climate with topography
typical of the Basin and Range province. Yucca Mountain is made of
layers of ashfalls from volcanic eruptions that happened more than 10
million years ago. There are two major aquifers beneath Yucca Mountain.
Regional ground water in the vicinity of Yucca Mountain is believed to
flow generally in a south-southeasterly direction. The DOE plans to
build the repository about 300 meters below the surface and about 300
to 500 meters above the water table. For more detailed descriptions of
Yucca Mountain's geologic and hydrologic characteristics, and the
disposal system, please see chapter 7 of the 2001 BID (Docket No. OAR-
2005-0083-0050) and the preamble to the proposed rule (64 FR 46979-
46980, August 27, 1999, Docket No. OAR-2005-0083-0041).
We proposed standards for Yucca Mountain on August 27, 1999 (64 FR
46976). In response to our proposal, we received more than 800 public
comments and conducted four public hearings. After evaluating public
comments, we issued final standards (66 FR 32074, June 13, 2001). See
the Response to Comments document from that rulemaking for more
discussion of
[[Page 49019]]
comments (Docket No. OAR-2005-0083-0043).
1. What Are the Elements of EPA's 2001 Standards?
We are issuing today's proposal to respond to a ruling by the U.S.
Court of Appeals for the District of Columbia Circuit (``the Court'')
that vacated portions of 40 CFR part 197. Sections I.B (``Legal
Challenges to 40 CFR part 197'') and I.C (``Ruling by U.S. Court of
Appeals for the District of Columbia Circuit'') discuss aspects of the
legal challenges on which the Court ruled. This section summarizes some
of the key provisions and concepts in 40 CFR part 197 to provide a
context to better understand the basis for the legal actions and
today's proposed action, which is described in Section II of this
document (``How Will EPA Address the Decision by the Court of
Appeals?'').
The standards issued in 2001 as 40 CFR part 197 included the
following:
A standard to protect the public during storage operations
at the Yucca Mountain site;
An individual-protection standard to protect the public
after disposal from releases from the undisturbed repository;
A human-intrusion standard to protect the public after
disposal from releases caused by a drilling penetration into the
repository;
A set of standards to protect ground water from
radionuclide contamination caused by releases from the repository after
disposal;
The requirement that compliance with the disposal
standards be shown for 10,000 years;
The requirement that DOE continue its projections for the
individual-protection and human-intrusion standards beyond 10,000 years
to the time of peak (maximum) dose, and place those projections in the
Environmental Impact Statement (EIS);
The concept of the Reasonably Maximally Exposed Individual
(RMEI), defined as a hypothetical person whose lifestyle is
representative of the local population, as the individual against whom
the disposal standards should be assessed; and
The concept of a ``controlled area,'' defined as an area
immediately surrounding the repository whose geology is considered part
of the natural barrier component of the overall disposal system, and
inside of which radioactive releases are not regulated.
We emphasize that today's proposal is narrowly focused to respond
to the Court ruling. Most sections of our 2001 rule are unaffected by
the Court's ruling and are not implicated in today's proposal. We are
requesting and will respond to comments only on those provisions we are
proposing to change today.
a. What Is the Standard for Storage of the Waste? (Subpart A,
Sec. Sec. 197.1 Through 197.5)
Section 801(a)(1) of the EnPA calls for EPA's public health and
safety standards to apply to radioactive materials ``stored or disposed
of in the repository at the Yucca Mountain site.'' The repository is
the excavated portion of the facility constructed underground within
the Yucca Mountain site. The storage standard, therefore, applies to
waste inside the repository, prior to disposal.
The DOE also will handle, and might store, radioactive material
outside the repository prior to subsurface emplacement. Therefore, our
standards will provide public health and safety protection for surface
management and storage activities on the surface of the Yucca Mountain
site and in the Yucca Mountain repository. The combined doses incurred
by any individual in the general environment from these activities must
not exceed 150 [mu]Sv (15 mrem) committed effective dose equivalent per
year (CEDE/yr).
b. What Are the Standards for Disposal? (Subpart B, Sec. Sec. 197.11
Through 197.36)
Subpart B of our 2001 rule consisted of three separate standards
(or sets of standards) that apply after disposal, which are discussed
in more detail in the appropriate sections of this document (e.g.,
Section II.A, ``How Will Elements of the Disposal Standards be
Affected?''). For additional detail, see the preamble to the June 2001
rulemaking (66 FR 32074, June 13, 2001). The disposal standards are:
An individual-protection standard;
A human-intrusion standard; and
Ground-water protection standards.
i. What Is the Standard for Protection of Individuals? (Sec. Sec.
197.20 Through 197.21)
The first standard is an individual-protection standard. It
specifies the maximum dose that a reasonably maximally exposed
individual (RMEI) may receive from releases from the Yucca Mountain
repository.
Our individual-protection standard set a limit of 150 [mu]Sv (15
mrem) CEDE/yr. This limit corresponds to an annual risk of fatal cancer
within the range that NAS suggested as a ``reasonable starting point
for EPA's rulemaking'' (NAS Report p. 5, Docket No. OAR-2005-0083-
0076). The NAS s suggested risk range corresponds to approximately 2 to
20 mrem CEDE/yr.
The standard described above applies for a period of 10,000 years
after disposal, and is to be measured against exposures to the RMEI at
a location outside the controlled area (in the ``accessible
environment'').
aa. Who Represents the Exposed Population?
To determine whether the Yucca Mountain disposal system complies
with our standard, DOE must calculate the dose received by some
individual or group of individuals exposed to releases from the
repository and compare the calculated dose with the limit established
in the standard. The standard specifies, therefore, the representative
individual for whom DOE must make the dose calculation as the RMEI. It
was left to NRC to define the details, beyond those which we specified,
necessary for the dose calculation. NRC has further defined the RMEI as
an adult (10 CFR 63.312(e)) and specified that the average
concentration of radionuclides in well water ingested by the RMEI be
based on a water demand of 3,000 acre-feet per year (10 CFR 63.312(c)).
The Reasonably Maximally Exposed Individual (RMEI)
The approach we chose (the RMEI) embodies the intent of the
internationally-accepted concept to protect those individuals most at
risk from the proposed repository but specifies one or a few site-
specific parameters at their maximum values. The characteristics of the
RMEI are defined from consideration of current population distribution
and ground-water usage, and average food consumption patterns for the
population downgradient from Yucca Mountain in Amargosa Valley, Nevada.
Our RMEI is a theoretical individual representative of a future
population group or community termed ``rural-residential'' (see Chapter
8 of the 2001 BID for a description of this concept, Docket No. OAR-
2005-0083-0050). We assume that the rural-residential RMEI is exposed
through the same general pathways as a subsistence farmer. However,
this RMEI would not be a full-time farmer. Rather, the RMEI might do
personal gardening and earn income from other sources of work in the
area. Under our standard, the RMEI will have food and water intake
rates, diet, and physiology similar to those of individuals living in
Amargosa Valley, Nevada. We assume that all of the drinking water and
some of the food (based upon surveys) consumed by the RMEI is from the
local area. Similarly, we assume that local food production
[[Page 49020]]
will use water contaminated with radionuclides released from the
disposal system. We believe this lifestyle is conservative but similar
to that of most people living in Amargosa Valley today.
Location of the RMEI. The location of the RMEI is a basic part of
the exposure scenario. We require that the RMEI be located in the
accessible environment (i.e., outside the controlled area) above the
highest concentration of radionuclides in the plume of contamination.
Based upon a review of available site-specific information (see Chapter
8 of the 2001 BID, Docket No. OAR-2005-0083-0050), we identified the
southern edge of the Nevada Test Site as the southernmost extent of the
controlled area. The actual compliance point will be determined through
the licensing process. (Even if the RMEI were to be located north of
this line of latitude, the RMEI must still have the characteristics
described in Sec. 197.21.) As discussed in Section I.B (``Legal
Challenges to 40 CFR part 197'') and I.C (``Ruling by the U.S. Court of
Appeals for the District of Columbia Circuit''), the location of the
RMEI was a subject of the Court decision, was upheld, and is not a
subject of today's proposal.
bb. How Far Into the Future Must Performance Be Assessed?
In 2001, we established a compliance period of 10,000 years. Under
the 2001 standards, the peak dose within 10,000 years after disposal
would be required to comply with the individual-protection standard. In
addition, we required calculation of the peak dose beyond 10,000 years,
but within the period of geologic stability. We required DOE to include
the results and bases of the additional analyses in the EIS for Yucca
Mountain as an indicator of the future performance of the disposal
system. The rule did not, however, require that DOE meet a specific
dose limit after 10,000 years. The compliance period was a subject of
the Court decision and is the primary subject of today's proposal.
ii. What Is the Standard for Human Intrusion? (Sec. Sec. 197.25
Through 197.26)
We adopted NAS's suggested starting point for a human-intrusion
scenario. As NAS recommended, our standard required a single-borehole
intrusion scenario based upon Yucca Mountain-specific conditions. The
intended purpose of analyzing this scenario ``* * * is to examine the
site- and design-related aspects of repository performance under an
assumed intrusion scenario to inform a qualitative judgment'' (NAS
Report p. 111). The assessment would result in a calculated RMEI dose
arriving through the pathway created by the assumed borehole (with no
other releases included). Consistent with the NAS Report, we also
required ``that the conditional risk as a result of the assumed
intrusion scenario should be no greater than the risk levels that would
be acceptable for the undisturbed-repository case'' (NAS Report p.
113). We interpreted NAS's term ``undisturbed'' to mean that the Yucca
Mountain disposal system is not disturbed by human intrusion but that
other processes or events that are likely to occur could disturb the
system.
The DOE is not required to use probabilistic performance assessment
for the human-intrusion analysis, as it is for the individual-
protection standard. However, if it chooses to do so, we required that
the human-intrusion analysis of disposal system performance use the
same methods and RMEI characteristics for the performance assessment as
those required for the individual-protection standard, with the
exception that the human-intrusion analysis would exclude unlikely
natural features, events, and processes (FEPs).
The DOE must determine when the intrusion would occur based upon
the earliest time that current technology and practices could lead to
waste package penetration without the drillers noticing the canister
penetration. In general, we believe that the time frame for the
drilling intrusion should be within the period that a small percentage
of the waste packages have failed but before significant migration of
radionuclides from the engineered barrier system has occurred because,
based upon our understanding of drilling practices, this period would
be about the earliest time that a driller would not recognize an impact
with a waste package.
The compliance standard for human intrusion parallels that for the
individual-protection scenario. If the intrusion were to occur at or
earlier than 10,000 years after disposal, DOE must demonstrate a
reasonable expectation that annual exposures incurred by the RMEI
within 10,000 years as a result of the intrusion event would not exceed
150 [mu]Sv (15 mrem) CEDE. However, if the intrusion occurred after
10,000 years, or when earlier intrusions result in exposures projected
to occur after 10,000 years, DOE would not have to compare its results
against a numerical standard, but would have to include those results
in its EIS.
iii. What Are the Standards To Protect Ground Water? (Sec. Sec. 197.30
Through 197.31)
We established separate ground-water standards as a means to
protect the aquifer as both a resource for current users and a
potential resource for larger numbers of future users either near the
repository or farther away in communities comprised of a substantially
larger number of people than presently exist in the vicinity of Yucca
Mountain. The standards DOE must meet are equivalent to the
radionuclide Maximum Contaminant Levels (MCLs) established for drinking
water.
To implement the ground-water protection standards in Sec. 197.30,
we required that DOE use the concept of a ``representative volume'' of
ground water (Sec. 197.31). Under this approach, DOE must project the
concentration of radionuclides or the resultant doses within a
``representative volume'' of ground water for comparison against the
standards. We selected a value of 3,000 acre-ft/yr as a ``cautious, but
reasonable'' figure for the representative volume. Section 197.31 also
describes two methods by which DOE may calculate radionuclide
concentrations in ground water. See the preamble to the 2001 rulemaking
for more discussion of the representative volume and approaches for
calculating radionuclide concentrations for compliance purposes.
As with the individual-protection standard, compliance with the
ground-water protection standards must be determined at the point of
highest concentration in the plume of contamination in the accessible
environment. The controlled area was defined in the same way as for the
individual-protection standard. The ground-water protection standards
were a subject of the Court decision, were upheld, and are not a
subject of today's proposal.
c. What Is ``Reasonable Expectation''? (Sec. 197.14)
An important provision of our standards is the establishment of the
principle of ``reasonable expectation'' to guide implementation of our
standards and provide context for evaluating projections against the
numerical compliance standards discussed above. It is a critical
element in implementing our standards, but its importance might easily
be overlooked or misunderstood. We use the concept of ``reasonable
expectation'' in these standards to reflect our intent regarding the
level of ``proof'' necessary for NRC to determine whether the projected
performance of
[[Page 49021]]
the Yucca Mountain disposal system complies with the standards (see
Sec. Sec. 197.20, 197.25, and 197.30). In issuing our 2001 standards,
we noted that this term is meant to convey our position that
unequivocal numerical proof of compliance is neither necessary nor
likely to be obtained for geologic disposal systems. We believe
unequivocal proof is not possible because of the extremely long time
periods involved and because disposal system performance assessments
require extrapolations of conditions and the actions of processes that
govern disposal system performance over those long time periods.
The primary means for demonstrating compliance with the standards
is the use of computer modeling to project the performance of the
disposal system under the range of expected conditions. These modeling
calculations involve the extrapolation of site conditions and the
interactions of important processes over long time periods,
extrapolations that involve inherent uncertainties in the necessarily
limited amount of information that can be collected through field and
laboratory studies and the unavoidable uncertainties involved in
simulating the complex and time-variable processes and events involved
in long-term disposal system performance. Overly conservative
assumptions made in developing performance scenarios can bias the
analyses in the direction of unrealistically extreme situations, which
in reality may be highly improbable, and can deflect attention from
questions critical to developing an adequate understanding of the
expected features, events, and processes (``Assumptions, Conservatisms,
and Uncertainties in Yucca Mountain Performance Assessments,'' Sections
11 and 12, July 2005, Docket No. OAR-2005-0083-0085). The reasonable
expectation approach focuses attention on understanding the
uncertainties in projecting disposal system performance so that
regulatory decision making will be done with a full understanding of
the uncertainties involved. Thus, realistic analyses are preferred over
conservative and bounding assumptions, to the extent practical.
B. Legal Challenges to 40 CFR Part 197
Various aspects of our standards were challenged in lawsuits filed
with the U.S. Court of Appeals for the District of Columbia Circuit in
July 2001. Oral arguments were conducted on January 14, 2004. These
challenges and the outcome are described in the following sections.
1. Challenges by the State of Nevada and Natural Resources Defense
Council
The State of Nevada, the Natural Resources Defense Council (NRDC),
and several other environmental and public interest groups challenged
several aspects of our final standards on the grounds that they were
insufficiently protective and had not been adequately justified.
Specifically, they claimed that:
EPA's promulgation of standards that apply for 10,000
years after disposal violates the EnPA because such standards are not
``based upon and consistent with'' the findings and recommendations of
the NAS. NAS recommended standards that would apply to the time of
maximum risk and stated that there is ``no scientific basis for
limiting the time period of the individual-risk standard to 10,000
years or any other value.''
The size of the controlled area defined by EPA, which
represents the maximum extent of the disposal system and inside which
DOE need not demonstrate compliance with the EPA standards, rests on
inappropriate assumptions regarding the ability of people to live
closer to the repository and violates the Safe Drinking Water Act
provisions against endangering sources of drinking water.
EPA's definition of ``disposal'' in 40 CFR 197.12 deviates
from the definition in the NWPA by inserting the qualifying phrase
``for as long as reasonably possible,'' suggesting that the Yucca
Mountain disposal system would be held to a lesser standard of
protection because it would not have to provide ``permanent
isolation.''
2. Challenge by the Nuclear Energy Institute
The Nuclear Energy Institute (NEI) is a trade organization
representing nuclear power producers, who collect a surcharge from
ratepayers for the Nuclear Waste Fund (established by the NWPA, see 42
U.S.C. 10222). NEI challenged the ground-water protection provisions in
40 CFR 197.30 on several grounds, including that:
They conflict with the direction in the EnPA that EPA
issue standards ``based upon and consistent with the findings and
recommendations of'' NAS and that EPA's ``standards shall prescribe the
maximum annual effective dose equivalent * * * from releases * * * from
radioactive materials stored or disposed of in the repository.'' NEI
argued that EPA's ground-water standards: (1) were in a form other than
effective dose equivalent (EDE); (2) were not recommended by NAS, which
stated that such standards were not ``necessary to limit risks to
individuals'' (NAS Report p. 121); and (3) were not limited to releases
from the repository because they require that DOE consider natural
background when determining compliance.
The science underlying the ground-water standards uses the
outdated ``critical organ'' methodology, which results in inconsistent
risk estimates and is inconsistent with other radiation-protection
standards.
EPA justified its ground-water standards on cost grounds
without conducting a thorough cost-benefit analysis; NEI believes such
an analysis would show that the ground-water standards provide no
benefit to public health but will increase the cost and slow the
construction of the repository.
EPA is inappropriately applying drinking water standards,
which were derived to apply to customers of public water supplies
(i.e., ``at the tap'') to ground water.
C. Ruling by the U.S. Court of Appeals for the District of Columbia
Circuit
Oral arguments for the challenges described above were heard on
January 14, 2004. The challenges to EPA's standards were consolidated
with challenges to NRC's licensing requirements, DOE's siting
guidelines, and the Presidential recommendation of the Yucca Mountain
site and the subsequent Congressional resolution. The Court's ruling
was handed down on July 9, 2004. The Court upheld EPA's Yucca Mountain
rule in all respects, save for the regulatory compliance period.
1. What Did the Court of Appeals Rule on the Issue of Compliance
Period?
The Court upheld the challenge to EPA's 10,000-year compliance
period, ruling that EPA's action was not ``based upon and consistent
with'' the NAS Report, and that EPA had not sufficiently justified its
decision to apply compliance standards only to the first 10,000 years
after disposal on policy grounds. Nuclear Energy Institute v.
Environmental Protection Agency, 373 F.3d 1 (D.C. Cir. 2004) (NEI)
(Docket No. OAR-2005-0083-0080). On that point, the Court stated that:
NAS's conclusion that EPA ``might choose to establish consistent
policies'' is of little importance * * * And although our case law
makes clear that a phrase like ``based upon and consistent with''
does not require EPA to hew rigidly to NAS's findings, EnPA Section
801(a) cannot reasonably be read to allow a regulation wholly
inconsistent with NAS recommendations. (NEI, 373 F.3d at 30.)
Similarly, the Court rejected EPA's reasoning that the requirement
of 40
[[Page 49022]]
CFR 197.35 that DOE project performance to the time of peak dose and
place those projections in the Environmental Impact Statement (EIS)
addressed the intent of the NAS recommendation by ensuring that
assessments would not be arbitrarily cut off at some earlier time:
Although EPA's addition of this provision might well represent a
nod to NAS, it hardly makes the agency's regulation consistent with
the Academy's findings. NAS recommended that the compliance period
extend to the time of peak risk, yet EPA's rule requires only that
DOE calculate peak doses and expressly provides that ``[n]o
regulatory standard applies to the results of this analysis.'' (Id.
at 31, emphasis in original)
While the Court suggested that under different circumstances the
Agency's standard might have been upheld, it nevertheless rejected the
Agency's limitation of the compliance period to 10,000 years:
In sum, because EPA's chosen compliance period sharply differs
from NAS's findings and recommendations, it represents an
unreasonable construction of section 801(a) of the Energy Policy
Act. Although EnPA's ``based upon and consistent with'' mandate
leaves EPA with some flexibility in crafting standards in light of
NAS's findings, EPA may not stretch this flexibility to cover
standards that are inconsistent with the NAS Report. Had EPA begun
with the Academy's recommendation to base the compliance period on
peak dosage and then made adjustments to accommodate policy
considerations not considered by NAS, this might be a very different
case. But as the foregoing discussion demonstrates, EPA wholly
rejected the Academy's recommendations. We will thus vacate part 197
to the extent that it requires DOE to show compliance for only
10,000 years following disposal. (Id. at 31.)
Finally, the Court concluded that ``we vacate 40 CFR part 197 to
the extent that it incorporates a 10,000-year compliance period'' * * *
(Id. at 100.) The Court did not address the protectiveness of the 150
Sv/yr (15 mrem/yr) dose standard applied over the 10,000-year
compliance period, nor was the protectiveness of the standard
challenged. It ruled only that the compliance period could not be found
consistent with or based upon the NAS findings and recommendations, and
therefore was contrary to the plain language of the EnPA.
a. What Were NAS's Findings (``Conclusions'') and Recommendations on
the Issue of Compliance Period?
As the Court noted, NAS stated that it had found ``no scientific
basis for limiting the time period of the individual-risk standard to
10,000 years or any other value,'' and that ``compliance assessment is
feasible * * * on the time scale of the long-term stability of the
fundamental geologic regime--a time scale that is on the order of 10\6\
years at Yucca Mountain.'' As a result, and given that ``at least some
potentially important exposures might not occur until after several
hundred thousand years * * * we recommend that compliance assessment be
conducted for the time when the greatest risk occurs'' (NAS Report pp.
6-7).
However, NAS also stated ``although the selection of a time period
of applicability has scientific elements, it also has policy aspects
that we have not addressed. For example, EPA might choose to establish
consistent policies for managing risks from disposal of both long-lived
hazardous nonradioactive materials and radioactive materials' (NAS
Report p. 56).
2. What Did the Court of Appeals Rule on Other Issues Related to EPA's
Standards?
The Court did not sustain any of the other challenges lodged by
Nevada, NRDC, or NEI. Instead, the Court found that:
In defining the controlled area, EPA's conclusions
regarding the likely extent of the future population and their
exposures were reasonable. Further, the provisions of the Safe Drinking
Water Act do not apply at Yucca Mountain (by virtue of the EnPA
statement that EPA's standards ``shall be the only standards applicable
to the Yucca Mountain site''). (NEI, 373 F. 3d at 32-38.)
EPA is not bound to follow the NWPA definition of
``disposal'' because the enabling authority for this action is the
EnPA, which does not require that NWPA definitions be used and does not
itself define ``disposal.'' Therefore, EPA acted reasonably ``in
filling that statutory gap.'' (Id. at 38-39.)
EPA's interpretation of the EnPA as permitting separate
ground-water standards is reasonable because: (1) The EnPA does not
restrict EPA to establish only EDE standards, but requires that EPA
``establish a set of health and safety standards, at least one of which
must include an EDE-based, individual-protection standard''; (2) NAS
made no ``finding or recommendation'' either for or against a ground-
water standard, so consistency with NAS is not at issue; and (3) ``Part
197 * * * does not regulate background radiation * * * the rule
requires only that DOE take background levels into account when
measuring permissible releases of radionuclides from the repository.
Therefore, part 197 could not possibly run afoul of EnPA's focus on
released radiation.'' (Id. at 43-48.)
NEI's arbitrary and capricious arguments in NEI were the
same as the arguments that NEI had raised in a challenge to EPA's
radionuclide MCLs under the Safe Drinking Water Act, which the Court
had rejected only one year previously in City of Waukesha v. EPA. (Id.
at 48-49.)
EPA ``unremarkably'' concluded that ground-water
protection standards represent sound pollution prevention policy and
will encourage a more robust repository design. This reasoning
prevailed with the Court on both the cost-effectiveness and ``at the
tap'' challenges. (Id. at 49-50.)
II. How Will EPA Address the Decision by the Court of Appeals?
As promulgated, 40 CFR part 197 contained four sets of standards
against which compliance would be assessed. The storage standard
applies to exposures of the general public during the operational
period, when waste is received at the site, handled in preparation for
emplacement in the repository, emplaced in the repository, and stored
in the repository until final closure. The three disposal standards
apply to releases of radionuclides from the disposal system after final
closure, and include an individual-protection standard, a human-
intrusion standard, and a set of ground-water protection standards.
In today's action, we are not proposing to revise all of these
standards, only those affected by the Court decision. Therefore, we are
proposing to revise only the individual-protection and human-intrusion
standards, along with certain supporting provisions related to the way
DOE must consider features, events, and processes (FEPs) in its
compliance analyses. In addition, we are proposing to adopt updated
scientific factors for calculating doses to show compliance with the
storage, individual-protection, and human-intrusion standards, as
described in more detail in Section II.C.6. We are not proposing to
change any aspect of the ground-water protection standards. We are
providing notice and requesting public comment only on our proposed
revisions to 40 CFR part 197. With the exception of the updated factors
for calculating doses for the storage standard, we are not requesting
and will not consider public comment on either the storage or ground-
water protection standards. Furthermore, we are not requesting, nor
will we consider, comments on those aspects of the individual-
protection and
[[Page 49023]]
human-intrusion standards to which no changes are proposed.
We are proposing to address the Court's decision by revising
elements of our standards to incorporate the time of peak dose into the
determination of compliance. We are also proposing to further delineate
how DOE should incorporate features, events, and processes that may
take place over very long times into its calculation of peak dose,
consistent with our ``reasonable expectation'' standard.
A. How Will Elements of the Disposal Standards be Affected?
The Court's ruling vacated only one aspect of 40 CFR part 197, the
10,000-year compliance period. Thus, we considered the language and
reasoning of the Court's decision to determine its applicability to
each element of the disposal standards. The three main components of
the standards are discussed in the following sections. We also
considered the need to modify certain other aspects that would
influence how DOE would conduct its performance assessments beyond
10,000 years. These aspects are discussed in more detail in Section
II.D (``How Will Today's Proposal Affect the Way DOE Conducts
Performance Assessments?'').
1. Individual-Protection Standard
The Court's decision clearly affects the compliance period for the
individual-protection standard, which is the primary standard for
public health and safety called for by the EnPA. The legal challenge
and the Court's response left no doubt that the compliance period for
the individual-protection standard was at issue and the decision
centered on the NAS's recommendation regarding the compliance period
for the individual-protection standard. Therefore, as described in
Section II.C, we are proposing today to modify the individual-
protection standard to incorporate a compliance measure effective at
the time of peak dose, in addition to the 15 mrem/yr standard
applicable for the first 10,000 years after disposal, which we are
retaining.
Section I.A.1.b.i discusses other elements of the individual-
protection standard, specifically the definition of the controlled area
and the use of the RMEI as the representative exposed person. We are
not modifying the definition of the controlled area, which was upheld
by the Court. We have described the maximum extent of the area, using
current conditions and relatively near-term plans for development. The
actual compliance point will be determined through the licensing
process, and DOE will have to justify its reasons for selecting a
particular location to NRC.
Similarly, we are not proposing to alter the description of the
RMEI as a person having a ``rural-residential'' lifestyle as reflected
in today's population. We have described at length our reasons for
using current characteristics as an appropriate means to avoid
excessive speculation about which of the infinite number of possible
future lifestyles would be most representative over very long periods
(see 66 FR 32088-32094 (Docket No. OAR-2005-0083-0042) and Section 4 of
the Response to Comments document for the 2001 rulemaking (Docket No.
OAR-2005-0083-0050)). Some comments on our 1999 proposal disagreed with
our reasoning and choice of RMEI. We recognize that interested parties
may see an extension of the compliance period as justifying a different
description for the RMEI, at least for time frames well beyond 10,000
years. They may point to climate change scenarios as potentially making
the ``rural-residential'' lifestyle as it is defined in our 2001 rule
incompatible with climate change assumptions. It may be argued that
climate change could significantly affect the types of locally grown
food in the RMEI's diet, as well as the use of contaminated ground
water for irrigation or watering livestock, which would ultimately
influence exposures. NAS alluded to such a possibility, noting that one
effect of climate change could be ``a shift in the distribution and
activities of human populations'' (NAS Report p. 92). However, NAS also
concluded that ``there is no simple relation between future climatic
conditions and future population'' (NAS Report p. 92). We agree that it
is difficult to predict exactly how climate change, or other
evolutionary scenarios, would influence lifestyles, nor can we predict
the viability or distribution of agricultural activities compared with
those pursued today. In fact, we believe that the RMEI as a current
``rural-residential'' individual may be among the more conservative
possibilities. Given the importance of irrigation and other uses of
ground water in the Amargosa Valley region, it is likely that potential
exposures to contaminated ground water would be lower under many wetter
climate change scenarios where greater precipitation could reduce the
use of ground water for irrigation and other practices.
Some commenters might question whether it is important to have
internal consistency between climate/biosphere characteristics and RMEI
lifestyle and characteristics. We believe that it would be highly
speculative to select RMEI characteristics to correspond to some future
climate state. We require that DOE consider climate change within
10,000 years, and are proposing today also to require consideration of
climate change for much longer times (see Section II.D.2.d,
``Consideration of Climatological FEPs''). As noted above, we believe
the present-day RMEI represents a conservative choice if, as seems
likely, future climate in the Yucca Mountain region tends to be cooler
and wetter. Under wetter conditions, agricultural activities around the
site area would rely less on irrigation using well water. With less use
of contaminated ground water for irrigation, the contribution to the
RMEI dose from contaminated food would presumably be lowered or perhaps
eliminated. In counterpoint, under wetter conditions, it is possible to
speculate that individuals could live closer to the repository than is
considered for present-day conditions and potentially tap contaminated
ground waters closer to Yucca Mountain than at the RMEI location. We
believe that the RMEI, as presently defined for present-day conditions,
is a reasonably conservative approach for the dose assessments, and is
appropriate for wetter climate conditions. Assumptions regarding the
possible uses of ground water are quite speculative and have been
avoided to the extent possible in the setting of the standards (66 FR
32111). Therefore we are not redefining the RMEI characteristics in any
attempt to correlate them with climatic variations, primarily due to
speculation regarding the uses of ground water by man. As noted above,
this approach is consistent with the NAS's conclusion that there is no
exact correlation between potential climate changes and shifts in the
distribution and activities of human populations. Comments on the
definition of the controlled area and specification of the RMEI are
outside the scope of today's proposal. We will not consider or respond
to comments on these topics.
2. Human-Intrusion Standard
While the Court did not specifically address the human-intrusion
standard, we believe it is logical and defensible to modify it to
parallel the individual-protection standard. Like the individual-
protection standard, our provisions for human intrusion envisioned some
consideration of performance beyond 10,000 years. The 2001 standard
required that DOE determine when an intrusion by drilling would be
possible and assess the consequences. The resulting exposures
[[Page 49024]]
were then subject to the same compliance standard as the individual-
protection standard (15 mrem/yr at 10,000 years or earlier and dose
projections beyond 10,000 years to be compiled in the EIS). In
proposing revisions to the human-intrusion standard to conform to
changes we are proposing to make to the individual-protection
provisions, we are adhering to the NAS recommendation that ``EPA
require that the estimated risk calculated from the assumed intrusion
scenario be no greater than the risk limit adopted for the undisturbed-
repository case'' (NAS Report p. 12). In light of this recommendation,
and the Court's interpretation of how closely we must align with the
NAS recommendations to be deemed ``based upon and consistent,'' we
believe it is both prudent and reasonable to propose to revise the
human-intrusion standards to incorporate peak dose compliance measures
that conform to the proposed revisions for individual protection.
Aside from the application of dose standards at both 10,000 years
and the time of peak dose, the foundation of the proposed revised
human-intrusion standard is unchanged. DOE must determine the earliest
time at which it would be possible to penetrate waste packages by
drilling. The scenario described in Sec. 197.26 would still apply
(i.e., penetration of a single package, direct pathway to ground water,
etc.). The decision to apply a regulatory standard for the period of
geologic stability does not in any way affect the reasoning underlying
the selection of this scenario. It remains fully consistent with the
NAS conclusion that at Yucca Mountain ``there is no scientific basis
for estimating the probability of intrusion at far-future times'' (NAS
Report p. 106). Instead, NAS recommended that ``the result of the
analysis should not be integrated into an assessment of repository
performance based on risk, but rather should be considered separately.
The purpose of this consequence analysis is to evaluate the resilience
of the repository to intrusion'' (NAS Report p. 109). NAS further
suggested that EPA describe a ``stylized'' intrusion scenario based on
current drilling technologies, an approach we adopted in Sec. 197.26
and which will remain unchanged by today's proposal.
The circumstances of the intrusion scenario in Sec. 197.26 are
required to be developed based on present-day practices, in accordance
with the NAS recommendation. This approach was fully justified for the
reasons given by NAS and unchallenged for the 10,000-year time frame.
We find that maintaining the approach beyond 10,000 years is also fully
justified and consistent with the NAS for the same reasons. If
anything, it would be even more speculative to attempt to project
changes to the circumstances of the intrusion at time frames
potentially out to 1 million years. Furthermore, in keeping with the
purpose of the human-intrusion analysis as a test of repository
resilience, it is appropriate to continue to exclude unlikely natural
events and processes from the analysis.
The intrusion scenario requires consideration of package
degradation, premised on the assumption that drillers encountering an
intact package would cease drilling and releases would be avoided. We
believe that this assumption is equally valid both within and beyond a
10,000-year time frame. In our 2001 rule, DOE would not have been
required to demonstrate compliance with a dose limit if packages were
determined not to degrade sufficiently within 10,000 years to permit
intrusion (or, in any event, if the consequences of the intrusion were
not calculated to occur within 10,000 years). We are proposing to
modify our rule to require that DOE show compliance with a dose limit
regardless of when the consequences of the intrusion occur. Consistent
with the proposed revised individual-protection standard, DOE will have
to show compliance with a peak dose standard beyond 10,000 years, in
addition to a 150 [mu]Sv/yr (15 mrem/yr) standard applicable up to
10,000 years. The dose standard that applies to exposures to the RMEI
through the period of geologic stability will be the same as for the
individual-protection standard (see Section II.C.3, ``What Dose Level
is EPA Proposing for Peak Dose?''). Overall, this scenario continues to
represent a reasonable test that ``can provide useful insight into the
degree to which the ability of a repository to protect public health
would be degraded by intrusion'' (NAS Report p. 108). We are not
soliciting, and will not consider, comments on the overall intrusion
scenario or other aspects of the human-intrusion standard that are not
proposed to be changed.
3. Ground-Water Protection Standards
The Court's decision does not affect the ground-water protection
standards. The Court upheld our statutory reading of the EnPA as
providing the authority to establish such standards as the Agency
deemed necessary to supplement the individual-protection standard, as
well as the scientific basis of those standards. (See NEI, 373 F.3d at
43-48, Docket No. OAR-2005-0083-0080.) The Court further concluded that
our reasoning for including such a standard as a means to protect the
ground-water resource was sound and consistent with the Agency's
overall pollution prevention policies. Regarding consistency with the
NAS recommendations, the Court stated that:
Although we concluded earlier in this opinion that EPA violated
section 801's ``based upon and consistent with'' requirement by
adopting a 10,000-year compliance period, we reach the opposite
conclusion here because NAS treated the compliance-period and
ground-water issues quite differently. Whereas NAS expressly
rejected a 10,000-year compliance period, it said nothing at all
about the need to add a separate ground-water standard * * * Put
another way, NAS made no ``finding'' or ``recommendation'' that
EPA's regulation could fail to be ``based upon and consistent
with.''
NEI, 373 F.3d at 46-47.
As a result, we do not believe the Court's ruling regarding the
10,000-year compliance period applies to the ground-water protection
standards, which have the same compliance period. Further, unlike the
individual-protection and human-intrusion standards, we never
envisioned that DOE would project its compliance with the ground-water
protection standards beyond 10,000 years, even for inclusion in the
EIS. The Court decision leaves EPA with discretion in formulating the
provisions for ground-water standards. We believe (and the Court
agreed) that the application over 10,000 years of limits equivalent to
MCLs is a conservative but reasonable regulatory scheme that represents
sound pollution prevention policy. Furthermore, protection of public
health from releases to ground water over times beyond 10,000 years
will be provided by extending the individual-protection standard to the
time of peak dose, which accounts for transport and exposure through
all pathways. For these reasons, we are not proposing to modify the
ground-water protection standards, either by extending the period of
compliance or in any other respect. We are not requesting, and will not
consider, comments regarding any aspect of the ground-water protection
standards.
4. Reasonable Expectation
``Reasonable expectation'' is the compliance concept underlying our
disposal standards. That is, we require that DOE show a ``reasonable
expectation'' that the standards will be met. As discussed extensively
in our 2001 Yucca Mountain rulemaking, ``proof'' of disposal system
performance
[[Page 49025]]
in the traditional sense of the word cannot be attained for periods
extending into the thousands or hundreds of thousands of years (66 FR
32101-32103, June 13, 2001, Docket No. OAR-2005-0083-0042). In such
situations, it is a natural tendency to give greater emphasis to
aspects that may not be the most likely to occur, but have the
potential to significantly affect performance. This may be particularly
true in areas where physical data are limited. However, assessments
that are built around conservative assumptions at every decision point
may in fact result in highly unrealistic performance projections.
Simplifications and assumptions are involved out of necessity because
of the complexity and time frames involved, and the choices made will
determine the extent to which modeling simulations realistically
simulate the disposal system's performance. If choices are made that
make the simulations very unrealistic, the confidence that can be
placed on modeling results is very limited. The uncertainties involved
with these simplifications must be recognized. Overly conservative
assumptions made in developing performance scenarios can bias the
analyses in the direction of unrealistically extreme situations, which
in reality may be highly improbable, and can deflect attention from
questions critical to developing an adequate understanding of the
expected features, events, and processes. ``Reasonable expectation''
encourages the use of ``cautious, but reasonable'' assumptions and
discourages the reliance on highly conservative assumptions. It
recognizes that projections of disposal system performance over very
long times are best viewed as indicators of performance, rather than as
firm predictions. It further requires the applicant and regulator to
focus on the full range of outcomes and not to give greater weight to
certain projections simply because they are more conservative.
The concept of ``reasonable expectation'' was a guiding principle
in the formulation of our 2001 standards. We believe the concept is
equally applicable for periods well beyond 10,000 years, and is in fact
more important for very long time periods. In our view, it is
``reasonable'' to consider approaches for uncertainties in calculations
at several hundred thousand years that may differ from the approach for
uncertainties considered within 10,000 years after disposal. An
approach applying standards ``acceptable today for the period of
geologic stability would ignore this cumulative uncertainty and the
extreme difficulty of using highly uncertain assessment results to
determine compliance with that standard'' (66 FR 32098, June 13, 2001,
Docket No. OAR-2005-0083-0042). We therefore emphasize the primacy of
``reasonable expectation'' in compliance with 40 CFR part 197 and
retain it without change. However, we have considered how DOE and NRC
might need to approach the concept to account for the much greater
overall uncertainty in projections over periods as long as 1 million
years. Section II.B describes the overall concept of ``reasonable
expectation'' and our thoughts for today's proposal in more detail.
5. Effects of Uncertainty
We believe that the most problematic aspect of extending the
compliance period to peak dose is the uncertainty involved in making
projections over such long time frames, which we discussed in some
detail in our proposed and final rulemakings in 1999 and 2001,
respectively. This remains a critical factor in formulating today's
proposal, which we feel must be emphasized and explored in detail.
Although we refer generally to ``uncertainties'' throughout this
document, it may not always be clear to readers exactly what we mean by
this term, why their effects are difficult to manage, and why they
should have an impact on the decision-making process. It may be useful
to consider an analogous situation that will be readily familiar, such
as the tracking of hurricanes.
The strength and path of hurricanes are functions of factors such
as temperature, humidity, barometric pressure, and wind speed. There is
natural variation in these parameters, and their variation can make the
difference between a Category 5 storm (the most severe) striking a
populated coastal area and a tropical storm that remains out in the
ocean. When one views the projected path of a storm, the surrounding
envelope of possible paths expands as one looks into the future and may
spread over several hundred miles. The critical task in tracking the
storm is identifying which populated areas are in the path of the
storm, and whether they must be evacuated.
By this analogy, a 10,000-year dose projection might be comparable
to selecting a single town to evacuate when the storm is still two
hundred miles from landfall, while a peak dose projection might be more
like pinpointing the correct location when a tropical depression first
forms thousands of miles away, which may be weeks earlier. Regardless
of the level of rigor that can be applied to the technical calculation,
it is simply not possible to place the same level of confidence in the
two selections. We see similar difficulties in ``predicting'' the
``true'' behavior of the Yucca Mountain disposal system, or the
multiple engineered and natural components of that system, for periods
on the order of hundreds of thousands of years.
We are aware that some stakeholders dispute our position that
uncertainties increase significantly with time, and therefore believe
that uncertainty offers little justification for placing less
confidence in very long-term projections than can be placed in those
that apply over the relatively near term. Some stakeholders, for
example, suggest that uncertainty should have little impact on peak
dose projections and that DOE should be required to identify where
uncertainty, rather than reasonably expected performance, influences
dose projections (Docket No. OAR-2005-0083-0029 and 0033). They have
pointed to statements in the NAS Report to bolster this position, such
as: ``analyses that are uncertain at one time might not be so uncertain
at a later time; for example, the uncertainties about cumulative
releases to the biosphere that depend on the rate of failure of the
waste packages are large in the near term but are smaller later, when
enough time has passed that all of the packages will have failed'' (NAS
Report pp. 29-30); ``Because there is a continuing increase in
uncertainty about most of the parameters describing the repository
system farther in the distant future, it might be expected that
compliance of the repository in the near term could be assessed with
more confidence. This is not necessarily true'' (NAS Report p. 72);
``Detailed estimates of time for canister failure are less important
for much longer-term estimates of individual dose or risk'' (NAS Report
p. 85).
Although NAS pointed out that uncertainties associated with some
disposal system components will decrease over time (e.g., at some time
all waste packages will be degraded), our view, and the view of many
others (including NAS, as should be clear from the above citation:
``Because there is a continuing increase in uncertainty * * *''), is
that uncertainties generally increase with time, at least to the time
of peak dose. (See, for example, IAEA Draft Safety Requirements DS154,
``Geological Disposal of Radioactive Waste,'' Section A.7, page 37,
April 2005 (Docket No. OAR-2005-0083-
[[Page 49026]]
0051), which states, ``It is recognized that radiation doses to people
in the future can only be estimated and the uncertainties associated
with these estimates will increase farther into the future''; the
Nuclear Energy Agency report on ``The Handling of Timescales in
Assessing Post-Closure Safety,'' pp. 13-14 (Docket No. OAR-2005-0083-
0046), which states, ``These events and changes are subject to
uncertainties, which generally increase with time and must be taken
into account in safety assessments. Eventually, but at very different
times for different parts of the system, uncertainties are so large
that predictions regarding the evolution of the repository and its
environment cannot meaningfully be made''; and the Swiss National
Cooperative for the Disposal of Radioactive Waste (Nagra), which
states, in Technical Report 02-05 (pp. 27-28) (Docket No. OAR-2005-
0083-0075), ``HSK-R-21 [Swiss disposal regulation] acknowledges that
there is inevitable uncertainty in model calculations and the further
into the future predictions are made, the greater the uncertainty. The
implementer has to show what processes and events could affect the
repository over the course of time and then to derive and evaluate
potential evolution scenarios from these.'') For some aspects of the
system, such uncertainties can increase dramatically (``Assumptions,
Conservatisms, and Uncertainties in Yucca Mountain Performance
Assessments,'' Section 12.3, July 2005, Docket No. OAR-2005-0083-0085).
To repeat, we are in agreement with NAS that such projections can be
performed and even ``bounded'' to some extent. However, the central
question here is how the results of very long-term assessments can have
sufficient meaning to provide an adequate basis for a licensing
decision that the repository should or should not be approved.
NAS demonstrated some concern with this issue by recognizing that
the level of confidence that could be placed in projections was of key
importance, and offered constructive guidance in limiting or
considering the effects of uncertainties. Unfortunately, the NAS
statements on decreasing uncertainty regarding some disposal system
components do not draw a clear relationship to the time of peak dose at
which it recommended compliance be measured. While we generally agree
with these statements, we find that they are most relevant to times
after peak dose and, therefore, after the time frame most important
from a regulatory perspective. Returning to our hurricane analogy, it
is true that uncertainties eventually decrease; one might be able to
predict with equal confidence both the storm's location in two hours
and that in two weeks it will have completely dissipated. In this
sense, one can agree with the NAS's conclusion that ``it is not
necessarily true'' that long-term projections are more uncertain than
near-term projections. Nevertheless, relatively high confidence about
the endpoint of the hurricane has little impact on the ability to
predict where and when it might cause the greatest damage along its
path. Similarly, for Yucca Mountain, increasing confidence in certain
aspects of the system's components (e.g., the endpoint of the waste
packages, much like the endpoint of the hurricane) does not necessarily
inform estimates of peak dose.
NAS notes that ``uncertainties about cumulative releases'' that
``depend on the rate of failure of the waste packages'' will be
lessened at far future times when ``all of the packages will have
failed'' (NAS Report p. 28-29). The emphasis here on eventual failure
cannot help us when the direction is to assess peak dose. It is self-
evident and non-controversial that the engineered barrier system cannot
be expected to last forever. However, assumptions regarding ``the rate
of failure of waste packages'' are exactly the critical element in
estimating the timing and magnitude of the peak dose (``Assumptions,
Conservatisms, and Uncertainties in Yucca Mountain Performance
Assessments,'' Sections 12.3 and 12.4, July 2005, Docket No. OAR-2005-
0083-0085). Thus, identifying factors that would decrease overall
system uncertainty at times approaching 1 million years does not
adequately support a conclusion that uncertainties can be equally well
managed at the time of peak dose, even if that time is much less than 1
million years.
In addressing this larger question of how to consider long-term
projections in a regulatory process, we have considered guidance and
precedents from international programs. NAS provided important
scientific and technical reasoning for evaluating compliance at peak
dose, which we augment with guidance from sources who approached the
problem of uncertainty from the regulatory perspective. For regulatory
compliance over 10,000 years, we were able to identify several (albeit
limited) analogous regulatory programs in the U.S., including those for
the WIPP and EPA's underground injection control program (see the
preamble to the 2001 rulemaking, 66 FR 32098, Docket No. OAR-2005-0083-
0042). For time frames extending potentially to 1 million years, there
are no precedents in U.S. regulation. In response to the Court
decision, therefore, important sources for guidance and models for
contemplating regulations at such long times were other international
programs grappling with the same issues, namely disposal of highly
radioactive and long-lived waste. Throughout this document, we quote
extensively from a number of international sources, from both
multinational organizations (such as IAEA) and individual countries
(such as Sweden). We do this because we find ourselves in a situation
that is, if not unique, shared by a rather small circle. We have found
it useful to consult the ideas of those faced with a similar situation.
In general, they reinforce two points we emphasize throughout this
document. The first, which we have already discussed, is that
uncertainties generally increase with time. The second point is that
projections at those longer times cannot be viewed with the same level
of confidence as shorter-term projections, and may in fact be viewed as
more qualitative indicators of disposal system performance.
For example, the IAEA has stated that, for periods lasting from
about 10,000 to 1 million years, ``While it may be possible to make
general predictions about geological conditions, the range of possible
biospheric conditions and human behaviour is too wide to allow reliable
modelling * * * Such calculations can therefore only be viewed as
illustrative and the `doses' as indicative'' (IAEA-TECDOC-767, ``Safety
Indicators in Different Time Frames for the Safety Assessment of
Underground Radioactive Waste Repositories,'' p. 19, 1994, Docket No.
OAR-2005-0083-0044). Also, ``[t]he utility of individual numerical
indicators will vary greatly and, given the large uncertainties,
considerable caution is needed to avoid any suggestion or expectation
that any given indicator of disposal system performance can be an
accurate estimate of future reality. Such an indicator typically
provides only an estimate of what might happen under certain assumed
conditions * * * The aim of the assessment is not to predict the actual
performance of the disposal system * * * but rather to reach reasonable
assurance that it will provide an adequate level of safety'' (IAEA-
TECDOC-975, ``Regulatory Decision Making in the Presence of Uncertainty
in the Context of the Disposal of Long Lived Radioactive Wastes,'' pp.
22, 24,
[[Page 49027]]
1997, Docket No. OAR-2005-0083-0045). Finally, ``[c]are has to be
exercised in applying the criteria for periods beyond the time where
the uncertainties become so large that the criteria may no longer serve
as a reasonable basis for decision making'' (IAEA Draft Safety
Requirements DS154, ``Geological Disposal of Radioactive Waste,''
Section A.7, p. 37, April 2005, Docket No. OAR-2005-0083-0051).
The Nuclear Energy Agency (NEA) states that ``[t]here is an
increasing consensus among both implementers and regulators that, in
carrying out safety assessments, calculations of dose and risk should
not be extended to times beyond those for which the assumptions
underlying the models and data can be justified * * * Eventually, but
at very different times for different parts of the system,
uncertainties are so large that predictions regarding the evolution of
the repository and its environment cannot meaningfully be made'' (``The
Handling of Timescales in Assessing Post-Closure Safety,'' pp. 10, 13,
2004, Docket No. OAR-2005-0083-0046). Similarly, the Swedish Radiation
Protection Authority (SSI) has proposed draft guidance for the disposal
of SNF, stating that ``[f]or very long periods * * * [t]he intention
should be to shed light on the protective capability of the repository
and to provide a qualitative picture of the risks'' (p. 7, Docket No.
OAR-2005-0083-0048). This draft guidance is intended to supplement
SSI's standards (SSI FS 1998:1, September 28, 1998, Docket No. OAR-
2005-0083-0047), which require that ``[f]or the first thousand years
after disposal, the assessment of the repository's protective
capability shall be based on quantitative analyses of the impact on
human health and the environment'' (Sec. 11), but do not specify
quantitative analyses as the basis for longer-term assessments (``shall
be based on various possible sequences for the development of the
repository's properties, its environment and the biosphere,'' Sec.
12).
We acknowledge that detailing the effects of uncertainty is itself
uncertain. We recognize that knowledge is not absolute up to 10,000
years, with uncertainties burgeoning shortly beyond that time. We also
recognize that there can be considerable uncertainty in measurements of
current conditions. Further, we concur with NAS that uncertainties can
be qualitatively different for different aspects of the assessment. For
example, NAS points out that human behavior can be projected for a few
decades at most, while the geologic record can be studied for evidence
of processes that have occurred over millions of years (and are still
occurring today). However, the assessment of Yucca Mountain's
performance depends not only on the ability to project large-scale
geologic processes, such as seismicity and volcanism, but also the
gradual evolution of complex saturated and unsaturated zone
characteristics, such as the chemistry of infiltrating water or the
direction and connectivity of a fracture-flow system.
B. How Does the Application of ``Reasonable Expectation'' Influence
Today's Proposal?
Under today's proposal, projecting disposal system performance
involves the extrapolation of physical conditions and the interaction
of natural processes with the wastes for unprecedented time frames in
human experience, i.e., possibly hundreds of thousands of years. In
this sense, the projections of the disposal system's long-term
performance cannot be confirmed. Not only is the projected performance
of the disposal system not subject to confirmation, the natural
conditions in and around the repository site will vary over time and
these changes are also not subject to confirmation, making their use in
performance assessments equally problematic over the long-term. In
light of these fundamental limitations on assessing the disposal
system's long-term performance, we believe that the approach used to
evaluate disposal system performance must take into account the
fundamental limitations involved and not hold out the prospect of a
greater degree of ``proof'' than in reality can be obtained.
There are several fundamental components to be established in
setting up and analyzing disposal system performance scenarios. A model
must be created that translates the physical processes operating at the
site into mathematical statements, such as ground-water flow equations,
that can calculate the movement of radionuclides through the various
components of the disposal system and into the accessible environment.
A model may be very generic or highly sophisticated and tailored to
capture distinct aspects of a particular site. Two additional steps are
necessary in order to develop dose projections. First, the possible
performance scenarios themselves and associated assumptions must be
established, and second, the distribution of expected values for the
parameters involved in the performance calculations must be determined.
The scenarios are developed from an understanding of the natural
processes, the engineered barrier design, and the interactions of the
engineered barrier system with the repository environment. The range of
expected parameter values for the analyses is based upon the results of
site characterization studies, laboratory testing, and expert judgment.
For both of these components, unrealistic and perhaps extreme choices
can be made that would, in effect, give false expectations of disposal
system performance, or hide important uncertainties that would, in
reality, have important consequences on the performance projections
(the model itself may also have conservatisms built into it, which may
be even more difficult to identify). If extreme assumptions are made in
defining the scenario, a de facto ``worst-case'' scenario is developed
at the outset and analyses using the upper end of the range of
parameter values result in performance projections that are in fact
extreme cases, rather than representing the full range of expected
performance. Effectively, such a restrictive approach results in
emphasis on what would be the conservative extremes of the probability
distributions for the performance assessments and analyses rather than
if a realistic approach were taken. In such a case, the regulatory
judgment would be focusing on extreme situations, rather than on
evaluating safety under reasonably expected conditions. On the other
hand, if the scenario were defined more realistically and the same
distribution of parameter values used, the resultant distribution of
doses would be closer to the actual expected performance and regulatory
decisions could be made with confidence that the assessments represent
a more realistic range of expected performance. Including multiple
``worst-case'' assumptions in setting up the performance scenarios,
combined with selecting conservative values for site-related parameter
distributions, actually corresponds to assessing very low-probability/
high-consequence scenarios that can then easily be mistaken as
expected-case analyses. Under the reasonable expectation approach,
expected case as compared to conservative and worst-case assessments
are more explicitly identified and the uncertainties presented more
directly so that the reasoning behind regulatory decisions can be more
easily understood and defended. We note that this approach was also
recommended by a joint NEA-IAEA peer review of DOE's TSPA to support
its site recommendation, which states in Section 4.1.3 (``Realism or
conservatism''):
[[Page 49028]]
At a fundamental level, it is useful to resort to a
probabilistic analysis of a system evolution in time if a realistic
model can be attempted but legitimate uncertainties persist.
However, if the starting model is built a priori to be conservative,
exercising it probabilistically has little or no added value, as one
would still obtain conservative results. In the TSPA-SR a hybrid
conservative/probabilistic methodology is used, which causes
assumptions and reality to be mixed in a confusing way. In the
future it may be appropriate to present: (i) A probabilistic
analysis based on a realistic or credible representation; and (ii) a
set of complementary analyses with different conservatisms, in order
to place the best available knowledge in perspective. These
ancillary analyses could be given a probabilistic weight as well.
This should satisfy the regulatory requirements whilst providing a
better basis for dialogue and decision-making.
``An International Peer Review of the Yucca Mountain Project TSPA-
SR,'' pp. 54-55, 2002, Docket No. OAR-2005-0083-0062, emphasis in
original.
In making its decisions, the primary task for NRC is to examine the
projections put forward by DOE to determine ``how much is enough'' in
terms of the information and analyses presented, i.e., how NRC
determines when the analyses provide an acceptable level of confidence
and the results can be interpreted in a way meaningful for regulatory
compliance. In 40 CFR part 197 as originally promulgated, we did not
have specific measures in our standards on how to make that judgment.
NRC, as the implementing agency, must be satisfied with DOE's
presentation; therefore, we concluded those specific measures of
satisfaction were appropriate for NRC to determine. Neither did EPA
specify: (1) Confidence measures for such judgments or numerical
analyses; (2) analytical methods that must be used for performance
assessments; (3) quality assurance measures that must be applied; (4)
statistical measures that define the number or complexity of analyses
that should be performed; or (5) any assurance measures in addition to
the numerical limits in the standards. We specified only that the mean
of the dose assessments must meet the exposure limit.
We anticipate that if these very long-range performance projections
(beyond 10,000 years) indicate that repository performance would
degrade dramatically under a wide range of conditions at some point in
time, that this would become a concern in the licensing decision. If
such a dramatic deterioration were projected to occur close to the
regulatory time period it would be a more pressing concern for
licensing decisions than if it were to occur many hundreds of thousands
of years into the future (remembering that the uncertainty in
performance projections increases with time). With the initial issuance
of 40 CFR part 197, EPA elected to leave the handling of the very long-
term projections of performance as an implementation decision for the
regulatory authority, but to impose the requirement that such analyses
be performed and reported in the EIS. The degree of ``weight'' that
should be given to these very long-term assessments, we said, is an
implementation decision that should be left to NRC to determine, by
balancing the projected performance and the inherent uncertainties in
these projections against the projected dose levels (2001 Response to
Comments, p. 7-13, Docket No. OAR-2005-0083-0043).
We propose to continue this general approach of not specifying the
bases or mechanisms for a compliance decision, except that the post-
10,000-year analyses are now proposed to be part of the 40 CFR part 197
standards with a quantitative limit imposed.
As noted earlier, the conceptual framework of ``reasonable
expectation'' as promulgated in our 2001 rulemaking is applicable even
when extending the compliance period to peak dose. In fact, we believe
it becomes even more important as the level of confidence that can be
placed in numerical projections decreases over time. However, we are
not proposing to expand or modify the definition in Sec. 197.14 to
account for the greater uncertainty between 10,000 years and the time
of peak dose (within 1 million years of disposal). The existing
definition describes principles that are applicable for both shorter
and very long time frames (although the implications of these
principles may be different, depending on the time frame). To provide
insight into our interpretation of reasonable expectation at very long
times, we provide additional information in the remainder of this
section and throughout our discussion of the proposed changes for NRC
to consider as it implements our peak dose standard. We believe such
guidance will be useful, particularly in the context of handling long-
term FEPs, as discussed in Section II.D of this document.
We emphasize that parameters and scenarios should be included in
the performance assessment even if they are not among the more highly
conservative approaches. There is a tendency in long-term assessment to
introduce conservatisms and to focus on the higher-end dose
projections, while discounting lower dose projections that may actually
be just as probable or perhaps represent higher-probability scenarios.
We stress that DOE should work to ensure that the results express the
full range of possible outcomes within the bounds of credible scenarios
and parameter values. Less conservative scenarios (i.e., lower
projected doses) should not be eliminated unless they are deemed to be
highly improbable. Of course, the compliance measure will be expressed
as a specific statistical measure of the results, not the entire range
of results. The entire range of results is context to be used to assist
the licensing authority in judging the likelihood of the facility to
meet the standards. In that context, the results of the performance
assessments are not to be biased by an overemphasis on low-probability
scenarios at the expense of results for the entire spectrum of
reasonably credible and supportable scenarios and parameter values. Our
position is that the reasonable expectation approach accounts for the
inherent uncertainties involved in projecting disposal system
performance by taking into account a large spectrum of possible
parameter values rather than making assumptions that reflect only
conservative to very conservative values. We also emphasize that the
uncertainties in site characteristics over long time frames, and how
the long-term projections of expected performance of the disposal
system were made, need to be well understood before regulatory
decisions are made. We stress again the purpose of the assessments as
expressed by IAEA: ``The aim of the assessment is not to predict the
actual performance of the disposal system * * * but rather to reach
reasonable assurance that it will provide an adequate level of safety''
(IAEA-TECDOC-975, p. 24, Docket No. OAR-2005-0083-0045). NAS agrees
that ``[t]he results of compliance analysis should not, however, be
interpreted as accurate predictions of the expected behavior of a
geologic repository'' (NAS Report p. 71, Docket No. OAR-2005-0083-
0076).
In Section II.D of this document (``How Will Today's Proposal
Affect the Way DOE Conducts Performance Assessments?''), we propose to
limit speculation over the long compliance period now being addressed
by requiring compliance within a performance assessment that continues
to emphasize the most significant features, events, and processes. The
purpose is to provide a reasonable test of performance over a range of
conditions. To do so, we propose to eliminate very unlikely features,
events, and processes, and the scenarios
[[Page 49029]]
including them, from consideration and specify this in the standards.
We believe this is consistent with a finding of the NAS: ``It is always
possible to conceive of some circumstance that, however unlikely it may
be, will result in someone at some time being exposed to an
unacceptable radiation dose * * * The challenge is to define a standard
that specifies a high level of protection but that does not rule out an
adequately sited and well-designed repository because of highly
improbable events'' (NAS Report pp. 27-28). We have chosen to do this
by continuing to place reasonable constraints on the scenarios that
need to be examined. We believe this is consistent with another finding
of the NAS: ``We conclude that the probabilities and consequences of
modifications generated by climate change, seismic activity, and
volcanic eruptions at Yucca Mountain are sufficiently boundable so that
these factors can be included in performance assessments that extend
over periods on the order of about 10\6\ years'' (NAS Report p. 91).
Typically, as we discuss elsewhere in this document, the term
``boundable'' implies a ``worst case'' approach (i.e., a ``bounding
analysis'') to assessing the limits of disposal system performance. We
do not believe such an approach is appropriate and are not proposing to
adopt it. Instead, in this context, we interpret ``boundable'' as
referring to limits that may be placed on the scenarios so that they
will represent a reasonable test of disposal system performance over
the very long term, but not be driven by extreme assumptions or endless
speculation. Thus, we view our treatment of these ``modifiers'' as
comparable to our specification of a ``stylized'' scenario for human
intrusion, and consistent with the NAS statement that ``[i]t is
important that the `rules' for the compliance assessment be established
in advance of the licensing process'' (NAS Report p. 73).
In our 1999 preamble to proposed 40 CFR part 197, we said that if
we were to regulate longer than 10,000 years, we would expect the
licensing judgment to be less strict in relying on dose projections
compared to 10,000 years (64 FR 46998, August 17, 1999, Docket No. OAR-
2005-0083-0041): ``We note that if the compliance period for the
individual-protection standard extended to the time of peak dose within
the period of geologic stability (which NAS estimated to be 1 million
years for the Yucca Mountain site), this [reasonable expectation] test
would allow for decreasing confidence in the numerical results of the
performance assessments as the compliance period increases beyond
10,000 years. For example, this means that the weight of evidence
necessary, based upon reasonable expectation, for a compliance period
of 10,000 years would be greater than that required for a compliance
period of hundreds of thousands of years.'' Given the increased
uncertainty that is unavoidable in the capabilities of science and
technology to project and affect outcomes over the next 1 million
years, the concept of reasonable expectation underlying our standards
implies that a dose limit for that very long period that is higher than
the 15 mrem/yr limit that applies in the relatively ``certain'' pre-
10,000-year compliance period could still provide a comparable judgment
of overall safety. See Section II.C.3 (``What Dose Level is EPA
Proposing for Peak Dose?'') for a specific discussion of the dose limit
in today's proposal.
In formulating an approach to compliance out to the time of peak
dose, we have established 10,000 years as an indicator for times when
uncertainties in projecting performance are more manageable and for
which comparisons can be made with other regulated systems. We realize
that uncertainties exist within the initial 10,000-year period and that
10,000 years does not represent a strict dividing point between periods
over which projections can be made with certainty or not. Clearly, we
believe that calculations beyond 10,000 years have value, or we would
not have previously required DOE to include them in its EIS. However,
we also believe that over the very long periods leading up to the time
of the peak dose, the uncertainties in projecting climatic and geologic
conditions become extremely difficult to reliably predict and a
technical consensus about their effects on projected performance in a
licensing process would be very difficult, or perhaps impossible, to
achieve. This is one of the major reasons that the 10,000-year time
frame was originally selected in the generic standard for land disposal
of the types of waste intended for the Yucca Mountain repository (40
CFR part 191) (2001 Response to Comments, p. 7-17, Docket No. OAR-2005-
0083-0043). In such a situation, one might conclude that little or no
weight should be given to highly uncertain projections as a basis for a
licensing decision. Conversely, others might conclude that the
inability to produce highly reliable performance estimates should
preclude the possibility of licensing at all. Such a conclusion would
be inconsistent with any concept of permanent disposal, which
necessarily requires examination of time frames and events that cannot
be predicted with certainty. We believe that the performance
projections at Yucca Mountain, if constructed and interpreted
consistent with the concept of ``reasonable expectation,'' can provide
useful information on the facility's performance and can form a key
part of the basis for a licensing decision. Clearly NAS agreed, since
it recognized that significant uncertainties exist, yet nonetheless
recommended that projections to peak dose form the basis for EPA's
standards to be used in judging compliance for licensing the Yucca
Mountain disposal system. NAS further recognized that an approach akin
to reasonable expectation is warranted: ``No analysis of compliance
will ever constitute an absolute proof; the objective instead is a
reasonable level of confidence in analyses that indicates whether
limits established by the standard will be exceeded'' (NAS Report p.
71).
C. How Is EPA Proposing To Revise the Individual-Protection Standard
(Sec. 197.20) To Address Peak Dose?
In considering how to revise the individual-protection standard, we
have sought an approach that would be:
Responsive to the Court ruling;
Protective of public health and safety;
Reflective of the best science and cognizant of the limits
of long-term projections;
Implementable by NRC in its licensing process; and
Limited in scope and focused on aspects critical to
accomplishing the above goals.
In balancing these goals, we have carefully examined the NAS
recommendations and looked more broadly to international models and
guidance on long-term radioactive waste disposal. We believe today's
proposal satisfies these goals. We believe the first three are
straightforward and our reasoning outlined in the next sections will
clearly show how they influenced our proposal. The fourth point relates
to an essential purpose of our action that can sometimes be
overshadowed by emphasis on the NAS recommendations and the Court
ruling. As NAS stated, ``standards are only useful if it is possible to
make meaningful assessments of future repository performance with which
the standards can be compared'' (NAS Report p. 34). Ultimately, NRC
must be able to use our standards to judge whether DOE has provided
sufficient evidence that the disposal system will be protective of
public health and safety. While there are
[[Page 49030]]
significant scientific aspects to this decision, regulatory judgment
must bridge the gap between what science can show and the unprecedented
time frames involved. The licensing process must consider the
confidence that can be placed in performance assessments used to
represent disposal system evolution and the information necessary to
make a decision. Our ``reasonable expectation'' standard is critical to
making this judgment.
The last point above refers to the legal status of our rule.
Today's proposal is specifically targeted toward addressing the Court
ruling regarding the compliance period. Many other aspects of our rule
were either upheld by the Court or not challenged. As discussed in
Section II.A, we are not revisiting those issues.
In a similar vein, when considering potential approaches to address
the Court's decision, we did not feel constrained by our actions in the
2001 rulemaking. Nor do we believe that rejecting certain approaches in
that rulemaking creates a legal barrier to incorporating them into
today's proposal. Our preferred approach was rejected by the Court in
favor of a compliance standard applicable at the time of peak dose,
whenever it might occur within the period of geologic stability. In our
2001 rulemaking, we considered, discussed, and accepted comment on the
length of the compliance period, including consideration of the time of
peak dose. We ultimately chose not to establish a compliance period
applicable throughout the period of geologic stability. Thus, it is
difficult to see how we could satisfy the Court's ruling if we were not
permitted to reconsider or revise our previous conclusions.
1. Multiple Dose Standards Applicable to Different Compliance Periods
In balancing the considerations described above, the central
problem is to determine what is achievable in terms of the reliability
of dose projections. Our task was clearly presented by the Court, and
our starting position is to fulfill that task by proposing a compliance
standard at the time of peak dose, whenever it might occur within the
period of geologic stability. We have discussed at length our concerns
regarding the quality of very long-term projections and their
application in a licensing process; even in light of the Court
decision, those concerns remain. However, we also believe it is clear
that shorter-term projections do have sufficient reliability to serve
as the basis for regulatory decision-making. On the one hand, we do not
want to place more regulatory emphasis on peak dose projections than
can be justified; on the other, a standard effective at relatively
short times, where we believe such emphasis is warranted, is unlikely
on its own to be responsive to the Court ruling. We have sought to
reconcile these two extremes in order to satisfy all of the goals
outlined earlier.
In what we see as the best solution to this difficulty, today we
are proposing that the individual-protection standard consist of two
parts, which will apply over different time frames. One part of the
standard, which will apply over the initial 10,000 years after
disposal, consists of the 15 mrem/yr individual-protection standard
promulgated in 2001 as 40 CFR 197.20. The other part other part of the
standard, which is being proposed today, will apply beyond 10,000 years
to the time of peak dose up to a limit of 1 million years. We believe
this approach appropriately recognizes the relative manageability of
uncertainties at such disparate times, and the resulting level of
confidence that can be derived from performance projections.
There is no disagreement internationally that quantitative
projections are the most direct means of evaluating disposal system
performance, or that comparison of such projections with an acceptable
level of performance is a straightforward and transparent method of
assessing disposal system safety. However, there is also a general
consensus that reliance on quantitative projections to determine safety
may be misleading and incomplete, becoming more so at times very far
into the future. IAEA notes that ``[q]uantitative analysis is
undertaken, at least over the time period for which regulatory
compliance is required, but the results from detailed models of safety
assessment are likely to be more uncertain for time periods in the far
future'' (DS154, Section 3.48, p. 25, Docket No. OAR-2005-0083-0051).
Also, ``an indication that calculated doses could exceed the dose
constraint, in some unlikely circumstances, need not necessarily result
in the rejection of a safety case * * * In general, when irreducible
uncertainties make the results of calculations for the safety
assessment less reliable, then comparisons with dose or risk
constraints have to be treated with caution'' (DS154, Sections A.7,
A.8, pp. 36-37, Docket No. OAR-2005-0083-0051). As suggested by the
discussion of reasonable expectation in Section II.A.4, at longer time
periods, the quantitative projections should be considered less for
their strict numerical outcomes and more as one component in a
qualitative evaluation of the overall safety case.
In their book ``Principles and Standards for the Disposal of Long-
Lived Radioactive Wastes'' (2003, Docket No. OAR-2005-0083-0061),
Chapman and McCombie state that ``[a]n approach commonly used is to
calculate releases, doses or risks out to peak consequences--but to use
different approaches to judging acceptability in different time frames.
At far future times (>10 ka) [>10,000 years] * * * calculated doses may
then be more appropriately compared with less stringent limits than the
typical limits at shorter times'' (p. 79). They also present the
concept of ``time-graded containment objectives'' in which the first
1,000 years or so is characterized by ``total containment of all
activity in the repository.'' For the ``next one (or a few) hundred
thousand years * * * doses * * * are below the range of natural
background radiation.'' Finally, ``after this time * * * there is no
further containment objective: doses may be envisaged in the range of
those from natural background radiation.'' (p. 114)
Different countries have approached this situation in various ways,
and many national regulations are still evolving. For example, as
summarized by Chapman and McCombie in Table 5.1 (Docket No. OAR-2005-
0083-0061): Canada at one time limited quantitative compliance to
10,000 years, to be followed by qualitative evaluation, with special
attention to the rate of increase in projected risk; Germany takes a
similar approach in official guidance, but does not specify a time
frame in regulation; France requires quantitative compliance for
100,000 years, with the situation becoming ``hypothetical'' afterward;
Switzerland requires numerical compliance at all times. The Swedish
draft guidance referred to in Section II.A.5 states that ``[f]or long
periods of time, thousands of years and even longer, the risk analysis
should be successively regarded as an illustration of the protective
capability of the repository assuming certain conditions'' (p. 7,
Docket No. OAR-2005-0083-0048). We believe the approach proposed today,
outlined in the paragraphs below, is consistent with that trend.
First, we are retaining the standard promulgated in 2001 as Sec.
197.20, which requires that DOE demonstrate a reasonable expectation
that the RMEI will not incur annual exposures greater than 150 [mu]Sv
(15 mrem) (expressed as a committed effective dose equivalent) from
releases of radionuclides from the Yucca Mountain disposal system for
10,000 years after disposal. DOE will make this demonstration using the
arithmetic mean of performance
[[Page 49031]]
assessment results (see Section II.C.5, ``How Will NRC Judge
Compliance?'' for further discussion of the mean). We believe this is
appropriate, protective, and will maintain consistency with our generic
standards (now applied to the WIPP) and other precedents described
earlier. Further, NAS stated that the ``range [of 10-\5\ to
10-\6\ per year for risk] could therefore be used as a
reasonable starting point for EPA's rulemaking'' (NAS Report p. 49,
emphasis in original). By maintaining the 15 mrem/yr standard for
10,000 years we clearly establish a ``starting point'' for assessing
compliance that is consistent with both NAS and our overall risk
management policies, and serves as a logical foundation for us to
incorporate concerns regarding far future projections.
Because of the emphasis on peak dose as the key benchmark of safety
in both the NAS Report and the Court decision, some commenters may
question not only the need for a standard at such relatively short
times, but also whether it is legally permissible, given the Court's
decision. We believe there is ample justification for a separate
10,000-year standard on both counts. Taking the legal questions first,
there was no legal challenge and the Court made no ruling on the
protectiveness of our standard up to 10,000 years. Further, the Court
ruled that we must address peak dose, but did not state, and we do not
believe intended, that we could not have additional measures to bolster
the overall protectiveness of the standard. As the Court noted, the
EnPA requires that EPA ``establish a set of health and safety
standards, at least one of which must include an EDE-based, individual
protection standard'' (NEI, 373 F.3d at 45, Docket No. OAR-2005-0083-
0080), but does not restrict us from issuing additional standards.
Thus, as long as we issue ``at least one'' standard addressing the NAS
recommendation regarding peak dose, we are not precluded from issuing
other, complementary, standards to apply for a different compliance
period. The Court's concern was whether we had been inconsistent with
the NAS recommendation by not extending the period of compliance to
times longer than 10,000 years. NAS itself did not address the idea of
having separate standards to apply over different time periods. We
believe such a decision falls well within our policy discretion and in
that context the 10,000-year standard is analogous to our ground-water
protection standards.
An important reason for retaining a standard applicable for the
first 10,000 years is to address the possibility, however unlikely,
that significant doses could occur within 10,000 years, even if the
peak dose occurs significantly later, as DOE currently projects.
Examination of DOE's Total System Performance Assessments (TSPA)
for the site shows that the time of peak dose occurs in the hundreds of
thousands of years (FEIS, DOE/EIS-0250, Appendix I, Section 5.3,
February 2002, Docket No. OAR-2005-0083-0086). The waste packages
assessed in the TSPA are heavily engineered to provide corrosion
resistance under the conditions expected in the repository, and are
projected to remain essentially unbreached for periods well beyond
10,000 years. The scientific data that underlie these corrosion
resistance projections are laboratory tests on the metals, under
conditions intended to stress the metals and simulate their performance
in the repository. These testing methods are typical ``state-of-the-
art'' techniques for corrosion testing. However, it must be recognized
that the extrapolation of laboratory test results in a predictive sense
involves significant uncertainties, and our experience in verifying
such projections is only for time frames of decades in the case of
industrial applications (``Assumptions, Conservatisms, and
Uncertainties in Yucca Mountain Performance Assessments,'' Section 5,
July 2005, Docket No. OAR-2005-0083-0085). While DOE projects, based
upon the results of laboratory testing, that the waste containers will
maintain their integrity for thousands to tens of thousands of years,
it is not possible to claim unequivocally that no information will come
to light that might cause a reassessment of the containers' behavior
and its effect on disposal system performance. Although we believe that
significant doses within 10,000 years are highly unlikely, we also
believe it important to structure our regulations to preclude the
chance that protection at Yucca Mountain would be less than that
provided for WIPP or the Greater Confinement Disposal facility (GCD,
which is a group of 120-feet deep boreholes, located within NTS, which
contain disposed transuranic wastes). It would be inappropriate to
apply a standard designed to accommodate the uncertainties in
projections many tens to hundreds of thousands of years into the future
to projections within 10,000 years, when uncertainties are much more
manageable. The 15 mrem/yr dose limit is the measure against which
compliance would be judged during the initial 10,000-year period.
In today's action, we are proposing to add a standard of compliance
that would apply at the time of peak dose, if DOE determines that the
peak occurs at any time beyond 10,000 years but within 1 million years
(as recommended by NAS). Specifically, in addition to retaining the 15
mrem/yr standard applicable up to 10,000 years, we are proposing to
establish a separate numerical compliance standard against which the
median of peak dose projections would be compared (see Section II.C.3
for a discussion of the proposed dose limit and Section II.C.5 for a
discussion of the arithmetic mean and median). As discussed earlier, we
recognize that there is strong consensus in the international
radioactive waste community that dose projections extending for periods
into the many tens to hundreds of thousands of years can best be viewed
as qualitative indicators of disposal system performance, rather than
as firm predictions that can be compared against strict numerical
criteria. The primary concern, which we have also expressed, is
managing the uncertainties that become more prominent at longer time
frames.
Nevertheless, we believe that the best way to address the Court
decision is to establish a numerical compliance standard for the time
of peak dose so that a clear test for compliance decision-making can be
applied to the results of quantitative performance assessments. What we
are proposing is unprecedented in our national regulatory schemes, and
we remain greatly concerned about the ability of the implementing
agencies to manage the uncertainties in very long-term projections in
order to make comparisons with a numerical standard meaningful. We
discuss elsewhere in this document (see Sections II.B and II.D.2, for
example) ways in which NRC and DOE might temper the effects of
uncertainty in dose projections, e.g., through the selection of
parameter distributions or scenarios.
Some readers may note that we rejected similar approaches offered
in comments on our 1999 proposed rule. One commenter in particular
suggested that the dose standard could be increased over time, i.e., 15
mrem/yr up to 10,000 years, 150 mrem/yr from 10,000 to 100,000 years,
and 1.5 rem/yr from 100,000 to 1 million years (Docket A-95-12, Item
IV-D-35). As stated in our Response to Comments document published in
conjunction with the 2001 final rulemaking (p. 3-8, Docket No. OAR-
2005-0083-0043), we considered that our approach accomplished the same
goal as that offered by the commenter. While we did state that ``no
regulatory body that we are aware of considers doses of 150 mrem to be
acceptable,'' we also stated that ``the
[[Page 49032]]
uncertainties involved in very long-term assessments would make it more
difficult to judge compliance with any numerical standard,'' which we
still believe is true. It is clear that we struggled to reconcile the
competing claims of confidence in projections and intergenerational
equity. We sought an approach that would account for what we see as
potentially unmanageable uncertainties, but did not depart from levels
of risk that are considered protective today. Nevertheless, the Court's
decision puts us in the position of establishing a quantitative
standard at the time of peak dose. It is necessary for us to re-
evaluate potential approaches to doing so, including whether and under
what conditions a higher dose standard can be justified. We discuss an
approach similar to that offered by the commenter in Section II.C.2.c
(``Peak Dose Standard Varying Over Time'').
We are not requesting comment on the 15 mrem/yr standard or its
applicability for the initial 10,000-year period. The public record
reflects an exhaustive level of comment and consideration on these
points (see our 1999 proposed and 2001 final rulemakings, as well as
Sections 3 and 4 of the 2001 Response to Comments Document (Docket Nos.
OAR-2005-0083-0041, 0042, 0043, respectively). The Court did not
question the scientific basis of the 15 mrem/yr dose standard, the
protective nature of that limit, or its well-established precedents in
regulation for periods as long as 10,000 years (including its
implementation at WIPP and GCD), nor indeed were any of these aspects
of the rule challenged. Further, as noted above, the Court did not rule
that the 10,000-year compliance period had no value, only that it was
not by itself consistent with the NAS recommendation (``We will thus
vacate part 197 to the extent that it requires DOE to show compliance
for only 10,000 years following disposal,'' NEI, 373 F.3d at 31, Docket
No. OAR-2005-0083-0080).
We are requesting comment on the combination of the 15 mrem/yr
standard with a separate standard applicable beyond 10,000 years
through the period of geologic stability. We believe we have provided a
rational basis for taking this approach and that it is consistent with
the Court's position that we could have ``taken the Academy's
recommendations into account and then tailored a standard that
accommodated the agency's policy concerns.'' NEI, 373 F.3d at 26,
Docket No. OAR-2005-0083-0080.
2. What Other Options Did EPA Consider?
We considered a number of other approaches to respond to the
Court's decision, each of which had attractive qualities, as well as
disadvantages. These disadvantages generally relate to the difficulty
of implementation given the increasing complexity and uncertainty of
much longer-term projections.
a. Maintain the 10,000-Year Standard Alone Without Addressing Peak Dose
The Court suggested that, ``[h]ad EPA begun with the NAS
recommendation to base the compliance period on peak dosage and then
made adjustments to accommodate policy considerations not considered by
NAS,'' the 40 CFR part 197 standards issued in 2001 might have been
accorded more deference. NEI, 373 F.3d at 31, Docket No. OAR-2005-0083-
0080. However, it is not clear how EPA's earlier explanation of its
policy concerns might be reconciled with NAS's technical
recommendation. In view of this, we believe that the most direct and
responsive action to address the Court ruling is to revise our
standards to include consideration of the time when peak dose occurs.
Therefore, although we are retaining the previous 10,000-year
provisions as one component of our revised standards, we are also
proposing an additional measure to address the time of peak exposure
within the period of geologic stability beyond 10,000 years. We believe
that this approach, coupled with the selection of the dose standard to
apply at the time of peak dose (see Section II.C.3) and specification
of certain aspects of DOE's performance assessment (see Section II.D),
will adequately address our policy concerns.
b. Dose Standard To Apply at Peak Dose Alone
The second option we considered is simply to replace the 10,000-
year standard with one that applies at the time of peak dose, whenever
it might occur. This approach is attractive primarily because it would
be straightforward in responding to the Court decision. Although we
believe that 10,000 years has value as a precedent for safety
assessments, and are retaining that element of the standards, it is not
intrinsically significant as a demarcation point for addressing a peak
dose standard beyond 10,000 years. A peak dose standard alone (i.e.,
not in conjunction with the 10,000-year standard we are retaining)
would remove confusion on that point, but introduces additional
difficulties, as described in the following sections.
As discussed in Section II.C.4.a, we do not believe it is
reasonable or justifiable simply to extend the application of a 15
mrem/year dose limit over the entire period up to the time of peak
dose. Rather, at the time of peak dose, which could potentially occur
hundreds of thousands of years into the future, we believe rising
uncertainties justify adopting a different (higher) dose level.
However, as discussed in Section II.C.3, this approach, while more
cognizant of the effect of uncertainties and the dangers of relying on
specific numerical indicators at very long times, departs from our
previous standards of protectiveness in the event that peak doses occur
within relatively short time periods. Specifically, if peak doses occur
within 10,000 years, we would be in the position of measuring safety
against a dose level that we have explicitly rejected as not
sufficiently protective over that time frame, both in our generic
standards and in our earlier Yucca Mountain rulemaking. Further, there
would be a clear contrast between the level of protection offered to
the population in the vicinity of the WIPP and that offered the
population affected by Yucca Mountain. We recognize that our insistence
on maintaining a 15 mrem/yr standard over the initial 10,000 years
might appear inconsistent with our proposal, which could allow peak
doses shortly after 10,000 years at levels well above 15 mrem. However,
as discussed previously, we believe NRC has the authority, as part of
its licensing process, to consider the timing and magnitude of peak
dose in assessing the safety of Yucca Mountain. Furthermore, we do not
believe it is prudent to disregard the usefulness of a stringent
10,000-year measure simply because uncertainties at longer time frames
make it infeasible to conduct a performance assessment with the same
level of rigor. Our view on this point is discussed in Section II.A.1.
c. Peak Dose Standard Varying Over Time
We also considered a variation on our proposed approach, in which
the post-10,000-year dose level would rise incrementally as time and
the effects of uncertainty increase. This approach would provide
greater continuity with the 10,000-year standard and a gradual
transition as the role of uncertainty increases. The difficulty in this
approach is identifying criteria to define the timing and level of
these transitions, which would have to incorporate some appraisal and
comparison of the effects
[[Page 49033]]
of uncertainty at various times. Some of the advantages of this
approach are also captured by the statistical approach discussed in
Section II.C.2.e. We have not identified a defensible way to derive
transition levels or the times at which these dose level changes could
be made.
d. Standard Expressed as a Dose Target, Rather Than Limit
Although we have chosen to add a standard extending the compliance
period beyond 10,000 years, we believe that the most problematic aspect
of doing so is the uncertainty involved in making projections over such
long time frames, which we discussed in some detail in our proposed and
final rulemakings for 40 CFR part 197 in 1999 and 2001, respectively
(Docket Nos. OAR-2005-0083-0041 and 0042). To repeat, we are in
agreement with NAS that such projections can be performed and even
``bounded'' to some extent. However, we remain concerned about whether
and under what conditions results of very long-term assessments can
have sufficient meaning to provide the basis for a licensing decision
that the repository should or should not be approved.
One way to take these uncertainties into account is to establish a
more flexible compliance benchmark for very long time periods, one that
would represent a more qualitative ``target'' for dose assessments
rather than a strict numerical limit. This approach would be generally
consistent with several international programs. For example, the
Swedish Radiation Protection Authority (SSI) has proposed draft
guidance for the disposal of SNF, stating that ``[f]or very long
periods * * * [t]he intention should be to shed light on the protective
capability of the repository and to provide a qualitative picture of
the risks'' (p. 7, Docket No. OAR-2005-0083-0048). The Swedish
regulations themselves are not detailed regarding the way different
time periods should be addressed, although it is clear that times
beyond 1,000 years are seen differently than the period up to 1,000
years. For the first thousand years after closure, ``the assessment of
the repository's protective capability shall be based on quantitative
analyses of the impact on human health and the environment,'' but for
longer periods that assessment ``shall be based on various possible
sequences for the development of the repository's properties, its
environment and the biosphere'' (Sections 11 and 12, respectively,
Docket No. OAR-2005-0083-0047).
In some cases, this reasoning is also applied to near-surface
disposal facilities involving much shorter time frames. For example, in
the United Kingdom, ``[t]he Government therefore considers it
inappropriate to rely on a specified risk limit or risk constraint as
an acceptance criterion for a disposal facility after control is
withdrawn. It is, however, considered appropriate to apply a risk
target in the design process. However, if the estimated risk is above
the target, the Agency will need to be satisfied not only that an
appropriate level of safety is assured, but also that any further
improvements in safety could be achieved only at disproportionate cost
* * * In the very long term, irreducible uncertainties about the
geological, climatic and resulting geomorphological changes that may
occur at a site provide a natural limit to the timescale over which it
is sensible to attempt to make detailed calculations of disposal system
performance. Simpler scoping calculations and qualitative information
may be required to indicate the continuing safety of the facility at
longer times'' (UK Environment Agencies, ``Disposal Facilities on Land
for Low and Intermediate Level Radioactive Waste: Guidance on
Requirements for Authorisation,'' sections 6.14 and 8.23, Docket No.
OAR-2005-0083-0063). Thus, in the UK approach, estimated risks may be
allowed to exceed the numerical target if it is determined that further
restrictions in risk are impossible or impractical.
Our approach in the 2001 rulemaking, which required peak dose
projections to be placed in the Environmental Impact Statement, was
based on similar reasoning. It allowed NRC to evaluate those results
qualitatively, but did not prescribe that they be compared against a
dose limit. We also believe such an approach would be consistent with
our ``reasonable expectation'' standard, which intends to avoid a
narrow focus on numerical calculations and encourages consideration of
the totality of the assessment in the context of the overall safety
case (ICRP took the same view in its Publication 81, ``Radiation
Protection Recommendations as Applied to the Disposal of Long-Lived
Solid Radioactive Waste,'' stating that ``as the time frame increases,
some allowance should be made for assessed dose or risk exceeding the
dose or risk constraint. This must not be misinterpreted as a reduction
in the protection of future generations and, hence, a contradiction
with the principle of equity of protection, but rather as an adequate
consideration of the uncertainties associated with the calculated
results'' (Docket No. OAR-2005-0083-0087); similarly, IAEA states
``that calculated doses are less than the dose constraint is not in
itself sufficient for acceptance of a safety case * * * Conversely, an
indication that calculated doses could exceed the dose constraint * * *
need not necessarily result in the rejection of a safety case,'' DS154,
Section A.7, pp. 36-37, Docket No. OAR-2005-0083-0051). In considering
how to address peak dose in this standard, however, we believe it is
more implementable and will be viewed as more rigorous to set a
specific dose limit and provide direction concerning assumptions and
methodologies for peak dose calculations, and leave it to NRC to
consider the quantitative projections of peak dose as a particularly
important part of the ``full record before it'' that it will consider
in determining whether there is a reasonable expectation that the dose
limit will be achieved.
e. Standard Expressed as a Statistical Distribution
Finally, we considered a standard of compliance that would combine
features of the qualitative and quantitative approaches described
earlier. Rather than incorporating a specific numerical limit that must
be met by a single compliance measure (such as the median or arithmetic
mean of a distribution), this approach would be based upon the
characteristics of the distribution itself. It would take into account
the range of results for performance assessment by examining multiple
representative dose estimates such as upper and lower percentile
values. Under this formulation, DOE might have to show that some
percentage of the peak dose projections would remain within a certain
range of a reference dose level. For example, this standard might say
that at least 10% of peak annual dose results must be 15 mrem or lower,
and that no more than 10% of results can exceed some upper limit. Using
these parameters and assuming that DOE ran 100 assessments of system
performance using probabilistically-sampled input parameter values,
each resulting in a separately calculated ``peak'' dose, at least ten
of those results would have to be 15 mrem or lower and no more than ten
could be above the ``upper limit''.
This approach seems to address some of our concerns. First, it
recognizes growing uncertainties but constrains how much is acceptable
by specifying characteristics of the distribution that must apply at
all times without being overly affected by ``outliers.'' In fact, the
value of the projected peak dose is considered only in determining
where it falls in relation to the designated upper
[[Page 49034]]
and lower percentile measures. In this example, no more than 10% of the
results may exceed the ``upper limit'', but the amount by which they
exceed that limit is not taken into account (and similarly for doses
below 15 mrem/yr). Thus, projected doses of 1 rem/yr (1,000 mrem/yr)
would carry the same significance as much lower projected doses, as
long as both were higher than the ``upper limit''. As a result, this
approach might provide additional flexibility in judging the level of
conservatism appropriate to addressing uncertainties (and perhaps
compensate for conservatism) across a range of scenarios because the
results would not be disproportionately affected by low-probability
scenarios resulting in very high doses, as the arithmetic mean would
be. In addition, the lower dose threshold acts as a conservative
performance requirement in that it requires that the disposal system
provide a specified level of performance tied to the 15 mrem/yr dose
standard applicable to performance up to 10,000 years.
A firm base of assessments at lower levels (e.g., 15 mrem/yr) would
tie DOE's results to, and provide continuity with, the 10,000-year
projections. It could be reasonable to allow a small number of results
to exceed the ``upper limit,'' so long as the ``expected'' performance
remains within a given range (within about an order of magnitude of 15
mrem, if we were to use as the ``upper limit'' the value of 350 mrem/yr
we are proposing today). It should be kept in mind that even using the
mean of the distribution as the compliance measure allows for a
percentage of results to exceed the limit, depending to some extent on
how the distribution is skewed; this statistical approach offered for
discussion is simply more precise in specifying the percentage.
Second, while accounting for uncertainties, it can be linked to the
standards of safety established for geologic repositories at earlier
time frames. Percentile curves could be compared against reference
levels based upon well-established limits within the U.S. and
internationally, such as 15 mrem/yr, 25 mrem/yr, 30 mrem/yr, or 100
mrem/yr, or the 350 mrem/yr we are proposing today. This could provide
continuity with our approach at 10,000 years. It is reasonable to
assume that uncertainties will tend to become less manageable as time
increases, but there is no clear and predictable demarcation for when
uncertainties become ``unmanageable.''
Third, this approach would be consistent with our ``reasonable
expectation'' standard, which is intended to encourage DOE to focus on
``cautious, but reasonable'' scenarios and examine the full range of
results to obtain the best possible understanding of the long-term
behavior of the disposal system. In applying a standard that must
address times from 10,000 years up to 1 million years, it might be more
representative of system behavior to consider the entire distribution
of results that may occur over those times than to focus on a single
number as indicative of acceptable performance. Using this approach,
NRC would be assured that the bulk of the results will fall within
reasonable limits, may be better able to understand why results fall at
certain points along the continuum, and would have additional
flexibility to determine compliance within those limits.
We used a somewhat similar approach in developing the containment
requirements in 40 CFR 191.13(a). In that section of our generic
regulations, we required that calculations show that a disposal system
have no more than one chance in ten of exceeding the release limits,
and no more than one chance in 1,000 of exceeding ten times the release
limits. In establishing those requirements, we explained that the
release limits applied to ``those processes that are expected to occur
as well as relatively likely disruptions.'' The release limits
multiplied by ten applied to ``more likely natural disruptive events *
* * [and the] range of probabilities was selected to include the
anticipated uncertainties in predicting the likelihood of these natural
phenomena. Greater releases are allowed for these circumstances because
they are so unlikely to occur.'' In part 191, no release limits were
applied to even lower-probability (i.e., ``very unlikely'') events,
analogous to our approach of screening out very unlikely events at
Yucca Mountain: ``the Agency believes there is no benefit to public
health or the environment from trying to regulate the consequences of
such very unlikely events' (50 FR 38071, September 19, 1985, Docket No.
OAR-2005-0083-0064). We have successfully implemented this regulation
at WIPP.
While we see several potential positive aspects of this statistical
approach, we also have concerns regarding both the overall approach and
the ways in which it could give a misleading impression of disposal
system performance in a compliance demonstration. First, there is a
difficulty in defining exactly where percentile limits should be placed
and how they should be justified. Second, while the criteria we have
suggested would apply to the entire distribution of results, they would
essentially give the ``tails'' of the distribution a strong role in
determining whether the disposal system should be licensed. As we
discuss later in Section II.C.5 (``How Will NRC Judge Compliance?''),
we believe it is appropriate to consider an indicator of the ``central
tendency'' of the results as demonstrative of performance.
Our second concern relates to the idea that the calculated peak
dose values themselves are not explicitly incorporated into the
compliance determination through calculation of a separate statistical
measure, such as the mean. While this offers an advantage insofar as
the overall measure is not overly influenced by very high results, for
any defined set of cut-offs there is always the possibility that the
distribution will fall just outside the acceptable criteria. While
strictly speaking only the number of doses above the higher cut-off
level enters into the compliance demonstration, the magnitude of those
doses would also be important in the regulator's confidence in the
overall acceptability of the disposal system. Similarly, a distribution
that falls just outside the cut-offs could be judged ``better'' than a
distribution that meets the criteria, if a different measure such as
the mean or median were used for comparison. In considering a series of
100 realizations, for example, a distribution with 11 above, but only
slightly above, the ``upper limit'' and only nine at 15 mrem/yr or
lower (but with the next highest at only 16 mrem) would fail the test,
even if the bulk of the results were relatively low (say, below 100
mrem). However, a distribution with ten realizations significantly
higher than the ``upper limit'' (e.g., 500 mrem/yr and higher), ten at
15 mrem/yr, and most of the remaining doses well above 100 mrem/yr,
would pass the test, even though it is likely that the arithmetic mean
would be noticeably higher in the second case. Such a disparity might
also indicate the presence of high-dose scenarios in one distribution
that were not included in the other.
Therefore, we have chosen not to propose this approach for Yucca
Mountain. We are concerned that it will be less transparent to the
public and not give a clear indication of the necessary level of
performance. Further, upper and lower percentiles and dose limits must
be selected, as in the example above; the selection of all these values
would need to account for risk management and policy considerations. It
is difficult to identify a specific set of criteria that would lead to
the selection of one set of values over another.
[[Page 49035]]
3. What Dose Level is EPA Proposing for Peak Dose?
Having determined that it would be appropriate to propose a
numerical peak dose standard for the period of geologic stability
beyond 10,000 years, we must then determine the appropriate level for
that standard. We considered several factors in selecting the level
proposed today. First, and most significant, is the issue of
uncertainty in long-term projections. Uncertainties are problematic not
only because they are challenging to quantify, but also because their
impact will differ depending on initial assumptions and the time at
which peak dose is projected to occur. Further, the natural tendency in
modeling long-term processes is to introduce additional conservatisms
to help ensure that actual performance will be no worse than projected
performance. Thus, excessive conservatism in addressing uncertainty
drives assessments away from ``cautious, but reasonable'' assumptions
and may result in an unrealistic, overly pessimistic view of disposal
system performance. As we stated in our earlier rulemaking, ``[s]etting
a strict numerical standard at a level of risk acceptable today would
ignore this cumulative uncertainty and the extreme difficulty of using
highly uncertain assessment results to determine compliance with that
standard'' (66 FR 32098, June 13, 2001, Docket No. OAR-2005-0083-0042).
This raises a broader point regarding the significance of very-long
term projections and how they should be considered in the context of
repository safety. Leaving aside the uncertainties inherent in
projecting geologic characteristics over such periods, even a well-
characterized site will display natural variability in the parameters
that influence radionuclide transport. This natural variability exists
at every possible site and can be reduced (or at least better
estimated) by site characterization, but can never be eliminated, no
matter how stable the site. As assessments extend to longer time
periods, this natural variability will lead to an increasing spread of
results even if conditions do not change significantly (it may be
useful again for the reader to refer to the hurricane analogy discussed
in Section II.A.5, where the range of possible storm paths increases as
forecasts look farther ahead in time). Therefore, given the difference
in the level of confidence regarding the ``real'' performance of the
disposal system for projections at 250,000 years as at 10,000 years, we
believe that emphasizing incremental dose increases when such increases
are overwhelmed by fundamental uncertainties inappropriately takes
attention away from an evaluation of the overall safety of the disposal
system and its ability to contain and isolate wastes or respond to
disturbances. On that point, we have argued against viewing projections
as ``predictions'' of disposal system performance and have emphasized
that assessments should aim to provide a ``reasonable expectation''
that performance will be within acceptable limits (on this point, see
the NAS Report, for example p. 71: ``The results of compliance analysis
should not, however, be interpreted as accurate predictions of the
expected behavior of a geologic repository''). While there is a body of
experience in applying the ``reasonable expectation'' concept for
10,000 years, we are also considering its implications for time periods
in the hundreds of thousands of years (see Section II.B, ``How Does the
Application of ``Reasonable Expectation'' Influence Today's
Proposal?'').
We have also considered the potential impacts to future generations
that would be represented by a dose standard applied to periods up to 1
million years. Impacts on future generations could come in the form of
economic cost, health impacts, or a reduction in the options available
to make decisions to address the problems faced by those generations. A
number of regulatory and scientific bodies suggest that it is
appropriate to relate longer-term standards to background radiation
levels. NEA, for example, suggests that consideration of future
generations ``implies that the safety implications of a repository need
to be assessed for as long as the waste presents a hazard'' but that
such assessments need not focus on exposures: ``In view of the way in
which uncertainties generally increase with time, or simply for
practical reasons, some cut-off time is inevitably applied to
calculations of dose or risk. There is, however, generally no cut-off
time for the period to be addressed in some way in safety assessment,
which is seen as a wider activity involving the development of a range
of arguments for safety'' (``The Handling of Timescales in Assessing
Post-Closure Safety,'' p. 39, 2004, Docket No. OAR-2005-0083-0046,
emphasis in original). This reasoning supports the idea that dose
projections should be given progressively less weight in the overall
decision as time passes. We note that ICRP recently discussed a similar
concept. Specifically, ICRP suggests that future projected doses can be
weighted to take into account a variety of factors, and that
``[w]eights can also be assigned according to the time at which the
exposure will occur'' (``The Optimisation of Radiological Protection,''
draft for consultation, p. 29, April 2005, Docket No. OAR-2005-0083-
0052). Such an approach could involve giving doses in the far future
less weight, either in a numeric sense or in the context of the overall
safety case.
The National Academy of Public Administration (NAPA), in its 1997
report ``Deciding for the Future: Balancing Risks, Costs, and Benefits
Fairly Across Generations'' (Docket No. OAR-2005-0083-0087), recognizes
that each generation must consider not only how its actions will affect
future generations, but also the extent to which inaction will
compromise its own interests and negatively affect those same future
generations.
To inform decision-making, NAPA defined four basic principles:
Trustee: Every generation has obligations as trustee to
protect the interests of future generations;
Sustainability: No generation should deprive future
generations of the opportunity for a quality of life comparable to its
own;
Chain of Obligation: Each generation's primary obligation
is to provide for the needs of the living and succeeding generations.
Near-term concrete hazards have priority over long-term hypothetical
hazards;
Precautionary: Actions that pose a realistic threat of
irreversible harm or catastrophic consequences should not be pursued
unless there is some countervailing need to benefit either current or
future generations.
Under NAPA's approach, there is no absolute freedom of succeeding
generations to escape the effect of the preceding generations'
decisions. Rather, it is the responsibility of each generation to
consider those decisions and their consequences in the light of new
knowledge, technology, societal attitudes, and economic or other
factors. NAPA terms this the ``rolling present.'' As it relates to the
management of spent nuclear fuel, there is no question that the next
several generations may incur societal as well as economic costs,
whether it involves continued development of the Yucca Mountain
repository, development of interim storage facilities or expanded
storage at reactor sites, or decisions regarding the future use of
nuclear power. Application of the NAPA principles would lead each
generation to an approach that would best address the problem without
unduly limiting the options available to succeeding generations to
modify that approach or
[[Page 49036]]
to take other actions to address their needs.
In general, while there is wide agreement that future generations
should not be unduly compromised by the decisions of the current
generation, there is no clear consensus regarding the extent of the
claims held by future generations on the current generation (i.e., how
many generations should be considered, how to compare their interests
to those of the current generation, or what it means to ``compromise''
their ability to take action). The Swedish National Council for Nuclear
Waste (KASAM) concludes that increasing uncertainties ``means that our
capacity to assume responsibilities changes with time. In other words,
our moral responsibility diminishes on a sliding scale over the course
of time'' (Nuclear Waste State-of-the-Art Reports 1998, p. 27, Docket
No. OAR-2005-0083-0056). KASAM suggests that for the next 5 or 6
generations (roughly 150 years), we can apply a ``Strong Principle of
Justice'' so that these generations can be expected to achieve a
quality of life equivalent to that of the current generation. For a
further 5 or 6 generations, we may only be able to apply a ``Weak
Principle of Justice'' to ensure that these generations can at least
satisfy their basic needs. Beyond that point, the best we can do is
conduct ourselves today so as not to jeopardize future generations'
possibilities for life (the ``Minimal Principle of Justice''). In the
case of spent fuel disposal, these considerations lead to the idea that
a repository must provide reasonable protection and security for the
very far future, but this may not necessarily be at levels deemed
protective (and controllable) for the current or succeeding
generations.\2\
---------------------------------------------------------------------------
\2\ This sentiment, however, is not universal. Chapman and
McCombie point out that the Swiss radiation protection regulations
make the argument ``that since the current generation is the
beneficiary of nuclear power future doses should be less'' (p. 53).
They then acknowledge, however, that such arguments are complex,
noting that ``it has been pointed out that future generations do
indeed benefit from nuclear technology through the technical
advances made, the conservation of fossil reserves, the reduction in
greenhouse gases, etc.'' Further, they go on to write:
In addition, the inability to guarantee long-term or effectively
permanent institutional control over long-lived uranium mining
wastes disposed of at the earth's surface or over historical
``legacy wastes'' in countries where defence programmes have
resulted in large-scale contamination, means that we are implicitly
accepting (for this type of waste, and some NORM wastes) that future
generations may have lower levels of protection than today. This is
causing re-examination of the appropriate balance of radiological
protection standards for the future for these materials. The most
commonly accepted principle today for disposal of nuclear fuel cycle
wastes is that future generations must be protected for very long
times (at least 10,000 years) to at least reach the level of
protection expected by today's generations; for extremely long times
the growing tendency is to then make comparisons with natural
sources of radiation, such as ore bodies.
``Principles and Standards for the Disposal of Long-Lived
Radioactive Wastes,'' pp. 53-54, 2003, Docket No. OAR-2005-0083-
0061.
---------------------------------------------------------------------------
In any case, it is clear that quantitative regulatory limits cannot
be applied indefinitely. There is general agreement that assessments
(and corresponding regulatory safety limits or reference points) for
periods longer than 1 million years are of limited value in any case
(e.g., IAEA states that ``little credibility can be attached to
assessments beyond 106 years. Even qualitative assessments
will contribute little to the decision making process'' (``Safety
Indicators in Different Time Frames for the Safety Assessment of
Underground Radioactive Waste Repositories,'' IAEA-TECDOC-767, p. 19,
1994, Docket No. OAR-2005-0083-0044), and Sweden's draft guidance
states that ``[n]o account need be given for periods beyond a million
years after closure, even if'' peak exposures would be expected after
that time (p. 7, Docket OAR-2005-0083-0048).
In addition to examining international guidance and precedents, we
also reviewed the NAS's statements on the subject. As discussed in
detail later in this section, NAS refrained from recommending any
specific dose or risk limit for regulations, but instead suggested a
range of risks as a ``starting point'' for EPA's consideration.
Further, while NAS stated that a standard that ``could * * * apply
uniformly over time and generations * * * would be consistent with the
principle of intergenerational equity,'' it also recognized that other
approaches are possible: ``Whether to adopt this or some other
expression of the principle of intergenerational equity is a matter for
social judgment'' (NAS Report pp. 56-57).
In determining an appropriate level of protection for periods up to
1 million years, we believe it is appropriate to consider potential
exposures from the Yucca Mountain disposal system in the context of
exposures incurred by residents of other areas of the United States
from natural sources. Specifically, we believe it is reasonable to set
a standard that would represent a level of incremental radiation
exposure such that the total annual exposure of the RMEI could be
comparable to the total natural radiation exposures incurred now by
current residents of well-populated areas. Given the large
uncertainties surrounding the outcomes at these unprecedented time
frames, we believe such an action is justifiable and protective. Using
this approach, we are proposing to establish a standard of 350 mrem
(3.5 mSv) per year, which will limit total radiation exposures of the
RMEI to levels comparable to those incurred today from natural sources
by residents of a nearby western State.
We believe this level of protection appropriately blends the
concerns outlined above with current and historical thinking regarding
the acceptability of risks associated with background radiation, while
recognizing the conceptual difficulties inherent in regulating at times
potentially hundreds of thousands of years into the future. NAS
recognized that the level of protection was a matter best left to EPA
to establish through rulemaking: ``We do not directly recommend a level
of acceptable risk'' (NAS Report p. 49). Thus, the NAS Report does not
bind us to apply any particular dose limit in our Yucca Mountain
standards.
We note that a number of international scientific and regulatory
bodies and programs suggest natural sources of radioactivity serve as a
point of comparison when uncertainties become significant. For example,
the IAEA has stated that, for time frames extending from about 10,000
to 1 million years, ``it may be appropriate to use quantitative and
qualitative assessments based on comparisons with natural radioactivity
and naturally occurring toxic substances'' (``Safety Indicators in
Different Time Frames for the Safety Assessment of Underground
Radioactive Waste Repositories,'' IAEA-TECDOC-767, p. 19, 1994, Docket
No. OAR-2005-0083-0044). IAEA also suggests that ``[i]n very long time
frames * * * uncertainties could become much larger and calculated
doses may exceed the dose constraint. Comparison of the doses with
doses from naturally occurring radionuclides may provide a useful
indication of the significance of such cases'' (``Geological Disposal
of Radioactive Waste,'' DS154, Section A.7, p. 37, April 2005, Docket
No. OAR-2005-0083-0051). Similarly, in summarizing the results of a
workshop to assess long-term assessments, the NEA suggests that at time
frames when the ``system [is] responding to external change,'' a key
performance indicator could be ``comparison with background radiation
levels.'' At that workshop, the idea was presented that up to 100,000
years, ``a dose constraint derived from natural background levels is
prescribed'' and beyond that point ``the eventual redistribution of the
residual activity by natural processes remains indistinguishable from
natural regional variations in radiation levels'' (``The
[[Page 49037]]
Handling of Timescales in Assessing Post-Closure Safety: Lessons Learnt
from the April 2002 Workshop in Paris, France,'' pp. 33, 35, 2004,
Docket No. OAR-2005-0083-0046). Further, as regards low- and
intermediate-level waste disposal, the UK Environment Agencies
(consisting of the Environment Agency of England and Wales, the
Scottish Environment Protection Agency, and the Department of the
Environment for Northern Ireland) state that ``At times longer than
those for which the conditions of the engineered and geological
barriers can be modelled or reasonably assumed * * * Comparisons with
the ambient levels of radioactivity in the environment may also be
appropriate'' (``Disposal Facilities on Land for Low and Intermediate
Level Radioactive Wastes: Guidance on Requirements for Authorisation,''
section 6.22, 1996, Docket No. OAR-2005-0083-0063).
We therefore considered which natural sources of radioactivity in
the United States might provide similar reference points for a dose
standard beyond 10,000 years. Natural background radiation in the U.S.
averages roughly 300 mrem/yr, but varies significantly across the
country, from a low of about 100 mrem/yr in coastal areas to above 1
rem/yr (1,000 mrem/yr) in certain localized regions. For purposes of
this discussion, natural background radiation consists of external
exposures from cosmic and terrestrial sources, and internal exposures
from indoor exposures to naturally-occurring radon. Altitude and
geology are two of the primary variables accounting for regional
variations; however, there can be tremendous fluctuation even within a
city or county, primarily due to variations in radon emissions. These
fluctuations introduce some uncertainty in estimates of localized
background radiation levels, which are also affected by factors such as
the number and distribution of samples within a geographic area,
whether the samples are short-term or averaged over a longer period,
the structure of the building, the location of the sampling point(s)
within a building, and assumptions in translating measured
concentrations to estimated doses.
In order to assess total exposures and derive a dose limit, it is
necessary to establish levels of natural background radiation already
experienced in the vicinity of Yucca Mountain. We selected Amargosa
Valley as the point of comparison for this analysis. We believe this is
an appropriate approach, as the RMEI is defined as having a lifestyle
and diet representative of current residents of Amargosa Valley. It is
reasonable to consider total exposures in light of exposures already
incurred by people in the immediate vicinity of Yucca Mountain.
However, there are varying estimates of exposures from natural
background sources in that area. DOE estimates that the natural
background in Amargosa Valley is equivalent to the average across the
U.S., or 300 mrem/yr (FEIS, DOE/EIS-0250, Table 3-28, Docket No. OAR-
2005-0083-0086). However, that overall figure is highly dependent on
the radon contribution, which DOE also assumes is equivalent to the
average across the U.S., or 200 mrem/yr. Based on EPA radon studies, we
believe it is reasonable and somewhat conservative to assume that radon
exposures to residents of Amargosa Valley would be slightly higher (say
25%) than the national average (and possibly as much as 100 mrem/yr
higher than the statewide average), resulting in a radon contribution
to those residents of about 250 mrem/yr. Thus, combined with the cosmic
and terrestrial exposures estimated by DOE, we estimate total annual
natural background radiation at Amargosa Valley to be approximately 350
mrem/yr.\3\
---------------------------------------------------------------------------
\3\ Data from EPA studies in 1993 indicate that the total
average natural background exposure in the State of Nevada is 222
mrem/yr (``Assessment of Variations in Radiation Exposure in the
United States,'' 2005, Docket No. OAR-2005-0083-0077), which is
roughly 75% of the national average. Because data were not available
specifically for Amargosa Valley, we used the statewide average as a
starting point to estimate background radiation at Amargosa Valley.
The overall statewide average is significantly affected by estimated
exposures in Clark County (where Las Vegas is located), and not
necessarily representative of exposures closer to Yucca Mountain.
Clark County accounts for roughly two-thirds of the state's
population (Census Bureau, Nevada State Data Center, http://dmla.clan.lib.nv.us/docs/nsla/sdc/
). As outlined above, data support
the conclusion that average exposures in Clark County would be
significantly lower than in the rest of the state, primarily because
of indoor radon exposures. EPA's map of radon zones developed in the
early 1990s found Clark County to be the only county in Nevada
placed into the lowest emission category, in which average exposure
potential is less than 200 mrem/yr (``EPA Map of Radon Zones,'' EPA-
402-R-93-071, Docket No. OAR-2005-0083-0065). Most of the other
counties, including Nye County (where Yucca Mountain and Amargosa
Valley are located), fell into the intermediate category, in which
average exposure potential is estimated in the range between 200 and
400 mrem/yr.
---------------------------------------------------------------------------
To make the comparison with total exposures, it is also necessary
to consider what total exposures provide a reasonable reference point
for limiting releases from Yucca Mountain. As noted above, our goal is
to ensure that releases from Yucca Mountain will not cause total
exposures to the RMEI to exceed natural background levels with which
other populations live routinely. We selected the State of Colorado as
the reference point in meeting this goal. We considered several factors
in this selection. First, we must recognize that some incremental
exposure will be allowed; that is, it is a foregone conclusion that
even the most protective standard cannot be expected to reduce natural
background exposures, and clearly we cannot establish a negative
standard. Thus, the reference point would have to have a higher level
of background than does the area near Yucca Mountain. In addition,
because of the aforementioned complications in estimating localized
background radiation (due primarily to the radon component), we chose
to examine statewide averages, which are less uncertain. Of the states
with sufficient data, 32 have average background radiation levels
higher than Nevada. In selecting among these, we considered
characteristics such as geographic location and population. Our
preference is to choose a state in the western part of the country that
is fairly well-populated and might otherwise have characteristics
considered reasonably comparable to Nevada (such as radon potential,
surface water/coastal features, or size of major cities). We find that
Colorado best fits those criteria. According to the population data
(U.S. Census Bureau Statistical Abstract of the United States, July 1,
2004, http://www.census.gov/statab/ranks/rank01.html), Colorado ranks
22nd among all states in total population (Nevada is 35th). Colorado's
average annual background radiation is estimated at 700 mrem/yr (see
``Assessment of Variations in Radiation Exposure in the United
States,'' 2005, Docket No. OAR-2005-0083-0077, for both background
radiation and population information). Other states have comparable or
higher radon potential and higher background levels with which people
live routinely (background levels in North Dakota, South Dakota, and
Iowa, for example, are 789 mrem/yr, 963 mrem/yr, and 784 mrem/yr,
respectively), and might also be used for comparison. However, we
believe Colorado is more representative of the characteristics
exhibited by Nevada (and Amargosa Valley).
In view of these factors, we selected Colorado as our point of
reference. Thus, comparing Colorado's estimated average annual
background radiation of 700 mrem/yr to our estimate for Amargosa
Valley, we derive an incremental exposure level of 350 mrem/yr, which
we are proposing to establish today as the dose limit to
[[Page 49038]]
apply to the time of peak dose beyond 10,000 years.
The limit we are proposing today is somewhat higher than the
average natural background level of 300 mrem/yr across the U.S., which
places it above two other options we considered (see Sections II.C.4.b
and II.C.4.c). One option is the limit of 100 mrem/yr based on
international guidance for all sources of exposure except natural,
accidental, and medical. The other is 200 mrem/yr, which we derived
through a somewhat different way of looking at total background levels
nationwide. In our view, the 350 mrem/yr level and these other values
are within a range of values for which projections might well be
indistinguishable after several hundred thousand years. That is, when
taking increasing uncertainties into account in the very long term, the
effects of factors that would distinguish projections of 100, 200, and
350 mrem/yr within a 10,000-year time frame are more difficult to
identify clearly at very long times, so that such projections may be
qualitatively identical to each other and to the level of performance
represented by projections of 15 mrem/yr at 10,000 years. That is,
modest differences in basic modeling assumptions regarding such factors
as temperature inside the repository over the first few hundred years
after disposal can lead to differences in projected doses. Such
differences reflect uncertainties and changes in models, and should not
be interpreted as representing meaningful differences in the level of
safety that can be expected to be achieved. Given the difficulty in
estimating performance in the very far future, we would also view 350
mrem/yr as representing a satisfactory level of performance should it
be the ``true'' value at such long times.
We recognize that a standard based on variations in natural
background radiation would be higher than previous non-occupational
standards in the U.S. In our 2001 rulemaking, we justified the dose
limit of 15 mrem/yr and the 10,000-year compliance period in part
because they were consistent with other EPA policies. In particular, a
peak dose standard of 350 mrem/yr (and the time frame of up to 1
million years over which that standard could apply) may appear to some
to be a departure from the risk-management policies EPA has adopted and
applied in a variety of Agency programs, most notably in the Superfund
cleanup program. We believe the circumstances involved in today's
proposal are significantly different from the situations addressed
under Superfund or any other existing U.S. regulatory program, and that
it should be clear that comparisons between the two are inappropriate.
It should be clear that we are not arguing that most people take
into account levels of background radiation when deciding where to live
or work, or that it in any way plays a major role in their decision-
making. Rather, in establishing a standard to apply to the RMEI over
unprecedented times, we believe it is reasonable to consider exposures
incurred routinely today by people in other locations, which in our
view do not ``pose a realistic threat of irreversible harm or
catastrophic consequences'' to those people.
In that context, we note that EPA does not consider the risks from
such exposures to be excessive in the context of radon occurrence in
residences. As described earlier, radon exposures can vary widely even
in localized areas for a number of reasons. While average radon doses
are estimated to be roughly 200 mrem/yr, measurements indicate that
some exposures could be more than ten times that level in unique
situations. The concentration at which EPA recommends action be taken
to mitigate exposures is 4 pCi/l, which translates roughly to 800 mrem/
yr. The Agency further recommends that homeowners consider taking
action only if the measured concentration is between 2 and 4 pCi/l
(i.e., above 400 mrem/yr) (``A Citizen's Guide to Radon: The Guide to
Protecting Yourself and Your Family from Radon,'' EPA 402-K-02-006, May
2004, Docket No. OAR-2005-0083-0058). It should be understood that this
recommendation is not based solely on risk, but considers factors such
as the voluntary nature of the exposure, the application to private
property, and the capabilities of mitigation technology. The dose limit
proposed today is well below the ``action level'' recommended for
radon.
One way to provide context for comparisons with natural
radioactivity is to evaluate the radiotoxicity of the waste itself. In
particular, it has been suggested that assessment time frames could be
tied to the time necessary for the waste to decay to levels roughly
comparable to the uranium ore from which the fuel was derived, which is
often on the order of several hundred thousand years. For example, IAEA
states that ``[r]adiotoxicity indices are useful in putting the
potential hazards of radioactive waste disposal into perspective * * *
they are qualitative indicators of the time-scales of interest for
safety analysis'' (``Safety Indicators in Different Time Frames for the
Safety Assessment of Underground Radioactive Waste Repositories,''
TECDOC-767, p. 15, 2004, Docket No. OAR-2005-0083-0044). NEA takes a
similar position: ``radiological toxicity and comparison with natural
systems such as uranium ores offer a basis for a safety indicator that
can usefully complement dose and risk'' (``The Handling of Timescales
in Assessing Post-Closure Safety,'' p. 30, 2004, Docket No. OAR-2005-
0083-0046). Standards developed in Finland explicitly incorporate this
comparison by defining the ``farthest future'' for assessments as the
period when the activity in spent fuel becomes less than that in the
natural uranium from which the fuel was fabricated (NEA, p. 34, Docket
No. OAR-2005-0083-0046). Draft guidance for the Swedish program states
that assessments ``need not be extended beyond the point in time when
the initial content of the radioactive substances in the repository has
decayed to a level at which the potential of causing harmful effects or
other environmental consequences has decreased to insignificant
levels'' (p. 7, Docket No. OAR-2005-0083-0048). One technical paper
presented in the U.S. concludes that ``regardless of the assumptions
used, the risk to public health from a HLW or spent fuel waste
repository will always become less than that of the original uranium
ore deposit'' and that ``[c]onsidering the nature of the many barriers
to release that are included in the repository design, [it] should
easily be the case'' that this ``crossover time'' (the time at which
the radiotoxicity, or overall hazard, of the remaining waste will be
equivalent to that of the original ore used to make the fuel) will be
less than 10,000 years (``An Assessment of Issues Related to
Determination of Time Periods Required for Isolation of High Level
Waste,'' Proceedings of the Symposium on Waste Management at Tucson,
Arizona, February 26-March 2, 1989, Docket No. OAR-2005-0083-0049).
While it is clear that consideration of natural radioactivity is a
widely accepted concept for supporting safety assessments over very
long times, it should also be clear that we believe regulatory
standards for the Yucca Mountain disposal system based on background
exposures can be reconciled with considerations of impacts on future
generations, as outlined earlier in this section. Some international
statements regarding natural radioactivity reflect the lack of
consensus on what constitutes an undue burden. For example, NEA notes
that when ``the repository has become comparable to a natural system in
certain important aspects, this does not necessarily indicate a return
to unconditionally safe
[[Page 49039]]
conditions'' (NEA, p. 30, Docket No. OAR-2005-0083-0046).
However, Chapman and McCombie directly address this question,
stating that, at these very long times, ``There is no logical or
ethical reason for trying to provide more protection than the
population already has from Earth's natural radiation environment, in
which it lives and evolves * * * it must be recognized that man cannot
be expected over infinite times to do much better than nature. The
potential exists for natural uranium ore deposits, or spent fuel or HLW
repositories, to give rise locally to doses that are higher than the
global average for natural radiation, particularly if they are
eventually eroded in the near-surface environment. However people exist
today in many locations where doses are tens, even up to a hundred
times higher than the average. Thus, a repository is not providing,
globally, a novel source of exposure and does not at these long times
represent any unusual anomaly in the global environment'' (``Principles
and Standards for Disposal of Long-Lived Radioactive Wastes,'' pp. 114-
115, 2003, Docket No. OAR-2005-0083-0061).
We do not mean to suggest that uranium ore bodies are benign
entities, and there is certainly a difference between exposures
incurred by direct contact with the material and those incurred at a
distance after environmental transport of material has provided some
lowering of potential exposures by natural retardation processes. These
comparisons are relevant in the sense that exposures from longer-term
releases from the Yucca Mountain disposal system would not be expected
to be worse than those from natural features that are fairly common in
parts of the country. The exposures that might result from ore body
releases are highly dependent on the characteristics of the ore body
and surrounding environment, as well as the other assumptions applied
(measurements of releases from unmined ore bodies are limited; however,
some surficial radiation measurements from unmined ore bodies suggest
that a person at the site could easily receive several hundred mrem/yr
(``The Uranium District of the Texas Gulf Coastal Plain'', U.S.
Department of Energy Symposium Proceedings, CONF-780422, Vol. 2, 1978,
Docket No. OAR-2005-0083-0081). On this point, we stated in our 1985
final rulemaking for 40 CFR part 191 that ``estimates of the risks from
unmined ore bodies ranged from about 10 to more than 100,000 excess
cancer deaths over 10,000 years. Thus, leaving the ore unmined appears
to present a risk to future generations comparable to the risks from
disposal of wastes covered by these standards'' (50 FR 38083, September
19, 1985, Docket No. OAR-2005-0083-0064). In the terms of the
Precautionary Principle as presented by NAPA, exposures of this
magnitude that are projected to occur several hundred thousand years
into the future should not be considered to ``pose a realistic threat
of irreversible harm or catastrophic consequences'' (Docket No. OAR-
2005-0083-0087).
We recognize that meaningful distinctions are made today between
natural background radiation and additional incremental (and
involuntary) exposures caused by human activity. However, at long time
frames (potentially as long as 1 million years into the future), such
distinctions are less meaningful, and natural radiation levels can
serve as a reasonable and logical reference point for assessing
radiological impacts. We agree with NEA that a reasonable overall aim
``is to leave future generations an environment that is protected to a
degree acceptable to our own generation * * * this level of protection
will ensure that any radiological impacts due to disposal will not
raise levels of radiation above the range that typically occurs
naturally'' (NEA, p. 9, Docket No. OAR-2005-0083-0046). Our proposed
approach limits doses from the Yucca Mountain disposal system in the
far future to levels that represent variations in natural background
and are far below doses that can be projected from uranium ore bodies
or natural radiation in some locations in the U.S. and worldwide. Our
proposed limit is somewhat higher than the annual average background
radiation in the U.S. Using the reasoning described above, under this
standard the additional radiation exposure at the time of peak dose to
a resident of Amargosa Valley from the Yucca Mountain disposal system
would be no greater than what would be incurred if that person moved
today from the vicinity of Yucca Mountain to a nearby state. Using the
NAS suggestions as a starting point, and considering international
guidance and examples, we have derived the proposed dose limit to
balance competing factors highlighted by NAS and acknowledged by us as
important: the dual objectives to effectively address the effects of
uncertainty on compliance assessment and to adhere as closely as
possible to the relevant ethical principles, including a consideration
of impacts on future generations. We believe that our selection of a
350 mrem standard is reasonable and effectively addresses the factors
it is necessary to consider when projecting exposures very far into the
future. By applying over the entire period of geologic stability beyond
10,000 years (up to 1 million years), it will capture the peak dose
during that period. By doing so, our proposal is consistent with the
NAS recommendation to have a standard with compliance measured ``at the
time of peak risk, whenever it occurs within the limits imposed by the
long-term stability of the geologic environment, which is on the order
of one million years'' (NAS Report p. 2).
In all of our discussion of potential dose standards, we have
emphasized the importance of perspective in evaluating dose projections
at very long times. It is important to distinguish between effects that
are meaningful in assuring public health and safety and those that
simply illustrate a modeling exercise. We are proposing an approach to
setting a dose level derived from variations in current natural
background radiation in the U.S. that would relate potential exposures
to the RMEI to exposures incurred today by people in other locations
from sources of natural background radiation. Given the long times
involved in dose projections, and the significant uncertainties, we
believe that comparisons with natural sources of radiation are
appropriate.
Finally, from a regulatory perspective, we have also considered
that the peak dose limit would apply at any time after 10,000 years.
The limit we select must be credible both at times close to 10,000
years and times much further into the future. Readers may also question
whether a 350 mrem/yr standard can be considered credible at times
beyond but closer to 10,000 years. (We have acknowledged that
uncertainties are not immediately overwhelming and unmanageable for a
period up to 10,000 years.) We think it unlikely that the peak would
occur at a relatively early time beyond 10,000 years. However, should
that be the case, we believe that NRC has the authority to consider not
only the magnitude of the peak, but also the timing and overall trends
of dose projections as it evaluates the license application. NRC will
examine the full record before it in determining whether there is a
reasonable expectation that the standards will be met. As an
alternative, we might identify a sliding scale of compliance limits
applicable at different times, but, as discussed in Section II.C.2.c,
we do not believe there is a clear basis for doing so.
In addition to our proposed level of 350 mrem/yr, we took into
account the
[[Page 49040]]
factors described above in considering various options for the peak
dose limit, as discussed in the next section. Clearly, the competing
considerations described above are not easily resolved. While the final
standard may not be identical to any of these options, we believe that
they encompass the range of values we might reasonably select. We
request comment upon our proposed annual peak dose limit of 350 mrem
applicable beyond 10,000 years through the period of geologic
stability, the reasoning outlined above, and other ways in which we
might reconcile the various influential factors at very long times.
4. What Other Peak Dose Levels Did EPA Consider?
We considered several other dose options before selecting 350 mrem
as the value to propose. We request comment on the dose levels
discussed in the following sections.
a. Maintain the 15 Mrem/Yr Standard at Peak Dose
One approach would be simply to apply the same level deemed
protective at 10,000 years to peak exposures, whenever they might
occur. This approach has been recommended by some stakeholders (Docket
No. OAR-2005-0083-0022). Stakeholders have suggested defining the
``compliance period'' as the time from disposal to peak dose, stating
that ``[t]his new compliance period is absolutely required by [NAS],
which rejects any 10,000-year limitation on the compliance period.''
However, for the reasons discussed earlier, while we are proposing to
extend the compliance period throughout the period of geologic
stability, we have concerns that an approach that applies the same dose
level throughout that period would not adequately recognize the
complexities inherent in projections that could extend for several
hundred thousand years. As a result, we believe a 15 mrem/yr standard
at very long times would not be a meaningful indicator of disposal
system performance, and may in fact be misleading.
We disagree with the view that the Court's decision requires us to
amend our standards by extending both the compliance period and the
dose limit applicable at 10,000 years to the time of peak dose up to 1
million years, and forbids us to establish standards applicable at
intermediate times. The Court's decision reflected its judgment that
our 2001 standards were not consistent with the recommendations of NAS
as they related to the compliance period. Our goal in today's proposal
is to amend our standards so that they are clearly consistent with the
NAS recommendations, but also address the policy and other concerns we
raised in our 2001 rulemaking. Extending the compliance period directly
addresses NAS's primary recommendation. Regarding the dose limit
applicable at the time of peak dose, NAS stated ``we do not directly
recommend a level of acceptable risk'' (NAS Report p. 49). Similarly,
NAS offered no opinion on approaches involving multiple dose standards,
stating only that it viewed a 10,000-year standard by itself as
insufficient (NAS Report pp. 54-56). As the Court made clear in its
consideration of the ground-water protection standards, where ``NAS
made no `finding' or `recommendation' that EPA's regulation could fail
to be `based upon and consistent with' * * * [the EnPA] left [EPA]
free'' to promulgate standards to address its policy concerns. (NEI,
373 F.3d at 47, Docket No. OAR-2005-0083-0080.) In our view, the
standard applicable for the first 10,000 years and the derivation of a
different dose limit applicable beyond 10,000 years are both
permissible under our EnPA authority.
From a regulatory perspective, a compliance standard on the order
of 15 mrem/yr implies far more precision in projections for very long
times than can be supported and, as such, is inconsistent with the
``reasonable expectation'' approach. We have also discussed at length
the general agreement among international bodies and programs that
longer-term standards should recognize the uncertainties involved and
projections must be considered in a more qualitative manner, as one
element in the overall safety case. As such, we believe it is
inappropriate to portray that projections of incremental doses at such
low levels can be precisely controlled at far future times and to give
them excessive influence when they are not critical to improvements in
health and safety. Here again, we believe our statement from the 2001
rulemaking bears repeating: ``[s]etting a strict numerical standard at
a level of risk acceptable today would ignore this cumulative
uncertainty and the extreme difficulty of using highly uncertain
assessment results to determine compliance with that standard'' (66 FR
32098). From that perspective, holding the Yucca Mountain disposal
system to a 15 mrem/yr standard would not merely be an issue of
``fairness'' to very far future generations. Instead, by not
recognizing the factors that fundamentally affect dose projections at
such times, it would place those generations' interests in a much
higher regard, and by doing so would unreasonably constrain the current
and succeeding generations' abilities to pursue achievable solutions
they deem best suited to meet the interests of all generations. It
would, in other words, undermine the Chain of Obligation Principle by
giving ``long-term hypothetical hazards'' precedence over ``near-term
concrete hazards'' (``Deciding for the Future: Balancing Risks, Costs,
and Benefits Fairly Across Generations,'' 1997, Docket No. OAR-2005-
0083-0087). It is not simply a question of whether a 15 mrem/yr
standard could be met in actuality; rather, the question is whether
holding probabilistic assessments to such a level over hundreds of
thousands of years, when rising uncertainties exist in performance
projections and various high-consequence (but necessarily somewhat
speculative) scenarios must be considered in the analyses, represents a
reasonable test of the disposal system. We believe it does not.
b. 100 Mrem/Yr Standard at Peak Dose
In evaluating dose limits that might be responsive to the concerns
outlined above, we also considered 100 mrem/yr as a value that may be
appropriate for peak dose calculations. The value of 100 mrem/yr has
potential benefits in terms of precedent. The ICRP has since 1985
(Publication 45, ``Quantitative Bases for Developing a Unified Index of
Harm,'' Statement from the 1985 Paris Meeting of the ICRP, Docket No.
OAR-2005-0083-0087) recommended 100 mrem/yr as the principal overall
dose limit for public exposures from all sources excluding natural
background, medical, occupational, and accidental (these three man-made
sources can involve higher exposures, can involve greater understanding
of the reasons for exposure, and may require informed consent from the
exposed person). NRC requires that its licensees conduct operations so
that individual members of the public are not exposed above this level
(10 CFR 20.1301). We view this figure as representing a national and
international precedent as a generally-accepted benchmark for annual
public exposures from various sources.
The use of 100 mrem/yr can also be interpreted as protective of
future generations' interests, yet not so restrictive as to represent
an unreasonable standard for the very far future. We recognize that in
practice today, doses from any particular source of radiation are
generally kept to a fraction of the 100 mrem overall limit, in
recognition that a person may be exposed to more than one practice or
source. Given our current responsibility
[[Page 49041]]
to propose a peak dose standard, however, we would argue that
allocation to a single source at the time of peak dose could be
reasonable, as other contributors currently in the Yucca Mountain area
are negligible by comparison (FEIS, DOE/EIS-0250, Section 8.3.2, Docket
No. OAR-2005-0083-0086). Moreover, to assume (or even to estimate the
chance) whether, how, or where other radiation facilities could develop
in the far future would require immense speculation about the long-term
evolution of government programs, population demographics, and
technology. Relying on current conditions rather than speculating on
future sources of exposure to the local population, as recommended by
NAS, would justify allocating the entire 100 mrem to Yucca Mountain.\4\
---------------------------------------------------------------------------
\4\ This approach would also be consistent with the recent ICRP
draft for consultation on optimization of radiological protection,
which states ``the choice of the relevant dose constraint for
protection against exposures from the licensed facility under
consideration will depend largely on whether or not this facility is
the dominant source to the exposed public under consideration. If
the facility is the dominant source, a dose constraint of 1 mSv/a
[100 mrem/yr] would be the appropriate starting point for
optimisation of protection'' (``The Optimisation of Radiological
Protection,'' p. 45, April 2005, Docket No. OAR-2005-0083-0052).
---------------------------------------------------------------------------
Nevertheless, we have decided not to propose a peak dose standard
of 100 mrem/yr because over the very long-term, we believe that natural
background levels to which individuals are or could be currently
exposed provides a more reasonable context in which to judge the
performance of the Yucca Mountain disposal system, and because our
proposed approach appropriately reflects international guidance and
consensus on this issue. See Section II.C.3 (``What Dose Level Is EPA
Proposing for Peak Dose?'').
c. Peak Dose Standard Based on Regional Background Radiation Levels
We also considered an alternative approach also based on an
examination of natural background radiation levels across the country.
In this approach, rather than examining total background radiation at a
specific location (or State), as we did to derive the 350 mrem/yr level
we are proposing today, we have looked at average levels across many
States (``Assessment of Variation in Radiation Exposure in the United
States,'' 2005, Docket No. OAR-2005-0083-0077). One reason for taking
this approach is that it compares statewide averages calculated on a
common basis. Even so, the cautions we expressed in Section II.C.3
regarding the uncertainties and variation in background radiation
values remain relevant.
Using this approach, we arrived at a dose limit of 200 mrem/yr. As
with our proposed approach, our overall policy goal is to establish a
standard that would keep total exposures to the RMEI within the range
of exposures incurred by residents of other locations today from
natural background sources alone. We would not view 200 mrem/yr as
excessive in the context of exposures routinely encountered today,
particularly when considering the uncertainties in projecting potential
doses over the very long times involved (i.e., 10,000 to 1 million
years).
We started by considering States with higher average background
levels than Nevada. As with our proposed approach, we believe this is
reasonable because the limit we establish must represent some positive
incremental exposure to the RMEI. The data compiled in ``Assessment of
Variation in Radiation Exposure in the United States'' (Docket No. OAR-
2005-0083-0077) show that 32 States have higher average background
levels than Nevada's 222 mrem/yr. Rather than identify any particular
State as the reference point, as we did in the direct comparison with
Amargosa Valley, we averaged the values for those 32 States and
obtained an average background radiation level of 429 mrem/yr. We
compared this value to the statewide average for Nevada as an indicator
of more regional, rather than localized, differences. Thus, on average,
residents of those 32 States today receive roughly 200 mrem/yr more
from natural background radiation sources than a resident of Nevada.
Considering all of the factors involved in very long-term projections,
such a limit would represent a level of exposure consistent with that
routinely and normally incurred in other locations. However, we have
decided not to propose this approach today because our preference is to
use Amargosa Valley (and the RMEI as the person our standards are
designed to protect) as a point of reference, but we welcome comment on
both the approach and the dose level of 200 mrem/yr derived from it.
5. How Will NRC Judge Compliance?
We require that DOE use probabilistic performance assessment to
demonstrate compliance with the individual-protection standard in Sec.
197.20 (DOE may, but is not required to, use the same technique to show
compliance with the human-intrusion and ground-water protection
standards). With this method, DOE will obtain a distribution of
calculated dose results. This distribution will be influenced by
variations in parameter values as well as fundamental uncertainties and
the assumptions underlying the conceptual model(s) of disposal system
evolution. In making a compliance demonstration, DOE must satisfy NRC
that a specified portion of that distribution satisfies the dose
criterion. In our 2001 rulemaking, we specified in Sec. 197.13 that
the mean of the distribution of results be used to demonstrate
compliance with Sec. 197.20. In doing so, we intended that the
arithmetic mean (commonly known as the average) of the distribution be
used, but did not feel the need to be so specific. The arithmetic mean
is a well-understood measure that is used in many compliance
applications, including at WIPP. As discussed later, we intend to
retain the arithmetic mean for the compliance measure for the first
10,000 years after disposal.
However, for the period beyond 10,000 years, for which we must
consider assessing performance for as long as 1 million years (the
NAS's estimated period of ``geologic stability''), we realize that some
additional specification is necessary. Although we do not believe that
the basic approach to performance assessment should be affected, we
discuss in Section II.D certain aspects of that approach that may need
to be modified or given special attention to effectively address these
much longer times in a meaningful way. Similarly, we must consider
whether the arithmetic mean used for compliance at 10,000 years remains
the appropriate measure of compliance, particularly at very long times,
or whether another measure is more appropriate.
We believe that for these very long-term projections, a measure
that represents a ``central tendency'' in the distribution of
calculated doses is most appropriate to adequately consider the range
of uncertainty in making dose projections over such very long time
spans. Such a measure should not be strongly influenced by high or low-
end projections that represent low probability situations. Today we are
proposing to specify that compliance with the standard that will apply
beyond 10,000 years should be measured against the median of the
distribution of projected doses. The remainder of this section
discusses our rationale for this approach.
In general, the compliance measure we select must be meaningful and
representative of the entire distribution of calculated doses, but, as
we have stated, not easily influenced by results either at the very
high or very low end of the distribution. In conducting
[[Page 49042]]
performance assessments many assumptions and uncertainties must be
incorporated into the complex calculations. In constructing scenarios
for repository performance, there are uncertainties in describing how
the disposal system will perform and evolve over time, under the
influence of natural conditions and the effects of the repository
itself on the surrounding host rock. There are significant
uncertainties in predicting when discrete events, such as seismic
activity, will occur at and around the immediate repository location
and the possible effects of these events. Some scenarios incorporating
these uncertainties would be of low probability, and the results could
vary from relatively poor performance to exceptionally good performance
of the disposal system. The results of such low-probability situations
with dramatically different results than the majority of the
projections would show up in the ``tails'' of the dose results
distribution. While we believe such low-probability situations should
not be ignored in compliance decisions, neither do we believe they
should be given undue influence in judging compliance. Therefore, we
believe that the appropriate compliance measure should represent a
central measure for the dose projections, and should not be defined in
a way that it can be strongly affected by extreme results
(``outliers'') in the dose projections (``Assumptions, Conservatisms,
and Uncertainties in Yucca Mountain Performance Assessments, Sections
12.1 and 12.2, July 2005, Docket No. OAR-2005-0083-0085).
Today we are retaining, and more clearly specifying, the arithmetic
mean of the dose projections for compliance within the initial 10,000-
year period. We believe the arithmetic mean is a familiar and well-
understood statistical concept, and that its application in
probabilistic risk assessment is sufficiently established to support
our decision. In addition, while uncertainties are present in
performance assessments during this time frame, we believe that the
uncertainties increase in importance as the assessments stretch into
the extremely long time frames beyond 10,000 years but within the
period of geologic stability. In this sense, we believe that the
arithmetic mean (average value) of the dose projections can still be a
reasonably reliable measure of the total dose distribution during the
10,000-year period. More importantly, however, we believe it is
valuable to maintain consistency between the compliance measure used
for the first 10,000 years of disposal system performance for the Yucca
Mountain repository and the measure applied for any other geologic
disposal application under the authority of our generic regulation for
geologic disposal, 40 CFR part 191. We believe that the Yucca Mountain
disposal system should be required to meet the same level of
protection, and be evaluated under the same regulatory compliance
framework, as any other geologic disposal application for the 10,000-
year period considered in part 191, which has been applied to the WIPP
facility specifically and would apply to any other disposal system in
the future. In the unlikely event that performance assessments show
that the peak dose would occur within the 10,000-year period, we
believe that the same compliance measure and evaluation should be
applied for the Yucca Mountain disposal system as for any other
geologic disposal system.
However, we have noted repeatedly that extending the compliance
period to time frames well in excess of 10,000 years introduces
additional uncertainty in making disposal system performance
projections, since the natural system will continue to change through
time (see ``Assumptions, Conservatisms, and Uncertainties in Yucca
Mountain Performance Assessments,'' Section 12.5, July 2005, Docket No.
OAR-2005-0083-0085, and the 2001 BID, section 7.3.11, Docket No. OAR-
2005-0083-0050). We believe probabilistic calculations are the most
appropriate approach to assess the range of potential doses over very
long time frames, both for the period up to 10,000 years and after. The
probabilistic approach examines a spectrum of possible site conditions,
and allows the construction of conceptual models that address
reasonable alternative models of performance within that range of
possible physical and chemical conditions at the site. A distribution
of projected peak doses will result from these analyses, each
representing a possible ``future'' and a dose associated with the
specific parameter values chosen for each calculation. Only by
examining the relative effects of variations in the parameter values on
the calculated dose can the important Adriver'' parameters be
identified. Without these types of analyses, an understanding of the
disposal system's behavior in the long term would not be possible, and
a compliance case supporting a decision about the protectiveness of the
disposal system might not be a reasonable representation of potential
risks. We are proposing to require that DOE apply this general approach
for assessments regardless of time frame, although, as we have
discussed earlier, there is much agreement that the level of confidence
or meaning that can be placed in such analyses decreases over very long
periods. The challenge lies in defining a performance measure that
balances the uncertainties inherent in very long term projections and
provides a reasonable level of protectiveness.
Similarly, some discussion is warranted on the role of conservatism
in performance assessment. Excess conservatism in constructing
scenarios, i.e., making assumptions to include or exclude specific FEPs
and defining parameter value ranges, can easily lead to very high dose
estimates due to a compounding effect of very conservative assumptions.
Such excessive conservatism is misleading if incorporated in
assessments described as the Anominal'' or Abase case'' performance
projections. A simple arithmetic mean calculated for an excessively
conservative analysis would suggest that the ``most likely'' dose is
higher than if a more reasonable and realistic approach were taken to
framing the assessments. Recognizing that conservatism in long-term
performance projections may be unavoidable in practice, as discussed
below, we believe that a regulatory performance measure should not give
undue emphasis to high-end projections. It is always possible to
propose scenarios where releases are high, even though the probability
of these particular scenarios may be extremely small or very difficult
to estimate. The same reasoning also applies to scenarios that result
in very low releases in the very long term. The ``bounding'' approach
to assessments plays an important role in the light of the increasing
uncertainties. Once the time frame for performance projections is
extended into the very long term, the confidence that can be placed on
either the high- or low-end release scenarios becomes progressively
more difficult to estimate even though a ``bounding'' approach may
simplify calculations. Consequently, we believe that a performance
measure for these very long term assessments should not over emphasize
high-end release scenarios or ignore low-end release scenarios under
the motivation for conservatism in the assessments.
In addition, uncertainty and conservatism can influence one
another. Characterization of the site today yields a range of values
for important parameters that would have a dominant effect on
projecting doses from contamination plumes eventually released from the
repository, and these
[[Page 49043]]
data form the base of the parameter value distributions input to the
dose calculations. Attempting to project the evolution of these
parameter values over the 1 million year geologic stability period adds
additional uncertainty in their variations. To address these
uncertainties in parameter value estimation and scenario construction,
analyses of disposal system performance may be done Aconservatively,''
i.e., by selecting parameter values, constructing scenarios, and making
assumptions that deliberately overestimate projected doses. This
approach provides some confidence that uncertainties and other
potential negative influences have been adequately considered and that
regulatory decisions will not be based on overly optimistic views of
disposal system performance. However, the distribution of doses, as
well as peak doses, from such an approach will be biased toward high-
end values. As a result of making conservative assumptions and
parameter distributions, there is a very real possibility that high-end
projections could represent highly improbable situations in a physical
sense (``Assumptions, Conservatisms, and Uncertainties in Yucca
Mountain Performance Assessments,'' Sections 1 through 12, July 2005,
Docket No. OAR-2005-0083-0085). For such cases, arriving at a
compliance decision becomes complex and speculative. Thus, we believe
the appropriate measure of compliance for peak dose estimates is a
``central tendency'' measure which is not strongly influenced by low-
probability realizations giving either very high-end or low-end dose
estimates (``Assumptions, Conservatisms, and Uncertainties in Yucca
Mountain Performance Assessments,'' Sections 12.1 and 12.2, July 2005,
Docket No. OAR-2005-0083-0085).
The NAS also found this approach to have merit. NAS recommended
that regulatory decision making should consider the period when risks
are at their highest, whenever that occurs, i.e., the time of peak dose
(NAS Report pp. 2, 6). In defining ``risk,'' the NAS used the term
Aexpected value'' in referring to a probabilistic distribution of
projected doses (NAS Report p.65). NAS did not further define this term
in a statistical context, but elsewhere provided qualitative language
describing the overall goal: ``define the standard in such a way that
it is a useful measure of the degree to which the public is to be
protected from releases from a repository'' and ``does not rule out an
adequately sited and well-designed repository because of highly
improbable events'' (NAS Report pp. 27-28). NAS in its recommendations
did not speak explicitly to any particular performance measure to be
used in determining compliance with regulatory standards. This decision
was to be left to EPA in the course of rulemaking.
Disposal programs abroad also have to consider the role of
uncertainty in developing performance assessments. The U.S. is ahead of
most other geologic repository programs abroad in terms of having a
specific site that has been extensively characterized and for which
detailed performance assessments have been done. While other programs
have not reached the stage of developing specific regulatory criteria
for judging the acceptability of a particular repository site and
design, there is a general consensus that multiple lines of evidence
and analysis are desirable in establishing a safety case, and that
judgment plays a critical role in assessments of disposal system
performance as well as establishing and applying regulatory criteria
(IAEA-TECDOC-975, Docket No. OAR-2005-0083-0045). The joint NEA-IAEA
International Peer Review for DOE's TSPA-SR modeling highlighted the
difficulty of specifying the statistical measure of compliance, noting
that ``the TSPA nominal case is treated probabilistically yet it
involves a mixture of embedded conservatism and statistical analyses to
determine the mean, median and the various percentiles of the dose
distribution. The reported ``mean'' is therefore not the true mean in a
statistical sense. Moreover, that value is reported in the Executive
Summary of the TSPA-SR and elsewhere as the expected value of effective
dose, without any qualification. This stretches credibility especially
as the discrete numerical values are given for times in the far future.
The USDOE needs to indicate that, for compliance purposes, a
performance indicator has been chosen that is meant to illustrate the
safety of the system and argue the compliance with regulation.'' The
Peer Review Team further recommended that ``when a best estimate/best
knowledge probabilistic analysis is performed, the best estimate or the
most probable range of the calculated `dose' should also be given.''
(pp. 54-55, Docket No. OAR-2005-0083-0062)
In determining the ``expected value'' of performance, some
international efforts have considered the possibility of viewing the
performance assessment as separate representations of scenarios driven
by their relative likelihood, and which might be compared to different
regulatory standards. For example, regulatory agencies of France and
Belgium have developed a joint document that suggests preparation of
``reference evolution'' and ``altered evolution'' scenarios
(``Geological Disposal of Radioactive Waste: Elements of a Safety
Approach,'' p. 24, 2004, Docket No. OAR-2005-0083-0066). The reference
evolution scenarios would consider ``the most likely effects of certain
or very probable events or phenomena,'' while the altered evolution
scenarios ``take into account the least likely effects of these events
or phenomena'' as well as considering ``the consequences of events or
phenomena that are not integrated into the reference scenario, as the
likelihood of occurrence is lower.'' Under this approach, the reference
evolution scenarios might be directly compared to the dose constraint,
while the altered evolution scenarios ``must be appraised on a case by
case basis depending on'' various factors, and may then be ``compared
to different references * * * without this comparison constituting an
absolute acceptance criterion.'' This approach appears to go further
than that recommended by the TSPA-SR Peer Review Team (and discussed in
relation to our reasonable expectation principle in Section II.B). DOE
similarly identifies ``nominal'' and ``disruptive'' scenarios, but
aggregates the results for comparison with the relevant criteria.
As stated earlier, we required in our 2001 rulemaking that DOE use
the arithmetic mean of the distribution of results to demonstrate
compliance with the 10,000-year dose limit (and are today proposing to
clarify the use of that measure). However, in considering the much
longer times, we are concerned that the arithmetic mean is too easily
influenced by extremes in the distribution, particularly very high dose
projections resulting from scenarios that are unlikely to occur. A
conservative approach to constructing and evaluating performance
scenarios tends to generate high-end results and a simple averaging of
these results would drive the arithmetic mean to higher values that
would not be as representative overall of the actual distribution of
projected doses. Therefore, we do not believe the arithmetic mean will
satisfy the goals laid out earlier in this section for a performance
measure for periods well in excess of 10,000 years.
While typically the ``average'' of a series of values (i.e., a
distribution) is thought of as near the midpoint between the highest
and lowest values, if a somewhat conservative approach is taken or
there are significant outliers, it
[[Page 49044]]
is not unusual for the arithmetic mean to approach significantly higher
percentiles. In such cases, the regulatory compliance decision can be
influenced by the high-end doses of an overall set of very conservative
performance assessment results. In fact, for early occurrences of
disruptive events (human intrusion or igneous intrusion), DOE
assessments show that at some periods of time the arithmetic mean of
the projected doses can exceed the 95th percentile of the distribution
of TSPA results (FEIS, DOE/EIS-0250, Appendix I, Section 5.3, Docket
No. OAR-2005-0083-0086). While conservatism in assumptions is not the
only reason for the arithmetic mean to occur at a relatively high
percentile, in general we do not believe this can be reasonably
interpreted to be an adequate representation of central tendency for
the purpose of judging the performance of the Yucca Mountain disposal
system.
Thus, we found it necessary to consider what other statistical
measures might better represent the central tendency for performance
assessments at very long time frames. The identification of appropriate
statistical measures for central tendency of a dose distribution is
influenced by the shape of the distribution, when these estimates are
plotted for a particular point in time, or more specifically for the
peak dose values from each computer modeling simulation in the disposal
system performance assessments. We have examined three measures of
central tendency: the arithmetic mean, the geometric mean, and the
median. The degree to which they reliably represent the central
tendency of a particular distribution, and more importantly how well
they could serve as compliance measures, is discussed below. Like the
arithmetic mean we have discussed above, each measure has advantages
and disadvantages, and is dependent on the actual shape of the dose
distribution as to how well it would represent the central tendency
(``Assumptions, Conservatisms, and Uncertainties in Yucca Mountain
Performance Assessment,'' Sections 12.1 and 12.2, July 2005, Docket No.
OAR-2005-0083-0085).
The most familiar shape for a distribution is the bell-shaped
``normal'' distribution. In a normal distribution, the ``peak'' occurs
in the center of the distribution and the remaining values lie evenly
on both sides of the center value. A normal distribution is often seen
when values are relatively close together (i.e., the range of values
does not cover many orders of magnitude), and are produced from a
continuous function composed of additive terms. Because the values of
the distribution are evenly spread out around the central peak, the
distribution can be seen to be symmetrical; that is, one side is the
``mirror image'' of the other. The arithmetic mean can be easily
determined from such a distribution because an equal number of values
are found at the same distance from the peak (e.g., if the peak is at
10, there will be equal occurrences at 9 and 11, at 8 and 12, and so
on). Thus, the center line in a purely normal distribution represents
the arithmetic mean of the distribution. From the discussion earlier in
this section, it should be clear that the performance results do not
represent a purely normal distribution. In a purely normal
distribution, the arithmetic mean cannot be as high as the 60th
percentile, much less the 70th, 80th, or 95th percentile. It must
always be the 50th percentile. For this reason, we believe it likely
that at long times the arithmetic mean will be more strongly influenced
by higher-end estimates (estimates lower than zero are not possible)
and less representative of the overall distribution.
As an alternative, we considered whether the geometric mean of the
distribution would be an appropriate statistical measure. Referring
back to the shape of the distribution as an indicator of the measure of
central tendency, we noted earlier that the bell-shaped curve is the
most familiar shape. However, many measured quantities in nature show a
distribution skewed toward higher-end values, i.e., there is no
symmetrical distribution around a central value (``The Lognormal
Distribution in Environmental Applications,'' EPA/600/S-97/006,
December 1997, Docket No. OAR-2005-0083-0057). When data like these are
transformed by taking their logarithms and plotted on a logarithmic
scale, the data can appear ``normally'' distributed. Such distributions
are referred to as log-normal. For such ``transformed'' data, the
geometric mean is used as the measure of central tendency (that is, the
geometric mean of a log-normal distribution has a comparable
significance to the arithmetic mean of a normal distribution).\5\ The
fact that the arithmetic mean has been significantly higher than the
50th percentile in DOE's published performance assessment results
suggests those distributions might be log-normal in nature, which would
indicate the geometric mean as the appropriate statistical measure of
central tendency. As a point of comparison, the geometric mean of a
log-normal distribution is always lower than the arithmetic mean. This
makes the geometric mean attractive if we are concerned about the undue
influence of high-end estimates, as the geometric mean will always show
less influence than the arithmetic mean.
---------------------------------------------------------------------------
\5\ The formula for calculating the geometric mean (GM) for a
series of values, x1, x2, x3 . . .
. Xn, is GM = \n\ [radic] x1 * x2 *
x3 . . . . Xn, while the formula for
calculating the arithmetic mean (AM) is AM = (x1 +
x2 + x3 . . . xn)/n. For the GM
calculation no zeros are permissible, and the GM is always less than
the AM. For parameter values in a skewed distribution (skewed to
high-end values) that is transformed into a log-normal distribution,
the formula for the GM is expressed as ln GM = (1/n)(1n
x1 + 1n x2 + 1n x3 . . . . + 1n
xn). It can be seen that the GM of the log-transformed
values in a log-normal distribution is calculated in the same
fashion as the AM for a normal distribution. Both the AM and the GM
are measures of central tendency for their respective distributions
and equivalent to the median of the distributions as long as the
distributions are truly normal or log-normal.
---------------------------------------------------------------------------
However, there are some difficulties in using the geometric mean
that must be considered. One difficulty is related to the nature of the
geometric mean itself. Because the calculation involves the taking of
the logarithm, the distribution values are expressed in terms of their
exponential values, which may then be ``averaged.'' For example, the
logarithm of 100 is 2, because 100 = 10\2\ (or 10 to the 2nd power).
Similarly, the logarithm of numbers less than 1 are expressed as
negative numbers (e.g., the logarithm of 0.01 = -2, because 0.01 can
also be written as 10-2). Thus, in the same way that the
arithmetic mean might be affected by a few very large values in a
distribution, the geometric mean can be affected by very small numbers
whose logarithms are expressed as very large negative numbers.
In practical applications, this means that a distribution that
generally appears log-normal can contain some very small numbers
(outliers) that affect the geometric mean as a measure of central
tendency. Depending on how many and how small these outliers are, the
calculated geometric mean can also be very different from the 50th
percentile of the distribution. For Yucca Mountain, this situation
could represent a case where the waste packages remain essentially
unbreached through the geologic stability period, leading to very small
doses (and correspondingly high negative logarithms of those dose
values). This scenario might have a very low probability in reality,
but could influence the geometric mean, possibly causing it to be lower
than the 50th percentile of results calculated from all the performance
scenarios taken in total (and possibly very much lower). Alternatively,
extremely pessimistic scenarios for waste package releases could give
high-end outliers, although
[[Page 49045]]
the high-end projections may not affect the geometric mean as much
because the site's characteristics will not easily allow orders of
magnitude increase in releases to reach the RMEI. In terms of the
logarithmic values, the difference between 0.001 mrem and 0.1 mrem is
exactly the same as the difference between 1 mrem and 100 mrem (two
orders of magnitude), yet the difference in actual site performance is
clearly more significant between 1 mrem and 100 mrem. Thus, while it is
possible to have very low-dose estimates, micro-rem/yr and below, which
have large negative logarithms, there will not be correspondingly high
dose estimates in the tens to hundreds of thousands of rem/yr (with
equally high positive logarithms) to counterbalance the very low
numbers, and therefore these very low numbers could exert a stronger
effect on the geometric mean as an indicator of central tendency. In
such cases, the values of the geometric mean as a central tendency
performance measure could be compromised.
An additional complication exists for the regulator using the
geometric mean to judge compliance. Because the logarithm of the value
must be taken, dose projections of zero must be removed from
consideration altogether (the logarithm cannot be calculated). However,
extremely low (and highly influential) non-zero values may be retained
in the analyses, simply because computers are able to track them. That
is, projected doses that are in reality essentially indistinguishable
from zero can be calculated and carried through the analysis. If care
is not taken, projections could include doses such as 10-20
mrem/yr, which is meaningless in actuality (and clearly the logarithmic
value of -20 cannot be offset by any single high-end estimate). The
regulatory analyst is then faced with the prospect of ignoring
simulations that yield no dose, eliminating values below a certain
level (for very low dose estimates), or assigning some arbitrary value
to them simply to calculate a geometric mean. Eliminating small values
from consideration would not be consistent with our cautions (see
discussions on reasonable expectation) that low-end projections should
not be discounted in favor of higher estimates.
It is also not proven that the distribution of performance
assessment results is truly log-normal. As noted above, DOE's
previously published TSPA results indicate that the distribution of the
peak dose values is skewed to one side, so that values are not evenly
distributed around a central point (FEIS, DOE/EIS-0250, Appendix I,
Section 5.3, Docket No. OAR-2005-0083-0086). We have mentioned the role
of conservatism in framing dose assessments and biasing them to high-
end values, so this skewed distribution is not surprising. Such skewed
distributions often appear to be log-normal, for which the geometric
mean represents the central tendency. However, while we have some
confidence that future DOE results will be skewed toward the high end,
we cannot predict with certainty that the distributions are truly log-
normal, although we can say that they display two prominent
characteristics of log-normal distributions. First, the results span
several orders of magnitude, making the use of logarithmic conversions
effective in putting the values on a convenient scale. Second, its
derivation involves multiplicative functions which are imbedded in the
performance simulations, while normal distributions arise from simpler
functions that are additive in nature. Their actual shape will be
affected by DOE's modifications to the TSPA as it works through the
licensing process. The geometric mean may not actually represent the
best measure of central tendency if the distribution is not log-normal.
For these reasons, we are not proposing to use the geometric mean
as the measure of compliance at the time of peak dose. This brings us
to the third statistical measure we considered for these very long
times, the median of the distribution, for which 50% of the values lie
above and 50% lie below. The median is a simpler measure of central
value for any distribution of dose estimates. It is independent of the
shape of the distribution and therefore avoids concerns about how well
the performance assessment results may or may not strictly conform to
the normal or log-normal profiles, and attendant uncertainty about how
close the respective ``means'' are to the center of the distribution.
In this respect, the median is an attractive alternative to the
geometric or arithmetic means as a measure of central tendency that
will not be strongly influenced by high or low-end outliers in the
calculated projections. There is no need to eliminate or truncate
results at the low end, as there may be for the geometric mean.
Further, if the distribution includes many very low estimates, the
median could actually be higher than the geometric mean. As such, it is
also consistent with our reasonable expectation principle.
As an additional advantage, if the distribution ultimately falls
close to either a normal or log-normal distribution, the median
converges with the arithmetic or geometric mean, respectively. It can
be clearly seen that the median and arithmetic mean are identical for a
normal distribution, as the ``mirror image'' around the arithmetic mean
also shows that exactly half of the results fall on either side. For a
log-normal distribution, the same result can be seen when the initial
values are transformed by taking their logarithms. Since by definition
the transformed data takes on the shape of the normal distribution, the
geometric mean assumes the role of the arithmetic mean for that
transformed distribution and is coincident with the median. From the
formulas in footnote 5, it is evident that the geometric mean for log-
transformed data (a log-normal distribution) is calculated in the same
manner as the arithmetic mean for non-transformed data in a normal
distribution. This means that, if the performance assessment results
align closely with the defined normal or log-normal distributions, the
median will converge with the other statistically defined measures of
central tendency for those distributions. If the results are very
highly skewed toward a true log-normal distribution, the geometric mean
essentially equates to the median, but without the calculational issues
described earlier. If less conservatism is incorporated into the
analyses and the resulting distribution is less skewed so that it more
closely resembles a normal distribution, the arithmetic mean
essentially converges with the median and the performance measure
approaches that used to show compliance within 10,000 years.
These relationships between the arithmetic and geometric means and
the median are strictly correct only as long as the distributions fit
the profiles for either the normal or log-normal distributions. If the
actual shapes of the distributions differ to some degree from the ideal
defined shapes, the means, either arithmetic or geometric, will not be
coincident with the median values for the distributions, the degree of
departure being dependent on exactly how much the distributions depart
from the ideal ``normal'' or log-normal'' shapes. Deviations from the
ideal normal and log-normal distribution shapes and the effects on
these measures as representative of the central tendency for the
calculated dose projections, are of critical importance in selecting
the compliance measure. The likelihood of deviations discourages our
use of either the arithmetic or geometric mean at the time of peak
dose, but has limited effect on the use of the median.
Therefore, we propose to use the median of the dose distribution as
the
[[Page 49046]]
performance measure for compliance in the post-10,000-year period and
request comment on that decision. Readers may note that our 1999
proposal, as well as 40 CFR part 191, specified that DOE use the
(arithmetic) mean or median, whichever was higher. We determined that
the arithmetic mean would always be higher for periods up to 10,000
years. Thus, we specified the more conservative measure to apply up to
10,000 years. However, as noted above, the arithmetic mean may be
overly influenced by higher-end estimates. Therefore, we do not
consider it the appropriate measure for times in excess of 10,000
years.
In summary, we propose to maintain and clarify the use of the
arithmetic mean for compliance with the 10,000-year standard. We
believe this is appropriate because the shorter-term projections are
not as influenced by the uncertainties or variability in performance
scenarios seen at much longer times. Fewer very high-end estimates are
expected and, therefore, overall the distribution of doses would be
less skewed and more representative of ``expected'' performance.
Further, in the unlikely event that the peak dose is found to occur
within the first 10,000 years, the arithmetic mean would be consistent
with the statistical measure used in all other applications for
geologic disposal, i.e., 40 CFR parts 191 and 194 for the 10,000-year
time frame. We request comment on the clarification of the arithmetic
mean as the 10,000-year compliance measure. For the period extending
beyond 10,000 years, we propose to use the median of the distribution
of doses calculated from the performance assessments as the compliance
measure, and we request comment on this choice.
6. How Will DOE Calculate the Dose?
Our 2001 standards required DOE to calculate doses as an annual
committed effective dose equivalent (annual CEDE) to demonstrate
compliance with the storage, individual-protection, and human-intrusion
standards. Today we are proposing to modify that requirement in a way
that would incorporate updated scientific factors necessary for the
calculation, but would not change the underlying methodology.
Specifically, we are proposing to require DOE to calculate the annual
CEDE using the radiation- and organ-weighting factors in ICRP
Publication 60 (``1990 Recommendations of the ICRP''), rather than
those in ICRP Publication 26 (``1977 Recommendations of the ICRP'').
This point may seem straightforward to many readers. We wish to
incorporate the most recent science into the calculation of dose, so
why should we not do so? The complication arises from the terminology
employed in the EnPA and ICRP 60 (and the follow-on implementing
Publication 72, ``Age-Dependent Doses to Members of the Public from
Intake of Radionuclides: Part 5 Compilation of Ingestion and Inhalation
Dose Coefficients,'' 1996, Docket No. OAR-2005-0083-0087). Section
801(a)(1) of the EnPA explicitly requires our standards to ``prescribe
the maximum annual effective dose equivalent to individual members of
the general public.'' Thus, we are required by law to issue an
individual-protection standard presented as an effective dose
equivalent. The Court agreed with this reasoning when it stated that
the EnPA ``require[s] EPA to establish a set of health and safety
standards, at least one of which must include an EDE-based, individual
protection standard.'' (NEI, 373 F.3d at 45, Docket No. OAR-2005-0083-
0080.)
ICRP is an independent body formed to develop consensus
recommendations on appropriate radiation protection measures. In doing
so, ICRP considers the principles and scientific bases involved in
practices that involve the generation of radiation and radioactive
materials, as well as the use of those materials. Over the years, ICRP
recommendations have been adopted by regulatory authorities and other
scientific and advisory bodies, and have helped to provide a consistent
basis for national and international regulatory standards.
In 1977 and 1979, ICRP published Report Nos. 26 and 30 (``Limits
for Intake of Radionuclides by Workers''), respectively (Docket Nos.
OAR-2005-0083-0087). These two reports reflect advances in the state of
knowledge of radionuclide dosimetry and biological transport of
radionuclides in humans that occurred over the 20 years since ICRP's
1957 dose methodology recommendation (ICRP 2). This methodology, known
as the effective dose equivalent (EDE) methodology, is the basis for
dose calculations performed to demonstrate compliance with 40 CFR part
191 and envisioned to be applied (although not specified) in the 2001
version of 40 CFR part 197. The EDE methodology was first incorporated
into Federal Guidance in 1987, in ``Radiation Protection Guidance to
Federal Agencies for Occupational Exposure'' (52 FR 2822, January 27,
1987; Docket No. OAR-2005-0083-0078).
The basis of the EDE methodology is that each organ in the body
reacts to radiation differently, i.e., some are more likely than others
to react by developing a cancer. This methodology takes these
differences into account by assigning a ``weighting factor'' to each
organ relative to the whole body. The weighting factor reflects the
likelihood, that is, risk, of fatal cancer developing in that organ per
unit of dose. When added together, the risk-weighted doses incurred by
the individual organs of the body become the ``effective dose
equivalent.'' In this manner, the risk of radiation exposure to various
parts of the body can be regulated through use of a single numerical
standard.
ICRP 26/30 uses the term ``effective dose equivalent.'' ICRP 60/72,
which offers some improvements to the biokinetic models used in ICRP 30
and thereupon updates the organ-weighting factors based on more recent
scientific studies, uses the term ``effective dose.'' It may appear
from this difference in terminology that we cannot both fulfill our
statutory mandate and specify the use of the ICRP 60/72 factors.
However, we do not believe this is the case. First, ICRP made it
clear in Publication 60 that it was adopting the shorter nomenclature
for ease of use, but did not intend to change the underlying approach
of ICRP 26/30: ``The weighted equivalent dose (a doubly weighted
absorbed dose) has previously been called the effective dose equivalent
but this name is unnecessarily cumbersome, especially in more complex
combinations such as collective committed effective dose equivalent.
The Commission has now decided to use the simpler name effective dose,
E'' (ICRP Publication 60, p. 7, Docket No. OAR-2005-0083-0087).
Second, we have used the different terms interchangeably in various
applications over the years. Historically, this concept has been
referred to as effective dose equivalent, effective dose, and total
effective dose equivalent, depending on when the terms were used and
the weighting factors applied. The concept of a ``committed'' dose is
inherent in the methodology (and recognized by ICRP, as in the previous
citation), but we have applied the term to more explicitly acknowledge
the continuing dose contribution over a period of years from
radionuclides taken into the body through ingestion, inhalation, or
absorption.
For example, our standards in 40 CFR part 191 are written in terms
of committed effective dose (CED). These standards were finalized in
1993, after the publication of ICRP 60 and passage of the EnPA. At that
time, our most recent Federal Guidance Report No. 11, ``Limiting Values
of Radionuclide Intake and Air Concentration and Dose Conversion
Factors for Inhalation,
[[Page 49047]]
Submersion, and Ingestion'' (EPA-520/1-88-020, September 1988, Docket
No. OAR-2005-0083-0071), which was issued to serve as the basis for
regulations setting upper bounds on exposures in the workplace,
specified the ICRP 26/30 method to calculate CEDE. Appendix B of 40 CFR
part 191 also specified use of the ICRP 26/30 weighting factors, but to
calculate CED. Thus, we used two different (albeit similar) terms to
represent the use of an identical methodology and associated weighting
factors. From this, it should be clear that we have historically
considered CED and CEDE to represent essentially the same approach,
regardless of the weighting factors used.
In today's proposal, we are specifying in the definition of
effective dose equivalent in Sec. 197.2 that DOE will calculate annual
CEDE using the radiation- and organ-weighting factors in ICRP 60/72,
which we are proposing to be incorporated into a new Appendix A. This
represents the most recent science and dose calculation approaches in
the area of radiation protection, which we previously endorsed in our
Federal Guidance Report No. 13 (``Cancer Risk Coefficients for
Environmental Exposure to Radionuclides,'' EPA 402-R-99-001, September
1999, Docket No. OAR-2005-0083-0072). We believe this change is
appropriate and reflective of the direction of the international
radiation-protection community as well as EPA's own guidance.
Furthermore, we believe this approach is consistent with the intent and
direction of the EnPA. The EnPA directs us to prescribe our standard
for protection of individuals in the form of a general class of
standards known as effective dose equivalent standards. We have done
that by using a standard in the form of committed effective dose
equivalent, which is a member of the class of effective dose equivalent
standards. We request comment on this proposed change.
Regardless of the preferences of radiation experts, the public may
be unfamiliar with the differences between the two methods and ask
whether a given dose level (for example, 15 mrem/yr) is equally
protective when expressed under each method. The calculation of dose
from individual radionuclides may be affected in different ways,
depending on which organs they tend to affect and the pathway through
which they enter the body. For example, consider two radionuclides that
occur in the expected inventory at Yucca Mountain, such as technecium-
99 and neptunium-237. For a given intake, the dose from technecium-99
is higher using the ICRP 60/72 system than it is using the ICRP 26/30
system. On the other hand, the dose from a given intake of neptunium-
237 is lower using the latter system compared to that calculated using
the former. However, in the majority of cases, the effect of changing
from one system to the other is small (``Dosimetric Significance of the
ICRP's Updated Guidance and Models, 1989-2003, and Implications for
Federal Guidance,'' ORNL/TM-2003/207, August 2003, Docket No. OAR-2005-
0083-0070). Further, the overall factors used to convert dose to risk
remain unchanged by today's proposal. Therefore, the estimated risk
from a given radiation dose remains the same. Therefore, the 15 mrem/yr
standard incorporated into today's proposal represents the same level
of protection as the originally promulgated standards.
We have also considered whether to allow for the use of future
updates to the organ weighting or other factors. We believe this may be
appropriate because DOE will continue to perform projections for many
years, and the final demonstration before repository closure and
license termination may be decades or even more than one hundred years
into the future. A provision allowing such updates ensures that the
most current science can be applied at all times. Therefore, we are
including a provision in our proposed Appendix A allowing DOE to use,
with NRC approval, updated dose calculation factors. We have tried in
today's proposal to make clear the basis for our acceptance of the ICRP
60/72 factors as sufficiently validated to be incorporated into
rulemaking. To ensure that such factors that might be considered in the
future have been appropriately reviewed and accepted by the scientific
community, we propose that NRC may only approve factors that have been
issued by independent scientific bodies (such as ICRP and its successor
bodies) and incorporated by EPA into Federal Guidance. To ensure
compliance with the EnPA, we would also require that the new approach
be compatible with the effective dose equivalent methodology
incorporated into today's proposal. We request comment on this
approach.
Commenters may be aware that the NAS released in June 2005 the
latest in a series of studies on the Biological Effects of Ionizing
Radiation (BEIR VII, Docket No. OAR-2005-0083-0087). EPA is a major
sponsor of these studies, which we consider in developing our
regulations and Federal Guidance. The BEIR VII report reaffirmed that
evidence exists that even the smallest radiation dose may convey some
risk of incurring a cancer, and that risk increases proportionally to
the dose (i.e., if the dose doubles, the risk also doubles). This
approach, known as the ``linear non-threshold'' hypothesis, has served
for many years as the basis for all radiation protection regulation and
guidance, including those issued by EPA. Further, the linear non-
threshold approach is the source of the assumptions regarding the dose-
risk relationship underlying both our 2001 rulemaking and today's
proposal. Thus, the primary conclusion of the BEIR VII study does not
affect the revision of our Yucca Mountain standards.
For a detailed discussion of potential health effects related to
exposure to radiation, as well as further explanation of the underlying
relationship between radiation dose and cancer risk, see the preamble
to the 1999 proposed rule (64 FR 46978-46979, August 27, 1999, Docket
No. OAR-2005-0083-0041) and Chapter 6 of the 2001 BID (Docket No. OAR-
2005-0083-0050).
D. How Will Today's Proposal Affect the Way DOE Conducts Performance
Assessments?
We find it important to emphasize certain key aspects of the
performance assessment that will apply regardless of the time frame
involved. First, the overall purpose of our standards is to provide a
reasonable test of disposal system performance. The overall purpose of
the performance assessment is to provide a reasonable test for
compliance with those standards. A major part of providing that
reasonable test is determining which features, events, and processes
(FEPs) are to be included in the performance assessment performed by
DOE. Regardless of time frame, we find it reasonable to limit the
consideration of FEPs and scenarios (sequences or combinations of FEPs)
to those reasonably likely to occur and to affect the disposal system
during the compliance period. Finally, in addressing those scenarios,
it is also reasonable to further prescribe certain aspects of the way
they are considered (``stylizing''), particularly when their
characteristics are difficult to establish with confidence. This
section provides an overview of the performance assessment process and
addresses in more detail the key aspects just mentioned.
The long-term performance of the disposal system is assessed
through complex probabilistic computer simulations aimed at quantifying
the behavior of the disposal system over time. The change in the
compliance period does not affect fundamentally how the disposal system
performance
[[Page 49048]]
assessment simulations are constructed and executed. The performance
assessment takes into consideration the physical and chemical
characteristics of the disposal system, and imposes on that
characterization the events and processes expected to occur during the
compliance period. The DOE has already conducted and published many of
its performance assessment results focusing on periods up to 10,000
years to support its Viability Assessment, FEIS, and site
recommendation. While many of those projections did cover times up to 1
million years, DOE did not focus as much attention on the assumptions
and characterization of those longer-term processes and events, or
necessarily conduct those projections in a way suitable for
demonstrating compliance with a regulatory standard because there was
no quantitative standard past 10,000 years. Today we are proposing
certain provisions that will affect DOE's treatment of longer-term
projections for compliance purposes, but will not alter the
requirements issued in 2001 for compliance within 10,000 years.
The performance assessment is developed by first compiling listings
of features (characteristics of the disposal system, such as the
description of the disposal system geologic setting), events (discrete
events that can occur at the site, such as seismic events, i.e.,
earthquakes), and processes anticipated to be active during the
performance period of the disposal system (such as corrosion processes
operating on the metallic waste package). These items are collectively
referred to as ``FEPs'' (features, events and processes). These FEPs
are then used in combination to construct scenarios, which are
essentially potential ``futures'' for the disposal system. A scenario
describes one possible path along which the disposal system could
evolve from the time of closure through the time of peak dose.
Individual FEPs are components of scenarios and can be combined in
various ways; while some FEPs, such as infiltration of water through
the repository, will be included in nearly all scenarios, low-
probability FEPs may appear in only a few. Thus, a scenario can be
visualized as a time history for the disposal system, beginning, for
example, with precipitation over Yucca Mountain and water infiltration
into the subsurface above the repository, and ending with a dose
assessment for the down-gradient RMEI making use of the ground water
moving from beneath the site. Natural parameter variations (such as
differing ground-water movement rates through the repository and in the
aquifers below the repository) give rise to many potential ``futures''
for a particular scenario, depending on the exact parameter values
chosen from the distribution of possible values, for each computer
simulation of repository performance. For ease of calculations,
scenarios with similar characteristics may be grouped into scenario
classes. More extensive descriptions of the scenarios used to assess
disposal system performance for Yucca Mountain are detailed in DOE
documents supporting such analyses for various purposes (see the
Viability Assessment, DOE/RW-0508/V.3, Vol. 3, Chapter 1.3, December
1998, Docket No. OAR-2005-0083-0086, and the Science and Engineering
Report, DOE/RW-0539, Chapters 4.3 and 4.4, May 2001, Docket Nos. OAR-
2005-0083-0069).
Scenarios have differing probabilities, depending on the likelihood
of particular FEPs included in them. The dose results calculated for
individual scenarios are weighted as a function of their probability to
develop an overall distribution of doses with time that is the final
product of the analyses. From this distribution of doses, compliance
with the regulatory standard is determined in the licensing process.
In considering how to approach assessments potentially out to 1
million years, we have considered international consensus on the
qualitative nature of such calculations. Although also true at the
10,000-year time frame, for peak dose it is even more evident that the
performance assessment cannot be viewed as a predictor of future events
and resultant releases. Instead the goal is to design an assessment
that is a reasonable test of the disposal system under a range of
conditions that represent the expected case, as well as relatively less
likely (but not wholly speculative) scenarios with potentially
significant consequences. The challenge is to define the parameters of
the assessment so that they demonstrate whether or not the disposal
system is resilient and safe in response to meaningful disruptions,
while avoiding extremely speculative (and in some cases, fantastical)
events. As NAS notes, ``The results of compliance analysis should not
be interpreted as accurate predictions of the expected behavior of a
geologic repository'' (NAS Report p. 71).
We recognize that many uncertainties can be bounded, and methods
exist to take these uncertainties into account in evaluating compliance
of the disposal system. Examples include the use of cautious, but
reasonable, parameter values and assumptions that ensure the models err
on the side of conservatism, and the use of probabilistic models in
order to explore the range of possibilities of total system evolution.
We further recognize that it can be difficult to determine when
conservatism is appropriate and when it is excessive. However, as
discussed earlier in this preamble, we are concerned that systematic
conservatism in the face of uncertainties is inconsistent with the
concept of reasonable expectation embodied in our standards. This view
is also shared at the international level. A joint report by the IAEA
and the NEA concludes that ``[w]hen uncertainty exists there is a
tendency to skew the model or values of parameters towards
conservatism,'' which ``results in embedded conservatism'' (``An
International Peer Review of the Yucca Mountain Project TSPA-SR,'' p.
52, 2002, Docket No. OAR-2005-0083-0062). However, those aspects of the
disposal system and waste behavior that depend upon physical and
geological properties can be estimated within reasonable limits of
uncertainty.
Still, ``[e]ven in the initial phase of the repository lifetime, a
compliance decision must be based on a reasonable level of confidence
in the predicted behavior rather than any absolute proof'' (NAS Report
p. 72). For performance projections made past 10,000 years, the
confidence that can be placed in those projections decreases as time
increases. While NAS indicated that analyses of the performance of the
Yucca Mountain system dealing with the far future can be bounded, ``the
uncertainties in some of the calculations that might be required could
render further calculation scientifically meaningless'' (NAS Report p.
29). What is more, a different panel convened by NAS has recently
stated that uncertainties often become so large that the results of a
risk assessment must be deemed indeterminate (``Risk and Decisions
About Disposition of Transuranic and High-Level Radioactive Waste,''
NAS, p. 91, 2005, Docket No. OAR-2005-0083-0060). Regarding natural
processes and/or events that can occur during a large period of time,
it becomes necessary to restrict the scenarios available to include in
a performance assessment by not including events or processes that have
a nearly negligible probability of occurrence over the period of
geologic stability, or that introduce additional uncertainty without
providing significantly new or different information about the
performance of the disposal system.
It is neither useful nor necessary for EPA to require DOE to
predict or model every conceivable scenario that could occur at Yucca
Mountain. Rather, we
[[Page 49049]]
believe that it is our responsibility to design a reasonable test of
the disposal system's performance over a very long time period. This
implies that some possible performance scenarios should not be included
in the performance assessment because their probability of occurrence
is extremely low. As a means of restricting scenarios, in setting the
standards in 40 CFR part 197, the Agency outlined how to identify FEPs.
For purposes of the performance assessment, the value of considering a
particular FEP (or series of FEPs) diminishes if either its likelihood
of occurrence or its potential consequence is insignificant. Therefore,
a time frame and probability cut-off measure are needed to limit the
range of FEPs that could be included as candidates for the performance
assessment. Without such measures, the list of FEPs would be limitless,
bounded only by the imagination. The Agency determined that FEPs that
could occur with a probability equal to or greater than 1 in 10,000
over a period of 10,000 years would be sufficiently likely to occur, so
that they should be included among the FEPs available for selection in
any particular scenario. FEPs with lower probabilities could be
excluded from the analyses. This probability limit represents an annual
probability of occurrence of 10-\8\ (1 in 100 million). This
means, for example, an event with this minimum probability has only a
one-hundredth of one percent chance of happening in any given 10,000-
year period. This is an extremely conservative screening criterion.
Extending the regulatory compliance period to as much as 1 million
years and maintaining the annual probability cut-off of
10-\8\ would still mean that FEPs with only a one percent
chance of occurring over this time period must be considered. This
probability cut-off for screening FEPs for inclusion in the disposal
system performance assessment provides a robust test of compliance, in
that even FEPs with very low probabilities are not a priori excluded
from the assessments.
Given the conservative nature of this low probability cut-off for
initial FEPs screening efforts, the application of the screening
criteria still produce a large number of scenarios that could be
postulated, presenting perhaps an unmanageable task for the analyses
and ultimately the regulatory compliance decision. In the generic rule
for geologic disposal, 40 CFR part 191, and the 2001 rule for Yucca
Mountain, we provided a means to manage the situation, by allowing
individual FEPs or scenarios to be deleted from the licensing
performance assessment if they contribute little to the dose received
by the RMEI, i.e., their consequences are low--either due to the low
probability of the FEPs or the low doses calculated for the scenario.
In extending the regulatory performance period in the regulation to the
time of peak dose, a similar provision aimed at managing the scope of
the analyses is called for.
The need to maintain the assessment within a reasonable scope as a
way to manage uncertainties leads us to conclude that a strict
extension of the approach for 10,000-year assessments would not
accomplish this overall goal. If, for example, we required
consideration of events with a probability of occurrence of
10-\4\ over 1 million years `` an approach that has been
suggested by some stakeholders `` it would equate to an annual
probability of 10-\10\ (one in 10 billion), which
encompasses events nearly as remote as the ``big bang'' that created
our universe. No disposal system, and perhaps not even our planet
itself, would be expected to survive the effects of such an event, and
we therefore do not find it a useful indicator to distinguish between
safe or unsafe performance of the disposal system. There are an
unlimited number of possible futures, some of which would involve risks
from a repository and others that would not. We must balance these
factors to ``define a standard that specifies a high level of
protection but that does not rule out an adequately sited and well-
designed repository because of highly improbable events'' (NAS Report
p. 28).
In addition, NAS recommended ``against an approach under which a
large number of future scenarios are specified for compliance
assessment, since such an approach could be seen as putting both the
regulator and the applicant in the indefensible position of claiming to
have considered a sufficient number of scenarios and that all
reasonable future situations are represented in the analysis'' (NAS
Report p. 98). NAS explicitly recognized that ``[i]t is important that
the `rules' for the compliance assessment be established in advance of
the licensing process; that is, that the scenarios that might be
excluded from the integrated risk analysis be identified'' (NAS Report
p. 73). We emphasize that the purpose of making exposure scenario
assumptions is not to identify exhaustively every possible future, but
to construct a reasonable (or, as NAS put, a ``fair'') test of disposal
system performance for the protection of public health. This is the
case regardless of the time frame involved, and from that perspective
today's proposal will not alter the way in which DOE will approach its
performance assessments.
In addition to placing limits on the probability of FEPs that
should be considered, an additional tool to construct the test (or set
``the `rules' for the compliance assessment,'' as NAS stated) is to
specify how certain scenarios should be assessed. This ``stylizing'' of
scenarios is similar to the approach we took (and NAS recommended) to
defining the human-intrusion scenario. In a more general sense, NAS
acknowledged that establishing the ``rules'' ``requires using the
rulemaking process to arrive at a regulatory decision about certain
assumptions as part of the standard'' (NAS Report p. 34). The NEA has
also recommended exploring the possibility of using a similar stylized
approach to address uncertainties in the evolution of the surface
environment and the nature of future human actions (``The Handling of
Timescales in Assessing Post-Closure Safety,'' pp. 22-23, 2004, Docket
No. OAR-2005-0083-0046). This approach would avoid speculation
regarding the evolution of the geologic environment at times when the
hazards associated with the waste are reduced compared to when the
waste is emplaced.
Stylized approaches can be utilized to address associated
uncertainties in order to allow consideration of events that are deemed
potentially important to performance but whose characteristics are
difficult to establish with certainty. There is international consensus
that this approach may be used to define assumptions that are too
difficult to bound (NEA, p. 22, Docket No. OAR-2005-0083-0046). This
approach could therefore be used for the determination of the evolution
of the geological environment over long periods. As noted above, this
approach is similar to that recommended by NAS, and utilized by EPA in
examining human intrusion (NAS Report p. 108). The NAS determined that
it was technically infeasible to assess the probability of human
intrusion into a repository over the long term. It concluded that it
was not scientifically justified to incorporate a myriad of alternative
scenarios of human intrusion into a fully risk-based compliance
assessment that requires knowledge of the character and frequency of
various intrusion scenarios. Accordingly, NAS recommended that we
specify in our standards a typical intrusion scenario to be analyzed
for its consequences on the performance of the repository. The intent
of this ``stylized scenario'' is to avoid non-productive speculation on
the forms and frequencies of intrusion that can never be predicted,
while
[[Page 49050]]
allowing the ``robustness'' of the containment properties of the
repository to be evaluated by a scenario that is plausible, and
potentially causes some levels of exposure. The same factors must be
balanced in considering how to assess key geologic and other features
over very long time frames when it is exceedingly difficult to
establish exact parameters--or even distributions of parameter values--
with any certainty.
The modifications proposed in Section II.C (``How is EPA Proposing
to Revise the Individual-Protection Standard to Address Peak Dose?'')
would require DOE to project exposures to the RMEI until the time of
peak dose and subject them to a compliance determination. The key
aspects emphasized at the beginning of this section guide our
requirements for the scope of performance assessments both at 10,000
years and over times extending through the entire period of geologic
stability. However, their implementation carries different implications
for those different time periods, given the nature of uncertainties and
the types of events that can be envisioned to occur. To address these
implications, we are proposing four provisions that will affect the
judgment of compliance when that judgment is extended to periods up to
1 million years. Specifically, we are proposing:
A separate compliance standard for the peak dose beyond
10,000 years;
That compliance beyond 10,000 years be demonstrated using
the median of the distribution of results;
That FEPs and scenarios not included in the 10,000-year
analysis because of their limited consequence during that period need
not be considered in the peak dose calculations;
That scenarios involving climate change, seismic activity,
igneous activity, and general corrosion be explicitly considered in the
peak dose calculations.
We have already discussed the peak dose standard and the use of the
median to demonstrate compliance (see Sections II.C.3 and II.C.5). The
selection of FEPs (including general corrosion) is discussed in detail
in Section II.D.2.a (``Consideration of Likely, Unlikely, and Very
Unlikely FEPs''). Discussion of climate, seismic, and igneous scenarios
is included in Sections II.D.2.b, c, and d, respectively.
1. Performance Assessments Up To 10,000 Years After Disposal
Our 2001 rulemaking established a framework within which DOE would
conduct its performance assessments to show compliance with the 10,000-
year standard. The previous section touched on various aspects of this
framework. Essentially, the performance assessment involves three basic
steps: (1) Identify the FEPs and scenarios that might affect the Yucca
Mountain disposal system, along with their probabilities of occurrence;
(2) examine the effects of those FEPs and scenarios on disposal system
performance; and (3) estimate the dose consequences from those FEPs and
scenarios, weighted by their probabilities of occurrence. Today's
proposal will not affect this framework in any way.
We supplemented this basic framework with two additional
provisions. The first, the underlying principle of reasonable
expectation, we have discussed in detail in Sections II.A.4 and II.B.
The other important provision, touched on in the previous section,
establishes the approach to identifying FEPs and scenarios and their
probability of occurrence. We specified that FEPs or scenarios with a
probability of occurrence lower than 1 in 10,000 over 10,000 years need
not be considered in the performance assessment. FEPs or scenarios with
a higher probability of occurrence also need not be considered if they
would not significantly change the results of the performance
assessment. We are not proposing to alter this provision as it applies
to the 10,000-year standard. The standards in 40 CFR part 191 (the EPA
regulation that addresses geologic disposal generically) also used this
formulation as the means of determining FEPs for any prospective
disposal system. In developing 40 CFR part 197 in 2001, the Agency
determined that there was no reason, on a site-specific basis, to
depart from this conservative screening criterion. We also note that
NAS endorsed this same probability level in its specific discussion of
volcanism, and suggested that such a level ``might be sufficiently low
to constitute a negligible risk [of occurrence]'' (NAS Report p. 95).
Probabilities below this level are associated with events such as the
appearance of new volcanoes outside of known areas of volcanic activity
or a cataclysmic meteor impact in the area of the repository. We
believe there is little or no benefit to public health or the
environment from trying to regulate the effects of such very unlikely
events.
2. Performance Assessments for Periods Longer Than 10,000 Years After
Disposal
As discussed in the previous sections, we do not believe that DOE's
performance assessments need be changed fundamentally to accommodate an
extended compliance period. The general framework described in the
previous section applies equally well to periods beyond 10,000 years,
although we are proposing specific provisions to apply to this longer
period. We recognize, however, that there may be some confusion
regarding the conduct of assessments to demonstrate compliance at two
different times. DOE will not necessarily conduct one set of
assessments to show compliance with the 10,000-year standard, and a
separate set of assessments to show compliance with the peak dose
standard applicable at times beyond 10,000 years. Rather, DOE's overall
approach could be to run its dose assessments from the time of facility
closure to the end of the period of geologic stability (1 million years
after closure). The FEPs and scenarios selected for each individual run
would continue to operate, and the disposal system to evolve, over that
entire time period. DOE would extract the results necessary for
comparison with our regulatory standards.
As it is with the 10,000-year standards, the main purpose of the
post-10,000-year standards is to provide a reasonable test of the
performance of the disposal system. The NAS stated it another way:
``The challenge is to define a standard that specifies a high level of
protection but that does not rule out an adequately sited and well-
designed repository because of highly improbable events'' (NAS Report
p. 28).
In formulating our approach to an extended compliance period, we
began by reviewing the NAS report. NAS concluded that several gradual
and episodic natural processes or events have the potential to modify
the properties of the repository and the processes by which
radionuclides are transported. NAS concluded that the probabilities and
consequences of modifications generated by volcanic eruptions
(volcanism), seismic activity, and climate change are sufficiently
boundable so that these ``modifiers,'' as it termed them, can be
included (along with an undisturbed scenario) in performance
assessments that extend over the expected period of geologic stability
(on the order of 1 million years) in the Yucca Mountain region (NAS
Report p. 91). NAS considered the ``long-term stability of the geologic
environment at Yucca Mountain'' to describe the situation where
geologic processes such as earthquakes (and similar physical and
geological processes that could affect the performance assessment at
the Yucca Mountain site) are sufficiently quantifiable and the related
[[Page 49051]]
uncertainties boundable that the performance can be assessed (NAS
Report p. 67). Furthermore, NAS acknowledged that, conceptually, there
is a need for screening criteria to distinguish significant FEPs from
those that can be considered to have negligible effects (NAS Report,
for example, pp. 59, 61, 72, 95, 98). NAS suggested that certain levels
(including a probability cut-off of 10-8 per year) might be
appropriate, but made no recommendation on this issue.
We believe the three categories of FEPs identified by NAS deserve
special attention. We will require that DOE consider these FEPs in its
long-term projections. However, we are proposing to apply the same
overall probability threshold and handle such events in a stylized
manner to address only their most significant effects. In essence, DOE
must include such FEPs in the peak dose assessment, but need not assess
in detail every conceivable variation on those events. Thus, our
approach would require that DOE develop reasonable igneous, seismic and
climate change scenarios and assess the most likely and significant
impacts, with appropriate variability in its assumptions, on dose
projections. The NAS did not identify any other ``modifiers'' that it
expected could be addressed in a quantitative risk assessment covering
the period of geologic stability. In addition, NAS specifically
mentioned potential effects of these modifiers, but also noted that,
while possible, many of these effects would be so unlikely or limited
that they would not be expected to significantly affect disposal system
performance (NAS Report pp. 91-95). These igneous, seismic, and
climatological FEPs are discussed in more detail in the following
sections. We propose to specify certain significant aspects or
characteristics of the event or process to which DOE may limit its
analyses, and DOE will assess reasonable variations within those
bounds, considering such basic assumptions as severity and time of
occurrence. DOE must then evaluate the consequences on the disposal
system and resulting impacts to the RMEI. By varying the time of
occurrence within the probability framework, DOE can also address the
effects of these FEPs on the peak dose.
Having identified particular natural FEPs that should be considered
throughout the period of geologic stability, we then considered whether
there are FEPs affecting the engineered barrier system that should also
be identified. In reviewing DOE's published TSPAs and other relevant
information, we conclude that general corrosion of the waste packages
has been shown to be a potentially significant failure mechanism at
times in the hundreds of thousands of years (Yucca Mountain Science and
Engineering Report, DOE/RW-0539, Section 4.2.4, May 2001, Docket No.
OAR-2005-0083-0069). Unlike certain other corrosion processes, as
discussed in the next section, which may be more likely or faster-
acting at earlier times, general corrosion may not be a significant
factor within 10,000 years and could potentially be removed from
consideration at those times because of its limited consequence. Were
we simply to state that FEPs not included in the 10,000-year analyses
should not be included in the post-10,000-year analyses, there might be
some question as to whether DOE would need to consider general
corrosion at all. We believe it has been shown potentially to be of
sufficient importance that it should be included in those projections.
Therefore, we are proposing to remove any ambiguity by specifying that
DOE must consider general corrosion in its projections throughout the
period of geologic stability.
In general, we continue to believe that it is reasonable to require
DOE to exclude from performance assessments those FEPs whose likelihood
of occurrence is so small that they are very unlikely, or whose
consequence is minimal, as described above. We propose that this
probability threshold as expressed in our 2001 rule for the 10,000-year
compliance period be extended throughout the entire period to peak dose
(i.e., FEPs included in the 10,000-year assessments are included in the
assessments beyond 10,000 years), but with the inclusion of the long-
term impacts of seismicity, volcanism, and long-term climate change, as
consistent with the probability screening criteria described herein
(NAS Report p. 94). These are the natural events and processes that NAS
determined were reasonably boundable when compliance time frames at
Yucca Mountain are extended out to the period of geologic stability. We
also propose that DOE must consider the long-term effects of general
corrosion on the engineered barriers, particularly on waste package
integrity. This is an extremely inclusive standard. It captures
significant events in the life of the repository, and yet is not so
restrictive that no repository could ever pass, given that there would
be no limit to the speculation of scenarios that could occur during the
period of geologic stability.
As discussed further in the following sections, we have examined a
variety of events and feel confident that the screening analysis for
10,000 years--with the assurance that seismic, igneous, climate change,
and general corrosion scenarios are included--includes the appropriate
range and severity of FEPs to also serve as a reasonable test of
disposal system performance throughout the period of geologic
stability. We have not (and have not claimed to) conducted an
exhaustive or detailed analysis of variations or permutations of
scenarios out to the time of peak dose. In fact, this is precisely the
sort of unrestrained and speculative exercise we wish to avoid. We
recognize that some commenters may believe it is appropriate to
consider whether further analysis or new data could reveal that an
event excluded from the 10,000-year screening is important to
performance of the disposal system over the geologic stability period.
As discussed later, we do not believe such scenarios are either very
likely or very important to performance. Nor do we believe that this
approach inappropriately constrains NRC, as the licensing authority.
Rather, we consider this approach to be consistent with the NAS
position that conducting compliance assessments ``requires using the
rulemaking process to arrive at a regulatory decision about certain
assumptions as part of the standard'' (NAS Report p. 34).
a. Consideration of Likely, Unlikely, and Very Unlikely FEPs
Our individual-protection standards (Sec. 197.20) as promulgated
in 2001 required DOE to consider in the performance assessment FEPs
with a one in 10,000 or greater chance of occurring during 10,000
years. FEPs below this probability threshold are considered ``very
unlikely'' and can be discounted based on probability alone. We also
allowed NRC and DOE to remove from consideration FEPs with a higher
probability if their effects on performance assessment results were
determined to be insignificant. In addition, performance assessments
conducted to show compliance with the human-intrusion and ground-water
protection standards may exclude FEPs considered ``unlikely.'' We
specified that NRC was to determine the probability below which FEPs
would be considered unlikely. NRC set that figure at a probability of
occurrence of 1 in 10 over 10,000 years (equivalent to an annual
probability of 10-5) (67 FR 62634, October 8, 2002, Docket
No. OAR-2005-0083-0059).
In extending the period of compliance, we must consider whether our
threshold for probability screening
[[Page 49052]]
of ``very unlikely'' FEPs remains appropriate. We believe it does, and
are proposing to retain it for the extended compliance period. While we
are retaining the compliance standard of 150 [mu]Sv/yr (15 mrem/yr)
applicable to 10,000 years, we are also proposing to introduce a second
compliance standard of 3.5 mSv/yr (350 mrem/yr) for the peak dose
beyond 10,000 years, which could potentially apply up to 1 million
years. This may lead some commenters to suggest that the formulation
for FEPs screening should simply be extended by two orders of
magnitude, i.e., that very unlikely FEPs would have less than a one in
10,000 chance of occurring over 1 million years. This would recognize
that very low-probability FEPs would become more likely to be seen
simply with the passage of time (essentially by looking at many 10,000-
year periods, the cumulative probability, rather than annual
probability, would be increased). However, in our view, such a
formulation would be unjustified and unreasonable.
It is important to consider the real meaning of these probability
thresholds. A FEP screened in at the existing lower probability
threshold would have only a 0.01% chance of occurring through 10,000
years, yet still must be included in the FEPs considered for the
performance assessment. We question, then, whether the effort involved
in incorporating even less likely events into the ``FEP pool,'' with
the level of speculation likely to be attached to them, would be
rewarded with even minimal contribution to safety.
The threshold for very unlikely events suggested by NAS was an
annual probability of 10-8 (1 in 100 million per year),
which NAS equated to 1 in 10,000 over 10,000 years, stating that this
level ``might be sufficiently low to constitute a negligible risk''
(NAS Report p. 95). We consider these two expressions to be
functionally equivalent (and have explicitly included both in our
proposal today), but adopted the latter as more clearly tied to the
10,000-year compliance period. Even though the NAS statement above was
referring to volcanism, we believe that this probability threshold is a
generic consideration that is applicable to any risk at Yucca Mountain,
not just volcanism. If one extends the time period of the assessment to
1 million years, a FEP at this level would still have only a 1 in 100,
or 1%, chance of occurring within that time, but would still be
considered in the performance assessment process. We believe this is a
``cautious, but reasonable'' level, especially when considering the
confounding effects of uncertainties at such long time periods. In
fact, we are unaware of any international precedents for scrutinizing
FEPs of this low probability. Thus, we are proposing to retain the
10-8 annual probability threshold for very unlikely FEPs for
both the 10,000-year and post-10,000-year assessments.
Application of this screening criterion deserves some additional
discussion. For FEPs involving the natural barrier, an annual
probability of 10-8 theoretically indicates that to compile
a definitive list of all FEPs involving the natural barrier, the
geologic record at the site would have to be examined back to a time
frame of 100 million years to identify FEPs that would be projected to
occur at least once in that time period. For the Yucca Mountain site,
the volcanic rocks containing the repository are only on the order of
10 million years in age, indicating that essentially any FEP that could
be identified in the geologic record during the 10 million year time
frame would have an annual probability higher than 10-8, and
would be included in the list of FEPs for scenario construction. We
believe that the Quaternary period, extending back approximately 2
million years, is a sufficiently long period of the geologic record to
allow DOE to make reasonable estimates of natural FEPs (see 66 FR
32100). Observed FEPs from that period, as well as other that can be
inferred, would be included in a 10-8 cut-off.
For FEPs involving the engineered barrier, a similar logic applies.
However, the ``record'' to be examined to identify FEPs for the
performance of man-made materials and systems is much shorter than the
geologic record. Application of the 10-8 annual limit
ensures all relevant FEPs are considered for inclusion. For example,
corrosion processes for which there is accelerated testing and analog
information at longer time frames, could still be included in scenario
development. Even when such processes would have a low probability, the
conservative probability cut-off threshold would still assure they are
considered in scenario development. For such processes, however, when
probabilities of occurrence over long times may be difficult to assign,
the decision to consider them may be based solely on consequence.
By contrast, were we to stretch the probability threshold by two
orders of magnitude, to an annual probability of 10-10 (one
in 10 billion per year), we would be introducing an unprecedented level
of conservatism into the performance assessments. At such a level, the
performance assessment would be required to consider geologic events
likely to have never happened, since the age of the Earth itself is
estimated at about 4.5 billion years (http://pubs.usgs.gov/gip/geotime/age.html
). Further, an event of this annual probability will not reach
even a 50% cumulative probability for another 500 million years (a
total of 5 billion years), or 500 times the period of geologic
stability at Yucca Mountain (defined by NAS as on the order of 1
million years). A probability threshold at that level would sweep in
cataclysmic volcanic and seismic events, as well as meteor impacts of
the type that extinguished the dinosaurs 65 million years ago. We
simply find it inconceivable that such events could be considered a
reasonable test of the repository, or that requiring them to be
analyzed would provide any benefit to public health and safety. To look
at it another way, an event at our current probability threshold of one
chance in 100 million per year would still be likely to occur only a
few times over an incremental 500 million year period, and roughly 50
times over the entire history of the earth, of which humans have been
present only 0.0001% of the time. Examining the geologic record at the
Yucca Mountain site for such a time period to identify FEPs would not
be meaningful. Even looking at the geologic record with the
10-8 probability is challenging. In fact, the volcanic rocks
that contain the repository were formed by very extensive volcanism
over an area of thousands of square kilometers. Using the annual
probability figure alone, it can be argued that such extensive
volcanism should be included in the list of FEPs for the performance
assessment. We strongly disagree. As emphasized by NAS, we reasonably
must confine ourselves to assessing performance of the existing
geologic setting. To remove such extreme assumptions, we addressed this
particular difficulty by recommending the geologic record through the
Quaternary (a period of approximately 2 million years) as the basis for
identifying FEPs for the performance assessment (66 FR 32100). Based on
this period as compared to the probability threshold we have
established, DOE must consider for its performance assessments events
that can be shown or reasonably inferred to have occurred during the
Quaternary, based on the physical conditions of the site and disposal
system.
If the same probability threshold applies at all times, as we are
proposing, then the FEP screening performed by DOE for its 10,000-year
projections would be expected to adequately represent those longer time
periods. We
[[Page 49053]]
believe it will, and do not believe it should be necessary for DOE to
re-examine its results to ``screen in'' FEPs it has previously analyzed
and rejected, or FEPs that might be expected to be more probable at
longer times, if such exist. Further, our view is that it would be an
endless task for DOE to analyze every FEP postulated to occur several
hundred thousand years into the future, simply because a scenario can
be invented to support it. Even if DOE were to exhaustively pursue each
nominated FEP, their effects are likely to be minimal at best,
especially when considering what are likely to be the much larger
effects of increasing uncertainties and large-scale scenarios such as
climate change. It should be clear, however, that FEPs selected for the
analysis will continue to unfold as the assessment continues, up to 1
million years. That is, for all FEPs included in the 10,000-year
analysis, DOE must project the effects of these FEPs continuing to
evolve over the course of the period of geologic stability, and account
for their contributions to the peak dose.
If we are starting from the basic screening for 10,000 years, it is
reasonable to examine the reasons why FEPs might have been excluded
from that screening when considering whether any warrant further
evaluation in the post-10,000-year performance analysis. We see three
general categories of FEPs (as opposed to the more specific seismic,
igneous, and climatic FEPs, which are addressed separately in the
following sections of this document) that may have been eliminated from
the full analysis:
FEPs Screened Out by Probability
The first category consists of FEPs determined to be ``very
unlikely'' to occur. As described above, these are FEPs that would have
a chance of occurrence of less than one in 10,000 over 10,000 years, or
an annual probability less than 1 in 100 million (10-8). We
see no reason to re-consider FEPs removed from the assessment based on
this criterion. Such a FEP would have to be more likely to occur at
some time in the future than it is now. This does not simply mean that
the cumulative expectation of an event or process having already
occurred is higher as time extends from 10,000 to 1 million years,
which would be the case for any low-probability FEP; rather, it means
that the probability itself would have to be higher at some later time
(for example, 10-9 annual probability until year 50,000,
then a 10-8 probability thereafter). We have not identified
natural FEPs that would be very unlikely for the first 10,000 years,
but would rise above that threshold within the period of geologic
stability (FEPs whose probability of occurrence is related to the
condition of the engineered barrier system are discussed later in this
section). It may be argued that a FEP may become more likely if certain
other FEPs have altered the site's characteristics in a particular way.
As a basis for requiring additional FEP screening, we would find such a
claim to be unreasonable and highly speculative. FEP probabilities are
derived in large part from examinations of the historic geologic and
climatic record going back millions of years. We suggested that the
Quaternary period might be an appropriate benchmark for such an
examination (66 FR 32100). Probabilities derived from such evaluations
are not amenable to that level of fine-tuning. Furthermore, DOE has
currently included FEPs which are at the boundary of the
10-8 threshold, such as volcanic events (estimated at 1.6 x
10-8). We would not view such an exercise as useful or of
value in the licensing process. We do not believe it is necessary or
appropriate for NRC to re-consider the probability criterion.
FEPs Screened Out by Consequence Within 10,000 Years
Our 2001 standards allow NRC to eliminate FEPs whose effects would
not significantly change the performance assessment results within
10,000 years. We are proposing today to take the same approach to the
peak dose projections, giving special attention to changes to the
magnitude of the peak dose. There is no reason for DOE to re-consider
FEPs for their effects on the 10,000-year projections, and we are aware
that some FEPs have been included whose effects are manifest at times
slightly beyond 10,000 years to give perspective on the shorter-term
evolution of the disposal system, such as slower-acting corrosion
mechanisms.
At issue, then, would be FEPs whose effects might not be evident or
as prominent until several tens or hundreds of thousands of years have
passed. Such FEPs might be considered to be either gradual, continuing
processes or episodic, disruptive events and processes. In general, we
believe that the 10,000-year assessments should adequately address the
more gradual processes and that the more significant of those processes
have been included in those assessments (for example, infiltration of
water through the repository and the processes leading to early failure
of waste packages heavily influence the 10,000-year assessments and
would do the same for peak dose projections). By the time those more
gradual processes would take effect, it is likely that the effects of
other processes would already be felt at a much higher level. One
fundamental purpose of probabilistic performance assessment is to give
proportionate emphasis to highly improbable events and processes. With
one exception (discussed below), we find it unlikely that any gradual,
continuing processes not already included through the screening for the
10,000-year assessments under our proposed rule could significantly
affect the projections over such long time periods. It is more likely
that their effects would be overwhelmed by other, higher-probability
(or faster-acting) processes operating over the same period.
The single such slow-acting process we have decided to include in
today's proposal is general corrosion of the engineered barriers,
particularly its effects on the waste packages. We recognize that DOE
has included general corrosion in its previous analyses for both the
10,000-year period and over the longer term. However, even though
general corrosion is significant to performance at longer times, it
might reasonably be considered insignificant within the first 10,000
years and could, thus, be screened out of the analysis to demonstrate
compliance with the 10,000-year standard. Under our overall approach,
were DOE to exclude general corrosion on the basis of consequence
within the first 10,000 years, longer-term projections could also
exclude this factor. We think such an exclusion over the period of
geologic stability would ignore a crucial factor in long-term
performance at Yucca Mountain. As we have noted, DOE's own analyses
point to general corrosion as the dominant waste package failure
mechanism, either alone or in combination with disruptive events
(igneous events are assumed to be less dependent on prior degradation
of waste packages). Without general corrosion assumed to act, a large
proportion of the waste packages could be assumed to remain intact even
up to or beyond 1 million years. Other corrosion mechanisms, such as
localized corrosion, are highly correlated with temperature and would
be expected to operate early in the assessment period, when
temperatures inside the repository are likely to be very much higher.
Stress-corrosion cracking is another mechanism that is somewhat
correlated with temperature, but is of more importance in situations
involving mechanical failure, such as rockfall resulting from seismic
events. Their longer-term impact is likely to be
[[Page 49054]]
greatly reduced after the repository cools. The same is not true for
general corrosion. The rate of general corrosion is somewhat influenced
by temperature, but this process is expected to continue even when the
temperature is lower. Our proposed approach would eliminate any
questions regarding whether general corrosion should be considered for
the longer-term assessments.
Although general corrosion was not called out by NAS, as were the
three natural FEPs, we believe this approach to general corrosion is
consistent with NAS's overall expectations for the evolution of the
disposal system. We have already discussed in the context of
uncertainty NAS's expectation that a significant proportion of the
waste packages would fail over the period of geologic stability and
that, while peak doses might occur much later, significant releases
could be anticipated within the first 10,000 years (see Section II.A.5,
``Effects of Uncertainty''). For example, NAS suggested that some
uncertainties will be lower ``when enough time has passed that all of
the packages will have failed'' (NAS Report p. 29-30); that
``uncertainties in waste canister lifetimes might have a more
significant effect on assessing performance in the initial 10,000 years
than in performance in the range of 100,000 years'' (NAS Report p. 72);
that ``[d]etailed estimates of time for canister failure are less
important for much longer-term estimates of individual dose or risk''
(NAS Report p. 85); and that ``[i]nflow of air through failed canisters
and oxidation of waste prior to infiltration of water * * * would
probably affect estimates of 10,000-year cumulative releases more than
estimates of longer-term doses and risks'' (NAS Report p. 86). Further,
NAS clearly identified corrosion as the dominant process leading to
waste package failure and recognized its importance in projecting peak
dose: ``Radionuclide releases from an undisturbed repository * * * can
occur through * * * degradation and failure of the waste canister
through corrosion'' * * *'' (NAS Report p. 26--see also pp. 68, 82,
85). We also believe our proposed approach to general corrosion is
consistent with both NAS's advice to use ``cautious, but reasonable''
assumptions and our principle of reasonable expectation, as general
corrosion represents a potentially significant failure mechanism
leading to radionuclide releases.
Regarding natural FEPs, we are proposing that DOE explicitly
evaluate the effects of seismic, volcanic, and climatological FEPs in
its assessments beyond 10,000 years, as discussed in the following
sections. It should be understood, however, that these FEPs may also be
considered within the 10,000-year period if warranted by probability or
consequence. The probabilities of seismic and igneous events beyond
10,000 years will be the same as those probabilities within 10,000
years. Events that DOE judges fall below the 10-8
probability threshold need not be included in either the 10,000-year or
post-10,000-year assessments. Such events might include seismic
episodes above a certain magnitude. There is more certainty that the
climate will experience significant changes over the period of geologic
stability, and therefore we require it to be considered at all times.
The effects of climate change on Yucca Mountain's performance, however,
are likely to be minimal within 10,000 years, and potentially more
significant at longer times when most of the waste packages are
breached.
FEPs Screened Out by Condition of the Engineered Barrier System Within
10,000 Years
We are aware that DOE has identified certain FEPs that were
eliminated from consideration within 10,000 years because it was deemed
impossible for them to occur while the engineered barrier system
remains intact. We believe such FEPs should be considered as a special
case, as they depend on the condition of the engineered barrier system
rather than a strict probability of occurrence.
The prime example of the FEPs in this category is in-package
nuclear criticality. The possibility of this occurring at Yucca
Mountain was discounted within 10,000 years on the basis that the waste
packages would remain largely intact during that time (although a
certain level of premature failures was assumed). DOE stated that ``One
of the required conditions is the presence of a moderator, such as
water, in the waste package. The waste packages will be designed to
make the probability of a criticality occurring inside the waste
package extremely small'' (FEIS, DOE/EIS-0250, section I.2.12, p. I-21,
Docket No. OAR-2005-0083-0086). At some point beyond 10,000 years,
however, packages are anticipated to degrade sufficiently to allow
water inside, so the reason for screening out this FEP is no longer
credible. We understand that NRC has evaluated this possibility and
initial results suggest that the effects would not be significant
(``System-Level Performance Assessment of the Proposed Repository at
Yucca Mountain Using the TPA Version 4.1 Code,'' CNWRA 2002-005,
September 2002, Revised March 2004, Appendix G, Docket No. OAR-2005-
0083-0067). More recently, NRC staff analyses regarding the potential
effects of a criticality event within the waste package indicated that
the effects would be more significant within the first 10,000 years
after disposal than at longer times (``Estimating In-Package
Criticality Impact on Yucca Mountain Repository Performance,''
International High Level Radioactive Waste Management Conference, Las
Vegas, Nevada, March 30-April 2, 2003, Docket No. OAR-2005-0083-0082).
Therefore, we do not require that DOE consider in-package criticality
beyond 10,000 years if it has not been considered for the first 10,000
years. To the extent DOE's waste package assumptions make such a
scenario credible within the initial 10,000 years, however, it would be
appropriate to include it in the post-10,000-year projections.
There may be other FEPs that fall within this category. However,
this illustrates the very possibility we wish to avoid. It is possible
to generate complex and vaguely-defined circumstances and insist that
DOE analyze them thoroughly. We see such an exercise as being of no
value. Rather, we believe it would be detrimental to the licensing
process, as well as contrary to our ``reasonable expectation'' concept
and the idea that performance assessments should represent credible
projections of disposal system safety.
Having considered the various types of FEPs that may have been
excluded from the 10,000-year analysis, our goal is to require an
appropriate consideration of FEPs in the analyses beyond 10,000 years.
We considered an approach that would provide NRC with broader
flexibility to consider previously excluded FEPs that it believes
should be included in the peak dose analyses, perhaps based on the
effect of the FEP on the magnitude of the peak dose. However, we
believe that any potential FEPs to be included are likely to be
overwhelmed by increasing uncertainties or larger-scale FEPs such as
climate change. For this reason, we do not believe the inclusion of
such FEPs will add materially to the understanding of the disposal
system's performance or will lead to a safer disposal system.
Furthermore, as stated earlier, we are guided by our reasonable
expectation principle in not requiring an exhaustive and completely
accurate prediction of repository conditions over a million-year
period. See Sections II.A, II.B, and II.C for discussions of the
[[Page 49055]]
relative confidence in calculations at very long times, and the need to
view those calculations in a more qualitative way. We aim to construct
a reasonable test of the disposal system that accounts for the possible
occurrence of significant FEPs at Yucca Mountain, and the system's
response to those stresses. We believe that proposing the continued
exclusion from peak dose calculations of events that are
inconsequential for 10,000 years, with the exception of general
corrosion and those identified by NAS, is consistent with this
approach.
To summarize our proposal for Sec. 197.36, we propose that DOE
continue to use the FEPs selected for compliance with the 10,000-year
projections in its projections for peak dose. This does not require
that DOE continue to define the characteristics of those FEPs in
exactly the same way it has previously (for example, in the FEIS).
Rather, DOE may continue to refine its representation of FEPs in the
analyses as its understanding of the factors involved improves. The
contribution to dose estimates of FEPs selected for the analyses must
be assessed throughout the period of geologic stability. We do require
that DOE explicitly consider the effects of seismic, igneous, and
climate change scenarios, within the overall probability constraints,
as described in more detail in the following sections. We also require
that DOE consider the effects of general corrosion throughout the
period of geologic stability. We have considered two approaches for
doing so. Under the first approach, consistent with our approach to
climate change outlined in Section II.D.2. DOE may apply a constant
representative corrosion rate throughout the period of geologic
stability. Under the second approach, consistent with our approach to
seismic and igneous FEPs outlined in Sections II.D.2.b and c, DOE may
apply corrosion rates as derived for the 10,000-year period, which may
be dependent on other factors, such as temperature within the
repository.
We have stated our concerns that the screening process should not
be used to put forward highly speculative and implausible situations
for DOE to analyze. It is our belief that the relevant FEPs are already
captured within the 10,000-year screening process, and that any others
would be overshadowed by other aspects of the longer-term modeling. We
believe our proposal to explicitly include certain FEPs important to
the longer-term projections appropriately balances these
considerations. We request comment on this approach.
b. Consideration of Seismic FEPs
The NAS stated, and we agree, that the effects of seismicity in the
area on (1) the repository and (2) the hydrologic regime are key
aspects to be considered during the period of geologic stability (NAS
Report p. 93). The effects of seismicity may result in (most
significantly) early waste package failure, an increase or decrease in
conductivity (movement of water) in the saturated or vadose zones, or a
shift in direction of fluid movement in the area (NAS Report pp. 92-
93). In addition, we believe the potential effects of seismic activity
on the structural stability of the repository itself (i.e., drift
collapse) may be important in projecting the failure of waste packages.
In order to reasonably assess the effects of seismicity at the
site, and yet also address the increasing uncertainty associated with
magnitudes of seismic events over the greatly increased time period, we
expect that DOE will take the rate of occurrence of seismic events
originally derived for the 10,000-year time period and extend the
calculations throughout the period of geologic stability. We are
proposing that DOE may limit its assessment of seismicity to the
effects on the disposal system of drift collapse and waste package
failure, i.e., effects on the engineered barriers that comprise an
essential component of the disposal system. At times sufficiently far
into the future, a wide range of possibilities could be proposed, and
some (for example, an earthquake of such an extreme magnitude that it
collapses all the drifts of the repository, allowing for complete
destruction of the facility), no matter how remote the probability,
could have far-reaching implications for the disposal system. By using
this approach, we can adhere to the basic premise that the risk
calculations reasonably predict the geologic environment at the
repository out to peak dose. We can also capture the potential effects
of seismicity and faulting at Yucca Mountain. By extending the
performance period to 1 million years, it is expected that more events
will occur, consistent with the established seismic hazard curve for
the site. No new types or classes of seismic or fault displacement
disruptive events can reasonably be anticipated. In the case of
seismicity, earthquakes are most likely to occur on the existing
network of active seismogenic fault sources under current tectonic
conditions. In the case of the fault displacement hazard, it is more
likely that fault slip will occur on existing faults that on newly
created ones.
DOE has developed a seismic hazard curve that describes the
seismicity to be expected at the site (``Seismic Consequence
Abstraction,'' MDL-WIS-PA-00003-Rev 00, 2003, Docket No. OAR-2005-0083-
0073). A seismic hazard curve determines what the probability is of any
particular strength of ground shaking. The goal of probabilistic
seismic hazard analysis is to quantify the rate (or probability) of
exceeding various ground-motion levels at a site (or a map of sites),
given all possible earthquakes. It is reasonable to assume that seismic
events will continue with activity rates and magnitudes predicted by
the seismic and fault displacement hazards for the site over the period
of geologic stability because the geologic record indicates relative
tectonic stability of the region over the past 10 million years. This
implies that there is continuity in the behavior of major geologic
events (such as earthquakes) over that entire time frame. Further, the
geologic record extending back millions of years has been used to
establish the hazard curves. There is not further data that
appropriately can be incorporated into the analysis, or used to justify
an adjustment of the estimates simply because they are to be projected
further into the future. It is expected that more events, such as
earthquakes and fault displacements, will occur with the extended
performance period, but that these events are much more likely to occur
on existing faults and seismic sources than on newly created ones.
Therefore, the rates and magnitudes considered in the probabilistic
calculations for 10,000 years can also be used to generate estimates of
seismicity out to the period of peak dose. These events should be
defined on an annual probability of occurrence. The magnitudes and
frequencies of potential seismic events should remain the same as in
the 10,000-year analysis; however, the analysis would be expected to
show greater consequences as potentially more major seismic events are
incorporated into the assessment as a result of extending the analysis
throughout the period of geologic stability as events occur at times
when packages are expected to be largely degraded and thus more easily
damaged.
The NAS stated that seismologic effects on the hydrology at Yucca
Mountain can also be bounded over the period of stability due to the
fact that the hydrology has been influenced by many similar seismic
events in the past (NAS Report p. 93). Seismic activity can account for
a number of changes in the
[[Page 49056]]
hydrology of the area, from the opening or closing of fractures and
large-scale changes in water levels to a shift in the direction of
ground-water flow in the region. It could also increase the potential
for enhanced movement of the radionuclides in the waste, because the
potential for increased rate of water movement could contribute to a
greater velocity of the ground water in the aquifer, which could reduce
the travel time of radionuclides out to the boundary of the controlled
area. However, we are proposing today that DOE's analysis for seismic
events may exclude the effects of seismicity on the hydrology of the
Yucca Mountain disposal system. In making this decision, we considered
the NAS's guidance as well as the relative effects of climate change on
the hydrology of the disposal system.
In its report, NAS observed that seismicity potentially can affect
the hydrologic regime by causing displacements and increasing
conductivity along existing fractures. NAS noted that such
displacements are likely to occur along existing fractures (as opposed
to creating new ones) and, further, that hydrology near Yucca Mountain
``has been conditioned by many similar seismic events over geologic
time'' (NAS Report p. 93). Since no major new flow paths would be
created, these statements imply that the most likely hydrologic effects
are changes in conductivity or a localized shift in the ground-water
flow. Nevertheless, NAS concluded that ``such displacements have an
equal probability of favorably changing the hydrologic regime'' (NAS
Report p. 93). We agree, and also conclude that predicting the
magnitude of changes in hydraulic conductivity--whether favorable or
unfavorable--or the details of localized changes in the direction of
ground-water flow is highly speculative, especially in view of the
highly fractured nature of the geology at Yucca Mountain.
However, we also agree with NAS that ``the effect of seismicity on
the hydrologic regime could probably be bounded'' (NAS Report p. 93).
The endpoint of most concern resulting from changes in flowpaths or
hydraulic conductivity would be the potential for greater movement of
water through the disposal system. As previously mentioned, this could
enhance movement of radionuclides from the waste. Importantly, this is
also the endpoint of concern for climate change scenarios. As discussed
in more detail in Section II.D.2.d, we are proposing that DOE must
consider climate change scenarios that result in an increased flow of
water through the disposal system. Unlike seismic events, such climate
change scenarios do not have the potential to favorably affect (i.e.,
reduce) the ground-water flow through the disposal system (at best,
they would have a neutral effect on overall performance). In addition,
the effects on water flow from climate change would be expected to
exceed any such effects resulting from seismicity. Thus, we believe
that our proposed requirements for DOE to consider climate change over
the period of geologic stability effectively bound the potential
hydrologic effects and no further analysis is required separately as
part of the seismic scenarios.
In contrast, the potential effects on waste package failure through
physical impact with other elements of the engineered barrier system or
drift collapse (rockfall) are not clearly captured in analyses of other
scenarios. Waste package failure is generally of importance because it
is the immediate step allowing water to contact the waste, leading to
release of radionuclides. Waste packages may be more vulnerable to
seismic effects if corrosion processes have weakened them. Seismic
events may cause the failure of the structures supporting the waste
packages, allowing them to be physically damaged through impacts with
other objects (i.e., if waste packages are no longer held in place,
they could collide with other packages or elements of the engineered
barrier system). The collapse of the emplacement drift itself could
also be significant at these longer times as pieces of rock fall onto
the already-weakened waste packages. Regarding waste package failure
caused by seismicity, NAS concluded that the rocks in the Yucca
Mountain area are so extensively fractured that future seismic events
are likely to occur along existing fractures rather than new ones (NAS
Report p. 93). By knowing the location of major fractures, DOE may be
able to minimize the adverse effects of seismicity. For example, DOE
can place waste packages away from these areas (fault avoidance),
thereby decreasing the risk of failure by seismic induced rock falls.
As can be seen by examples at the Waste Isolation Pilot Plant (WIPP),
engineering practices at repositories can be successful in reducing the
probability of adverse effects on isolation capabilities and DOE has
criteria for such practices at Yucca Mountain. Because faults are being
avoided by design, we do not believe DOE must assume they are not. In
the end, DOE might be able to show that seismic effects on waste
package failure ``could be reduced sufficiently to result in boundable
and probably very low risk,'' as postulated by NAS (NAS Report p. 93).
Our proposal would require that DOE specifically address waste package
failure resulting from seismic events causing damage to the engineered
barrier system, either through physical impacts within the drifts
through failure of the supporting structures or drift collapse so that
the significant effects identified by NAS will be fully considered.
There are other effects that can be envisioned from seismic events
near Yucca Mountain. Beyond the key aspects of seismicity discussed
above, however, we do not believe there are others that would be
expected to significantly affect performance (for example, from events
that are of low magnitude or sufficiently distant from the disposal
system), and NAS similarly identified none. The consideration of such
effects would unnecessarily complicate the development of the
performance assessment and the licensing process without contributing
information on the protective capabilities of the Yucca Mountain
disposal system. We believe they can reasonably be excluded from
analysis over the period of geologic stability.
Therefore, in conclusion, we propose that DOE evaluate the effects
of seismic activity throughout the period of geologic stability, but
limit those effects to those resulting in damage to the engineered
barrier system and ultimately the waste packages. The probability of
seismic events of different magnitude and duration for the period of
geologic stability will be the same as determined for the period within
10,000 years. We request comment on this approach.
c. Consideration of Igneous (Volcanic) FEPs
EPA recognizes that a volcanic intrusion into the repository,
although an unlikely event, could release a portion of the radioactive
inventory. We agree with the NAS that this possibility exists over the
period of geologic stability (NAS Report p. 94). While acknowledging
the complexity of the release of radionuclides from the repository,
given the known effects of the various types of past volcanic events
and the study of the cinder cones in the area, we believe it is
possible to develop reasonable estimates of the probability of
radionuclide release via volcanic episodes through the repository
through the period of geologic stability.
We agree with NAS that the probability of igneous events may be
great enough, and the potential
[[Page 49057]]
consequences significant enough, that they must be considered over the
period of geologic stability. An analysis of the probability is based
on extrapolations into the future of volcanic activity from the
geologic record, and on assumptions about the spatial distribution of
future volcanic eruptions in the Yucca Mountain region. Volcanism by
nature is an episodic event. In the Yucca Mountain region it has been
characterized as involving intermittent concentrated activity followed
by long periods of quiescence (NAS Report p. 94). For example, the
repository block tuffs are in the age range of approximately 11-12
million years old and were generated by large-scale volcanism involving
a large area around the site (``Site Environmental Report for the Yucca
Mountain Project Calendar Year 2003,'' PGM-MGR-EC-000005-REV 00,
Section 1.1, October 2004, Docket No. OAR-2005-0083-0086). This
material is made of layers of ashfalls from volcanic eruptions that
consolidated into the rock (of a type known as ``tuff''). Tuff has
varying degrees of compaction and fracturing depending on the degree of
``welding'' caused by temperature and pressure when the ash was
deposited. An event of this nature is not likely to be repeated during
the geologic stability period. It has been suggested by NAS, and fits
within our FEPs screening, that a probability of 10-8/yr,
which is a 1 in 10,000 possibility of a disruption (affecting the
repository, not simply a volcanic event in the region) over 10,000
years ``might be sufficiently low to constitute a negligible risk''
(NAS Report p. 95). Based on available information generated by DOE in
its TSPA (Yucca Mountain Science and Engineering Report, DOE/RW-0539,
Section 4.4.3, May 2001, Docket No. OAR-2005-0083-0069), the mean
annual probability of an igneous event within the Yucca Mountain
repository footprint is estimated at 1.6 x 10-8 per year
(which is slightly higher than a one in 10,000 possibility of a
disruption over 10,000 years). This probability, though extremely low,
is just within the regulatory threshold for inclusion of events with
very low probability of occurrence, but it can be assumed that this
probability will hold throughout the period of geologic stability (NAS
Report p. 94). For this reason, we are proposing to require that DOE
include consideration of igneous FEPs extending over the period of
geologic stability.
We also agree with NAS that reasonable estimates of the effects can
be developed (NAS Report p. 95). As with the seismic FEPs, we believe
this is best accomplished by limiting the analysis to those effects
most significant for performance. As we stated in our 2001 rule, the
geologic record is the best source of evidence for the frequency and
magnitude of natural features, events, and processes that could affect
repository performance, and the geologic record is best preserved in
the relatively recent past (66 FR 32100). Studies of the volcanic
history of the area in the recent past indicate a different type of
volcanic activity other than the intermittent layering volcanic
activity that produced Yucca Mountain has occurred (FEIS, DOE/EIS-0250,
Appendix I, Section 2.10, Docket No. OAR-2005-0083-0086). Basalt
volcanism, exemplified by the Lathrop Wells volcano, and other features
near the repository, appears to be the type of igneous activity, though
unlikely, that has some probability of occurring within the period of
geologic stability. By narrowing the type of events most plausible
during the period of stability, we can attempt to constrain the
uncertainty involved in using probabilistic analyses. The NAS noted
that the most significant effects are related to future events that
could intersect the repository (NAS Report p. 94).
Existing DOE calculations provide an example of analysis of such
disruptive igneous events. DOE states that, if igneous activity
occurred at Yucca Mountain, possible effects on the repository could be
grouped into three areas (FEIS, DOE/EIS-0250, Appendix I, Section 2.10,
Volcanism, Docket No. OAR-2005-0083-0086):
Igneous activity that would not directly intersect the
repository (can be shown to have no effect on dose from the
repository);
Volcanic eruptions in the repository that would result in
waste material being entrained in the volcanic magma or pyroclastic
material, bringing waste to the surface (resulting in atmospheric
transport of volcanic ash contaminated with radionuclides and
subsequent human exposure downwind); or
An igneous intrusion intersecting the repository (no
eruption but damage to waste packages from exposure to the igneous
material that would enhance release to the ground water and, thus,
enhance transport to the biosphere).
Based on studies of past activity in the region, probabilities for
different types of igneous activity have been estimated by DOE. Each
type of event was described in detail based on observation of effects
of past activities as embodied in the geologic record of the region.
These descriptions include geometry of intrusions, geometry of
eruptions, physical and chemical properties of volcanic materials,
eruption properties (velocity, power, duration, volume, and particle
characteristics). Most of the parameters describing the igneous
activity were entered in the modeling as probability distributions
(FEIS, DOE/EIS-0250, Appendix I, Section 2.10, Volcanism, Docket No.
OAR-2005-0083-0086).
DOE's current igneous activity scenario contains two separate
possible events: a volcanic eruption that includes exposure as a result
of atmospheric transport and deposition on the ground, and an igneous
intrusion ground-water transport event. In the volcanic eruption event,
a dike (or dikes) would intersect the repository and compromise all
waste packages in the conduit. Then, an eruptive conduit of an
associated volcano would intersect waste packages in its path. Waste
packages in the path of the conduit would be sufficiently damaged that
they provide no further protection, and the waste in the packages would
be entrained in the eruption and subject to atmospheric transport. In
the igneous intrusion ground-water transport event, the analysis
calculated releases caused by a dike (or dikes) intersecting
emplacement drifts, causing varying degrees of waste-package damage and
making the contents of the containers available for transport to the
RMEI through ground water. We believe these are the most significant
consequences that would result from a volcanic event through the
repository. Other results from igneous events--the occurrence of
distant events, potential drift instability, or changes in rock
fracturing--are secondary to the direct releases of radionuclides. In
addition, the response of the disposal system to such effects would
likely be captured by consideration of other FEPs (such as seismicity
or climate change). Therefore, we are proposing that DOE's
consideration of igneous events over the period of geologic stability
may be limited to events that intersect the repository, damage the
waste packages, and cause releases of radionuclides either directly to
the atmosphere and biosphere (i.e., an extrusive event) or to the
ground water. We expect that the same probability of occurrence for
these events used in the 10,000-year analysis be applied over the
period of geologic stability. Using this probability, it is very
unlikely that more than one igneous event would be included in a single
realization. However, the two types of events are very different in
terms of their potential effects and when those effects would be
greatest. We
[[Page 49058]]
believe this approach is appropriate, as described in the next
paragraph.
DOE's analysis of releases from waste packages entrained by magma
erupted on the surface assume the waste containers are breached by the
eruption itself and the wastes are available for dispersal by the
eruption. In this scenario, the doses would be highest if the eruption
happened early in the geologic stability period (before significant
decay of short-lived radionuclides that provide a dose through
inhalation as well as through deposition and uptake by plants), and are
lower if the event occurs at later times. Assuming waste packages are
breached during the event provides that the assessment is a ``worst
case'' in terms of potential doses because it does not depend on
assumptions regarding other waste package failure mechanisms, such as
corrosion. However, other analyses and laboratory experiments have been
presented suggesting that intact waste containers can withstand the
temperatures of the molten magma without melting or otherwise
sustaining significant damage (``Evaluation of the Igneous Extrusive
Scenario,'' Presentation to the Nuclear Waste Technical Review Board,
September 20, 2004, Docket No. OAR-2005-0083-0074). These analyses
suggest that an early eruption might not produce the highest doses
since the wastes could not be dispersed as easily. Under these
assumptions, an eruption considerably later in the geologic stability
period, when the waste containers have degraded considerably from
corrosion processes, is more likely to result in widespread dispersal
of the wastes. However, at the later times, the radionuclide inventory
in the wastes would have decreased from decay, and projected releases
would probably not exceed those estimated for the early eruption
scenario DOE performed. The existing assessments of the eruptive event
based on our previously issued regulations contain a number of
assumptions, which we believe has led to conservative assessments.
Under DOE's assumptions, the highest dose as a result of volcanic
eruptions would occur within the first 10,000 years because that is
when the radionuclide inventory is at its highest. We are not assuming
this approach will be retained in all details, and have structured our
proposed rule accordingly to ensure that igneous events are considered
over the period of geologic stability. However, we acknowledge that the
current approach, if retained, would meet our requirements and be
conservative. We request comment on our proposal.
d. Consideration of Climatological FEPs
The average of weather conditions over a long period of time is the
climate (http://www.cogsci.princeton.edu/cgi-bin/webwn), and it has been well
documented that climate can vary significantly over geologic time (NAS
Report p. 91). Climate controls the range of precipitation and
temperature conditions at Yucca Mountain. There are a number of
impacts, particularly on the hydrologic regime, that must be taken into
account. Run-on, run-off, and evapotranspiration of precipitation
influence the rate of infiltration into the subsurface. The greater the
amount of infiltration, or recharge, the greater the potential for an
increase in ground water to infiltrate into the repository, allowing
for an increase in the dissolution of the radionuclides. This could
lead to higher release rates from the waste. Consequently, it is
important to examine the effects of climate change throughout the
period of geologic stability.
At present the Earth is in an interglacial phase (NAS Report p.
91). Climate change historically has been cyclical: ``Over a million-
year time scale, however, the global climate regime is virtually
certain to pass through several glacial-interglacial cycles * * *''
(NAS Report p. 91). Similarly, the Yucca Mountain FEIS states: ``The
record shows continual variation, often with very rapid jumps, between
cold glacial climates (* * * pluvial periods) and warm interglacial
climates similar to the present. Fluctuations average 100,000 years in
length'' (FEIS, DOE/EIS-0250, p. 5-12, Docket No. OAR-2005-0083-0086).
NAS stated the following with regard to climate change at Yucca
Mountain:
During the past 150,000 years, the climate has fluctuated
between glacial and interglacial status. Although the range of
climatic conditions has been wide, paleoclimatic research shows that
the bounding conditions, the envelope encompassing the total
climatic range have been fairly stable (Jannik et al., 1991;
Winograd et al., 1992; Dansgaard et al., 1993). Recent research has
indicated that the past 10,000 years are probably the only sustained
period of stable climate in the past 80,000 years (Dansgaard et al.,
1993). Based on this record, it seems plausible that the climate
will fluctuate between glacial and interglacial states during the
period suggested for the performance assessment calculations. Thus,
the specified upper boundary, or the physical top boundary of the
modeled system, would be a conservative approach that captures the
most severe, detrimental performance effects of these variations
(especially in terms of ground-water recharge).
(NAS Report pp. 77-78.)
We are concerned about the possibility of over-speculation of
climatic change over such extremely long time periods, possibly out to
the next 1 million years. The NAS recognized this fact in its report,
stating ``Although the typical nature of past climate changes is well
known, it is obviously impossible to predict in detail either the
nature or the timing of future climate change. This fact adds to the
uncertainty of the model predictions' (NAS Report p. 77).
EPA agrees with the NAS statement and takes the position that it is
not useful to have unconstrained speculation on future climate during
the period of geologic stability, because it is possible to assume any
number of scenarios of climate over this large amount of time, and
there is very little evidence available to accept or refute most of
them. Because it is not possible to predict every situation that could
occur over such a long time, we feel that the best course, as outlined
below, is to construct a climate scenario that assumes reasonable
temperature and precipitation values, and allow this scenario to run
throughout the period of geologic stability.
Climate change differs from seismic and igneous events in that its
effects would not occur instantaneously, and it can affect multiple
portions of the disposal system with a very direct effect on
performance since the movement of water through the site is the primary
means for transporting radionuclides. These effects can persist for
very long time periods, even longer than the period of geologic
stability. Seismic events and volcanism, in contrast, are episodic
events; though the events occur relatively quickly and deliver their
consequences over the short term, the consequences themselves can be
very long-lasting and fundamentally change the geologic setting.
There are three major effects that climate change can impart on the
disposal system (NAS Report p. 91). The first is that increases in
erosion might significantly decrease the burial depth of the
repository. NAS pointed out that site-specific studies performed by DOE
indicate that an increase in erosion to the extent necessary to expose
the repository within the period of geologic stability is extremely
unlikely (NAS Report p. 91). Therefore, we do not believe it is
important or necessary to require DOE to assess the potential for
erosion from climate change.
The second change might be a shift in the distribution and
activities of human populations (NAS Report p. 92). A cooler, wetter
climate may provide a more hospitable environment,
[[Page 49059]]
increasing the population, and (some have argued) possibly changing the
parameters we have outlined for the RMEI. We are not proposing to
change the definition or characteristics of the RMEI. We have discussed
our reasoning for taking this approach in greater detail in Section
II.A.1 of this document. We do not believe that fixing the climate to
present-day characteristics is the appropriate way to circumvent the
difficulties in defining a biosphere applicable for 1 million years.
Our view is that evaluation of reasonable climate change is critical to
the integrity and meaning of peak dose projections. Further, as NAS
noted, ``there is no simple relation between future climatic conditions
and future population'' (NAS Report p. 92).
Finally, for extremely long time periods, major changes in the
global climate, for example a transition to a glacial climate, could
affect ground-water movement. NAS states ``Change to a cooler, wetter
climate at Yucca Mountain would likely result in greater fluxes of
water through the unsaturated zone'' (NAS Report pp. 91-92). NAS
observed that a doubling of the effective wetness (the ratio of
precipitation to effective evapotranspiration) could cause a
significant increase in recharge (NAS Report p. 91). This could affect
the rates of radionuclide release from the waste and transport to the
water table, although the location of the repository in the subsurface
would provide a time lag for climate change effects. NAS states, ``The
time required for unsaturated zone flux changes to propagate down to
the repository and then to the water table is probably in the range of
hundreds to thousands of years. The time required for saturated flow-
system responses is probably even longer. For this reason, climate
changes on the time scale of hundreds of years would probably have
little if any effect on repository performance, and the effects of
climate changes on the deep hydrogeology can be assessed over much
longer time scales'' (NAS Report p. 92).
In its current analysis of future climate states (``Future Climate
Analysis,'' ANL-NBS-GS-000008-Rev 00, 2000, Section 6.2, Docket No.
OAR-2005-0083-0068), DOE assumed that all future climates were similar
to current conditions or wetter than current conditions. The climate
model provides a forecast of future climates based on information about
past patterns of climates. The model represents future climate shifts
as a series of instant changes. During the first 10,000 years, there
are three changes, in order of increasing wetness, from present-day to
a monsoon and then to a glacial-transition climate. Between 10,000
years and 1 million years there are 45 changes between six climate
states incorporated in the TSPA model:
Interglacial Climate (same as present day)
Intermediate Climate (same as the Glacial-transition)
Intermediate/Monsoon Climate
Three stages of Glacial Climate of varying infiltration
rates
Precipitation that is not returned to the atmosphere by evaporation
or transpiration enters the unsaturated zone flow system. Water
infiltration is affected by a number of factors related to climate,
such as an increase or decrease in vegetation on the ground surface,
total precipitation, air temperature, and runoff. The infiltration
model uses data collected from studies of surface infiltration in the
Yucca Mountain region. It treats infiltration as variable in the
region, with more occurring along the crest of Yucca Mountain than
along its base. The results of the climate model affect assumed
infiltration rates. For each climate, there is a set of three
infiltration rates (high, medium, low) and associated probabilities.
This forms a discrete distribution that is sampled in the probabilistic
modeling. Whenever a particular climate state is in effect, the
associated infiltration rate distribution is sampled for each
realization of the simulation.
One of the issues associated with DOE's existing modeling efforts
on climate at very long times is that the analysis assumed
instantaneous changes between climate states. In other words, the
entire flow field was assumed to immediately switch from one climate
state to another. This approach is unrealistic because, as noted above,
it would likely take hundreds or thousands of years for increased
infiltration from a wetter climate to reach the underlying aquifer and
affect transport and flow patterns. DOE also assumed that the climate
change occurred at the same time for all realizations, which magnified
the effect of the instantaneous change of climate when looked at as a
probabilistic analysis. The result is that the doses calculated were
the product of the conservatism of the assumptions noted above (e.g.,
instantaneous climate shift, which was assumed to occur at the same
time for all realizations). Such assumptions are unlikely to produce
meaningful or realistic results.
We believe that an approach should be developed to answer several
basic questions about how climatological effects realistically will
impact the proposed repository until the time to peak dose. The
questions that concern us are:
1. How much total water will infiltrate into the repository over
this large amount of time?
2. Will more water infiltrate the repository over time when modeled
as a wave function (current DOE modeling) or as total average?
The answers to these questions assist in identifying conservative,
yet reasonable, conditions the repository may encounter over the period
of geologic stability. The amount of net infiltration into Yucca
Mountain has an effect on the disposal system performance because
higher net infiltration leads to the possibility that a greater
proportion of the repository will experience ground-water seepage. For
solubility-limited radionuclides in the waste, an increase in net
infiltration could lead to a higher release rate of radionuclides from
the disposal system, thereby affecting the potential dose to the RMEI
in the accessible environment. We do not believe that it is important
to know or predict with certainty precisely when the climate states
with peak precipitation occur during the modeling. There are too many
uncertainties and permutations available in trying to project a future
set of climate conditions, and it is difficult to place specific times
on when discrete pulses of precipitation should be injected into the
modeling (NAS Report p. 77). Instead, we believe that it is reasonable
to assume an average increase in precipitation over the entire time
from 10,000 years through the period of geologic stability, and to
model those consequences. An increase in average precipitation
throughout the period of geologic stability is a more reasonable
approach because it assumes a constant source of precipitation,
creating more downward flow that will eventually reach the repository.
This scenario need not be dominated by highs or lows in precipitation
over the time period and does not require speculation about the exact
timing or transient effects of shifts in climate. Rather, setting a
constant value somewhat higher than today's average annual rainfall and
extending it out to the time of peak dose would account for the greater
potential for available fluids at the time of the failure of the waste
packages. We believe that this approach provides a reasonable test of
the repository conditions out to the time of peak dose, and will give a
more conservative idea of potential fluid flow, as well as potential
for migration of radionuclides out of the repository.
[[Page 49060]]
We are proposing today that DOE, based on past climate conditions
in the Yucca Mountain area, should determine how the disposal system
responds to the effects of increased water flow through the repository
as a result of climate change. We believe that the nature and extent of
climate change can be reasonably represented by constant conditions
taking effect after 10,000 years out to the time of geologic stability.
We are proposing to explicitly require that DOE assume water flow will
increase as a result of climate change. We leave it to NRC as the
licensing authority to specify the values to be used to represent
climate change. However, we expect that a doubling of today's average
annual precipitation beginning at 10,000 years and continuing through
the period of geologic stability would provide a reasonable scenario,
given NAS's statements regarding potential effects on recharge (NAS
Report p. 92). NRC could also use the range of projected precipitation
values for different climate states and specify a reasonable long-term
average precipitation based on the duration of each climate state over
the period of geologic stability. We believe that either approach will
allow for a reasonable estimate of how water will impact the site
without subjecting the assessments to speculative assumptions that may
well be unresolvable, while providing a reasonable indicator of
disposal system compliance. NRC might choose to express the ground-
water flow effects directly as infiltration rates or other
representative parameters, avoiding the necessity of translating
precipitation and other climate-related parameters (e.g., temperature
or evapotranspiration rates) into infiltration.
Finally, we note that there are other potential effects of climate
change such as the formation of surficial ponds or changes in fauna and
flora (which could affect infiltration through changes in
evapotranspiration rates). NAS did not identify these as significant,
and also reiterated that speculation on the evolution of the biosphere
(aside from climate) is unwarranted and unproductive. We agree fully.
Therefore, in summary, we are proposing that DOE must include
consideration of climate change in its performance assessment for
compliance with the dose standard for the period of geologic stability.
The assessment may be limited to the effects of increased water flow
through the repository as a result of climate change. Climate change
may be represented by constant conditions, which NRC would specify in
regulation. We request comment on this proposal.
E. How Is EPA Proposing To Revise the Human-Intrusion Standard (Sec.
197.25) To Address Peak Dose?
As discussed in Section II.A.2, we believe it is logical and
defensible to modify the human-intrusion standard in Sec. 197.25 to
parallel the revisions we are proposing for the individual-protection
standard. We described in some detail in that section the reasons why
we believe that course of action to be appropriate, and briefly
summarize our proposal here. Like the individual-protection standard,
our provisions for human intrusion in the 2001 rule envisioned some
consideration of performance beyond 10,000 years. The exposures
resulting from the event were subject to the same compliance standard
as the individual-protection standard (15 mrem/yr at 10,000 years or
earlier coupled with compilation in the EIS if doses were projected to
occur after 10,000 years). In deciding to propose revisions to the
human-intrusion standard to conform to changes we are proposing to make
to the individual-protection provisions, we kept in mind the NAS
recommendation that ``the figure-of-merit for [the human-intrusion]
calculation should be the same as in the undisturbed case * * * EPA
should require that the conditional risk as a result of the assumed
intrusion scenario should be no greater than the risk levels that would
be acceptable for the undisturbed-repository case'' (NAS Report pp.
112-113).
The 2001 standard required that DOE determine when an intrusion by
drilling would be possible and assess the consequences. We believe it
is still appropriate for DOE to determine the time at which the
intrusion could occur. However, under our proposal today, consequences
at any time within the period of geologic stability would be subject to
a compliance demonstration. We are proposing to apply the same dose
limits to the human-intrusion scenario as we are proposing for the
individual-protection scenario. Thus, exposures incurred by the RMEI
within 10,000 years after disposal as a result of the intrusion must
comply with a standard of 150 [mu]Sv/yr (15 mrem/yr). Exposures after
that time within the period of geologic stability must comply with a
standard of 3.5 mSv/yr (350 mrem/yr). DOE must still use the same
assumptions regarding the RMEI as it used for the individual-protection
analysis.
We are not proposing to modify in any way the circumstances of the
intrusion described in Sec. 197.26. We believe those circumstances
continue to reflect two key points emphasized by NAS. First, ``there is
no scientific basis for estimating the probability of intrusion at far-
future times'' (NAS Report p. 106). Second, like future society, future
exploration technology cannot be predicted (NAS Report p. 107).
Therefore, there is no basis for assuming a different set of
circumstances to apply to intrusions beyond 10,000 years.
We request comment on our proposed changes to the human-intrusion
standard. We are not soliciting, and will not consider, comments on the
overall intrusion scenario or other aspects of the human-intrusion
standard that are not proposed to be changed.
F. Summary of Today's Proposal by Section
Today's proposal is limited in scope. We are proposing to amend
provisions only as necessary to address the Court ruling. Because of
the unique nature of the challenge facing us, in which we must craft a
regulatory standard to apply to times up to 1 million years, we have
chosen to discuss many aspects of our 2001 rule in this document. We
have done so because we believe it important that the public clearly
understand what actions we are proposing to take and why, as well as
reasons for not amending other provisions. In the listing that follows,
we identify only those provisions of the rule that we are proposing to
change today. We request public comment only on these proposed
amendments. We are not proposing to change any other provisions.
Therefore, we are not requesting, and will not respond to, public
comments related to those provisions, since they have been previously
established in rulemaking and are outside the scope of today's
proposal.
Subpart A--Public Health and Environmental Standards for Storage
Sec. 197.2, What definitions apply in subpart A?--Amends the
definition of Effective Dose Equivalent to specify that calculations be
performed using organ weighting factors in Appendix A.
Subpart B--Public Health and Environmental Standards for Disposal
Sec. 197.12, What definitions apply in subpart B?--Modifies the
definition of Performance Assessment to remove reference to 10,000
years. Modifies the definition of Period of Geologic Stability as
ending 1 million years after disposal.
Sec. 197.13, How is subpart B implemented?--Specifies that the
arithmetic mean of the distribution of projected doses is used to
determine
[[Page 49061]]
compliance within 10,000 years. Specifies that the median of the
distribution of projected doses is used to determine compliance beyond
10,000 years but within the period of geologic stability (for
Sec. Sec. 197.20 and 197.25 only).
Sec. 197.15, How must DOE take into account the changes that will
occur during the next 10,000 years after disposal?--Replaces references
to 10,000 years with ``period of geologic stability.''
Sec. 197.20, What [individual-protection] standard must DOE
meet?--Retains the standard of 15 mrem/yr to apply up to 10,000 years
after disposal. Adds a standard of 350 mrem/yr to apply beyond 10,000
years within the period of geologic stability.
Sec. 197.25, What [human-intrusion] standard must DOE meet?--
Retains the standard of 15 mrem/yr to apply up to 10,000 years after
disposal. Adds a standard of 350 mrem/yr to apply beyond 10,000 years
within the period of geologic stability. Removes references to time of
intrusion and to placement of results in EIS.
Sec. 197.35, What other projections must DOE make?--Section to be
deleted.
Sec. 197.36, Are there limits on what DOE must consider in the
performance assessments?--Addresses probability of features, events,
and processes in assessments used to comply with proposed Sec.
197.20(b). Adds provisions to address climate change, igneous, seismic,
and general corrosion scenarios.
Appendix A, Calculation of Committed Effective Dose Equivalent--
describes the method to calculate the dose for comparison with the
appropriate standards.
III. Statutory and Executive Order Reviews
A. Executive Order 12866: Regulatory Planning and Review
Under Executive Order 12866, [58 Federal Register 51735 (October 4,
1993)] the Agency must determine whether the regulatory action is
``significant'' and therefore subject to OMB review and the
requirements of the Executive Order. The Order defines ``significant
regulatory action'' as one that is likely to result in a rule that may:
(1) Have an annual effect on the economy of $100 million or more or
adversely affect in a material way the economy, a sector of the
economy, productivity, competition, jobs, the environment, public
health or safety, or State, local, or tribal governments or
communities;
(2) Create a serious inconsistency or otherwise interfere with an
action taken or planned by another agency;
(3) Materially alter the budgetary impact of entitlements, grants,
user fees, or loan programs or the rights and obligations of recipients
thereof; or
(4) Raise novel legal or policy issues arising out of legal
mandates, the President's priorities, or the principles set forth in
the Executive Order.
Pursuant to the terms of Executive Order 12866, it has been
determined that this rule is a ``significant regulatory action''
because it raises novel legal or policy issues arising out of the
specific legal mandate of Section 801 of the Energy Policy Act of 1992.
As such, this action was submitted to OMB for review. Changes made in
response to OMB suggestions or recommendations will be documented in
the public record.
B. Paperwork Reduction Act
This action does not impose an information collection burden under
the provisions of the Paperwork Reduction Act, 44 U.S.C. 3501 et seq.
We have determined that this rule contains no information collection
requirements within the scope of the Paperwork Reduction Act.
Burden means the total time, effort, or financial resources
expended by persons to generate, maintain, retain, or disclose or
provide information to or for a Federal agency. This includes the time
needed to review instructions; develop, acquire, install, and utilize
technology and systems for the purposes of collecting, validating, and
verifying information, processing and maintaining information, and
disclosing and providing information; adjust the existing ways to
comply with any previously applicable instructions and requirements;
train personnel to be able to respond to a collection of information;
search data sources; complete and review the collection of information;
and transmit or otherwise disclose the information.
An agency may not conduct or sponsor, and a person is not required
to respond to a collection of information unless it displays a
currently valid OMB control number. The OMB control numbers for EPA's
regulations in 40 CFR are listed in 40 CFR part 9.
C. Regulatory Flexibility Act
The Regulatory Flexibility Act (RFA) generally requires an agency
to prepare a regulatory flexibility analysis of any rule subject to
notice and comment rulemaking requirements under the Administrative
Procedure Act or any other statute unless the agency certifies that the
rule will not have a significant economic impact on a substantial
number of small entities. Small entities include small businesses,
small organizations, and small governmental jurisdictions.
For purposes of assessing the impacts of today's rule on small
entities, small entity is defined as: (1) A small business as defined
by the Small Business Administration's (SBA) regulations at 13 CFR
121.201; (2) a small governmental jurisdiction that is a government of
a city, county, town, school district or special district with a
population of less than 50,000; and (3) a small organization that is
any not-for-profit enterprise which is independently owned and operated
and is not dominant in its field.
However, the requirement to prepare a regulatory flexibility
analysis does not apply if the Administrator certifies that the rule
will not, if promulgated, have a significant economic impact upon a
substantial number of small entities (5 U.S.C. 605(b)). The rule
proposed today would establish requirements that apply only to DOE.
Therefore, it does not apply to small entities. Accordingly, I hereby
certify that the rule, when promulgated, will not have a significant
economic impact upon a substantial number of small entities. We
continue to be interested in the potential impacts of our proposed
rules on small entities and welcome comments on issues related to such
impacts.
D. Unfunded Mandates Reform Act
Title II of the Unfunded Mandates Reform Act of 1995 (UMRA), Pub.
L. 104-4, establishes requirements for Federal agencies to assess the
effects of their regulatory actions on State, local, and tribal
governments and the private sector. Under section 202 of the UMRA, EPA
generally must prepare a written statement, including a cost-benefit
analysis, for proposed and final rules with ``Federal mandates'' that
may result in expenditures to State, local, and tribal governments, in
the aggregate, or to the private sector, of $100 million or more in any
one year. Before promulgating an EPA rule for which a written statement
is needed, section 205 of the UMRA generally requires EPA to identify
and consider a reasonable number of regulatory alternatives and adopt
the least costly, most cost-effective or least burdensome alternative
that achieves the objectives of the rule. The provisions of section 205
do not apply when they are inconsistent with applicable law. Moreover,
section 205 allows EPA to adopt an alternative other than the least
costly, most cost-effective or least burdensome alternative if the
Administrator publishes with the final rule an explanation why that
alternative was not adopted. Before EPA establishes
[[Page 49062]]
any regulatory requirements that may significantly or uniquely affect
small governments, including tribal governments, it must have developed
under section 203 of the UMRA a small government agency plan. The plan
must provide for notifying potentially affected small governments,
enabling officials of affected small governments to have meaningful and
timely input in the development of EPA regulatory proposals with
significant Federal intergovernmental mandates, and informing,
educating, and advising small governments on compliance with the
regulatory requirements.
Today's proposed rule contains no Federal mandates (under the
regulatory provisions of Title II of UMRA) for State, local, or tribal
governments or the private sector. The proposed rule implements
requirements specifically set forth by the Congress in section 801 of
the EnPA and proposes radiological protection standards applicable
solely and exclusively to the Department of Energy's potential storage
and disposal facility at Yucca Mountain. The rule imposes no
enforceable duty on any State, local or tribal governments or the
private sector. Thus, today's rule is not subject to the requirements
of sections 202 and 205 of UMRA.
E. Executive Order 13132: Federalism
Executive Order 13132, entitled ``Federalism'' (64 FR 43255, August
10, 1999), requires EPA to develop an accountable process to ensure
``meaningful and timely input by State and local officials in the
development of regulatory policies that have federalism implications.''
``Policies that have federalism implications'' is defined in the
Executive Order to include regulations that have ``substantial direct
effects on the States, on the relationship between the national
government and the States, or on the distribution of power and
responsibilities among the various levels of government.''
This proposed rule does not have federalism implications. It will
not have substantial direct effects on the States, on the relationship
between the national government and the States, or on the distribution
of power and responsibilities among the various levels of government,
as specified in Executive Order 13132. Thus, Executive Order 13132 does
not apply to this rule. In the spirit of Executive Order 13132, and
consistent with EPA policy to promote communications between EPA and
State and local governments, EPA specifically solicits comment on this
proposed rule from State and local officials.
F. Executive Order 13175: Consultation and Coordination With Indian
Tribal Governments
Executive Order 13175, entitled ``Consultation and Coordination
with Indian Tribal Governments'' (65 FR 67249, November 9, 2000),
requires EPA to develop an accountable process to ensure ``meaningful
and timely input by tribal officials in the development of regulatory
policies that have tribal implications.'' This proposed rule does not
have tribal implications, as specified in Executive Order 13175. The
rule proposed today would regulate only DOE on land owned by the
Federal government. The rule proposed today does not have substantial
direct effects on one or more Indian tribes, on the relationship
between the Federal Government and Indian tribes, or on the
distribution of power and responsibilities between the Federal
Government and Indian tribes. Thus, Executive Order 13175 does not
apply to this rule. EPA specifically solicits additional comment on
this proposed rule from tribal officials.
G. Executive Order 13045: Protection of Children From Environmental
Health & Safety Risks
Executive Order 13045: ``Protection of Children from Environmental
Health Risks and Safety Risks'' (62 FR 19885, April 23, 1997) applies
to any rule that: (1) Is determined to be ``economically significant''
as defined under Executive Order 12866, and (2) concerns an
environmental health or safety risk that EPA has reason to believe may
have a disproportionate effect on children. If the regulatory action
meets both criteria, the Agency must evaluate the environmental health
or safety effects of the planned rule on children, and explain why the
planned regulation is preferable to other potentially effective and
reasonably feasible alternatives considered by the Agency.
This proposed rule is not subject to Executive Order 13045 because
it is not economically significant as defined in Executive Order 12866,
and because the Agency does not have reason to believe the
environmental health risks or safety risks addressed by this action
present a disproportionate risk to children. The public is invited to
submit or identify peer-reviewed studies and data, of which EPA may not
be aware, that assessed results of early life exposure to radiation.
H. Executive Order 13211: Actions That Significantly Affect Energy
Supply, Distribution, or Use
This rule is not a ``significant energy action'' as defined in
Executive Order 13211, ``Actions Concerning Regulations That
Significantly Affect Energy Supply, Distribution, or Use'' (66 FR 28355
(May 22, 2001)) because it is not likely to have a significant adverse
effect on the supply, distribution, or use of energy. The rule proposed
today would apply only to DOE. Construction, operation, and closure of
the repository at Yucca Mountain would fulfill the Federal government's
commitment to manage the final disposition of spent nuclear fuel from
commercial power reactors. However, there is no direct link between
operation of the repository and an increased use of nuclear power.
Other economic, technical, and policy factors will influence the extent
to which nuclear energy is utilized.
I. National Technology Transfer and Advancement Act
Section 12(d) of the National Technology Transfer and Advancement
Act of 1995 (``NTTAA''), Public Law 104-113, 12(d) (15 U.S.C. 272 note)
directs EPA to use voluntary consensus standards in its regulatory
activities unless to do so would be inconsistent with applicable law or
otherwise impractical. Voluntary consensus standards are technical
standards (e.g., materials specifications, test methods, sampling
procedures, and business practices) that are developed or adopted by
voluntary consensus standards bodies. The NTTAA directs EPA to provide
Congress, through OMB, explanations when the Agency decides not to use
available and applicable voluntary consensus standards.
In our original proposal (64 FR 46976, August 27, 1999), we
requested public comment on potentially applicable voluntary consensus
standards that would be appropriate for inclusion in the Yucca Mountain
rule. We received no comments on this aspect of the rule. The closest
analogy to consensus standards for radioactive waste disposal
facilities are our regulations at 40 CFR part 191. As discussed above
in this preamble, Congress expressly prohibited the application of the
40 CFR part 191 standards to the Yucca Mountain disposal facility, and,
therefore, the standards promulgated in 2001 and today's proposed
revisions are site-specific and developed solely for application to the
Yucca Mountain disposal facility.
[[Page 49063]]
List of Subjects in 40 CFR Part 197
Environmental protection, Nuclear energy, Radiation protection,
Radionuclides, Uranium, Waste treatment and disposal, Spent nuclear
fuel, High-level radioactive waste.
Dated: August 9, 2005.
Stephen L. Johnson,
Administrator.
The Environmental Protection Agency is hereby proposing to amend
part 197 of title 40, Code of Federal Regulations, as follows:
PART 197--PUBLIC HEALTH AND ENVIRONMENTAL RADIATION PROTECTION
STANDARDS FOR YUCCA MOUNTAIN, NEVADA
1. The authority citation for part 197 continues to read as
follows:
Authority: Sec. 801, Pub. L. 102-486, 106 Stat. 2921, 42 U.S.C.
10141n.
Subpart A--Public Health and Environmental Standards for Storage
2. Section 197.2 is amended by revising the definition of
``Effective dose equivalent'' to read as follows:
Sec. 197.2 What definitions apply in subpart A?
* * * * *
Effective dose equivalent means the sum of the products of the dose
equivalent received by specified tissues following an exposure of, or
an intake of radionuclides into, specified tissues of the body,
multiplied by appropriate weighting factors. Annual committed effective
dose equivalents shall be calculated using weighting factors in
accordance with appendix A of this part.
* * * * *
Subpart B--Public Health and Environmental Standards for Disposal
3. Section 197.12 is amended by revising paragraph (1) of the
definition of ``Performance assessment'' and the definition of ``Period
of geologic stability'' to read as follows:
Sec. 197.12 What definitions apply in subpart B?
* * * * *
Performance assessment means an analysis that:
(1) Identifies the features, events, processes, (except human
intrusion), and sequences of events and processes (except human
intrusion) that might affect the Yucca Mountain disposal system and
their probabilities of occurring;
* * * * *
Period of geologic stability means the time during which the
variability of geologic characteristics and their future behavior in
and around the Yucca Mountain site can be bounded, that is, they can be
projected within a reasonable range of possibilities. This period is
defined to end at 1 million years after disposal.
* * * * *
4. Section 197.13 is revised to read as follows:
Sec. 197.13 How is subpart B implemented?
(a) The NRC will determine compliance based upon the arithmetic
mean of the projected doses from DOE's performance assessments for the
period within 10,000 years after disposal:
(1) For Sec. 197.20 of this subpart; and
(2) For Sec. Sec. 197.25 and 197.30 of this subpart, if
performance assessment is used to demonstrate compliance with either or
both of these sections.
(b) NRC will determine compliance based upon the median of the
projected doses from DOE's performance assessments for the period after
10,000 years of disposal and through the period of geologic stability:
(1) For Sec. 197.20 of this subpart; and
(2) For Sec. 197.25, if a performance assessment is used to
demonstrate compliance.
5. Section 197.15 is revised to read as follows:
Sec. 197.15 How must DOE take into account the changes that will
occur during the period of geologic stability?
The DOE should not project changes in society, the biosphere (other
than climate), human biology, or increases or decreases of human
knowledge or technology. In all analyses done to demonstrate compliance
with this part, DOE must assume that all of those factors remain
constant as they are at the time of license application submission to
NRC. However, DOE must vary factors related to the geology, hydrology,
and climate based upon cautious, but reasonable assumptions of the
changes in these factors that could affect the Yucca Mountain disposal
system during the period of geologic stability, consistent with the
requirements for performance assessments specified at Sec. 197.36.
6. Section 197.20 is revised to read as follows:
Sec. 197.20 What standard must DOE meet?
(a) The DOE must demonstrate, using performance assessment, that
there is a reasonable expectation that the reasonably maximally exposed
individual receives no more than the following annual committed
effective dose equivalent from releases from the undisturbed Yucca
Mountain disposal system:
(1) 150 microsieverts (15 millirems) for 10,000 years following
disposal; and
(2) 3.5 millisieverts (350 millirems) after 10,000 years, but
within the period of geologic stability.
(b) The DOE's performance assessment must include all potential
pathways of radionuclide transport and exposure.
7. Section 197.25 is revised to read as follows:
Sec. 197.25 What standard must DOE meet?
(a) The DOE must determine the earliest time after disposal that
the waste package would degrade sufficiently that a human intrusion
(see Sec. 197.26) could occur without recognition by the drillers.
(b) The DOE must demonstrate that there is a reasonable expectation
that the reasonably maximally exposed individual will receive an annual
committed effective dose equivalent, as a result of the human
intrusion, of no more than:
(1) 150 microsieverts (15 millirems) for 10,000 years following
disposal; and
(2) 3.5 millisieverts (350 millirems) after 10,000 years, but
within the period of geologic stability.
(c) The analysis must include all potential environmental pathways
of radionuclide transport and exposure.
Sec. 197.35 [Removed and Reserved]
8. Section 197.35 is removed and reserved.
9. Section 197.36 is revised to read as follows:
Sec. 197.36 Are there limits on what DOE must consider in the
performance assessments?
(a) Yes, there are limits on what DOE must consider in the
performance assessments. The DOE's performance assessments conducted to
show compliance with Sec. Sec. 197.20(a)(1), 197.25(b)(1), and 197.30
shall not include consideration of very unlikely features, events, or
processes, i.e., those that are estimated to have less than one chance
in 10,000 of occurring within 10,000 years of disposal (less than one
chance in 100,000,000 per year). In addition, unless otherwise
specified in these standards or NRC regulations, DOE's performance
assessments need not evaluate the impacts resulting from any features,
events, and processes or sequences of events and processes with a
higher chance of occurrence if the results of the performance
assessments would not be changed significantly in the initial 10,000
year period after disposal.
[[Page 49064]]
(b) For performance assessments conducted to show compliance with
Sec. Sec. 197.25(b) and 197.30, DOE's performance assessments shall
exclude unlikely features, events, or processes, or sequences of events
and processes. The DOE should use the specific probability of the
unlikely features, events, and processes as specified by NRC.
(c) For performance assessments conducted to show compliance with
Sec. Sec. 197.20(a)(2) and 197.25(b)(2), DOE's performance assessments
shall project the continued effects of the features, events, and
processes included in paragraph (a) of this section beyond the 10,000-
year post-disposal period through the period of geologic stability. The
DOE must evaluate all of the features, events, or processes included in
paragraph (a) of this section, and also:
(1) The DOE must assess the effects of seismic and igneous
scenarios, subject to the probability limits in paragraph (a) of this
section for very unlikely features, events, and processes. Performance
assessments conducted to show compliance with Sec. 197.25(b)(2) are
also subject to the probability limits for unlikely features, events,
and processes as specified by NRC.
(i) The seismic analysis may be limited to the effects caused by
damage to the drifts in the repository and failure of the waste
packages.
(ii) The igneous analysis may be limited to the effects of a
volcanic event directly intersecting the repository. The igneous event
may be limited to that causing damage to the waste packages directly,
causing releases of radionuclides to the biosphere, atmosphere, or
ground water.
(2) The DOE must assess the effects of climate change. The climate
change analysis may be limited to the effects of increased water flow
through the repository as a result of climate change, and the resulting
transport and release of radionuclides to the accessible environment.
The nature and degree of climate change may be represented by constant
climate conditions. The analysis may commence at 10,000 years after
disposal and shall extend to the period of geologic stability. The NRC
shall specify in regulation the values to be used to represent climate
change, such as temperature, precipitation, or infiltration rate of
water.
(3) The DOE must assess the effects of general corrosion on
engineered barriers. The DOE may use a constant representative
corrosion rate throughout the period of geologic stability or a
distribution of corrosion rates correlated to other repository
parameters.
10. Appendix A to part 197 is added to read as follows:
Appendix A to Part 197--Calculation of Annual Committed Effective Dose
Equivalent
Unless otherwise directed by NRC, DOE shall use the radiation
weighting factors and tissue weighting factors in this Appendix to
calculate committed effective dose equivalent for compliance with
sections 20 and 25 of this part. NRC may allow DOE to use updated
factors issued after the effective date of this regulation. Any such
factors shall have been issued by consensus scientific organizations
and incorporated by EPA into Federal radiation guidance in order to
be considered generally accepted and eligible for this use. Further,
they must be compatible with the effective dose equivalent dose
calculation methodology established in ICRP 26/30 and continued in
ICRP 60/72, and incorporated in this Appendix.
I. Equivalent Dose
The calculation of the committed effective dose equivalent
(CEDE) begins with the determination of the equivalent dose,
HT, to a tissue or organ, T, listed in Table A.2 below by
using the equation:
[GRAPHIC] [TIFF OMITTED] TP22AU05.002
where DT,R is the absorbed dose in rads (one gray, an SI
unit, equals 100 rads) averaged over the tissue or organ, T, due to
radiation type, R, and wR is the radiation weighting
factor which is given in Table A.1 below. The unit of equivalent
dose is the rem (sievert, in SI units).
Table A.1.--Radiation weighting factors, wR \1\
------------------------------------------------------------------------
Radiation type and energy range \2\ wR value
------------------------------------------------------------------------
Photons, all energies...................................... 1
Electrons and muons, all energies.......................... 1
Neutrons, energy:
< 10 keV................................................. 5
10 keV to 100 keV........................................ 10
> 100 keV to 2 MeV....................................... 20
> 2 MeV to 20 MeV........................................ 10
> 20 MeV................................................. 5
Protons, other than recoil protons, > 2 MeV................ 5
Alpha particles, fission fragments, heavy nuclei........... 20
------------------------------------------------------------------------
\1\ All values relate to the radiation incident on the body or, for
internal sources, emitted from the source.
\2\ See paragraph A14 in ICRP Publication 60 for the choice of values
for other radiation types and energies not in the table.
II. Effective Dose Equivalent
The next step is the calculation of the effective dose
equivalent, E. The probability of occurrence of a stochastic effect
in a tissue or organ is assumed to be proportional to the equivalent
dose in the tissue or organ. The constant of proportionality differs
for the various tissues of the body, but in assessing health
detriment the total risk is required. This is taken into account
using the tissue weighting factors, wT in Table A.2,
which represent the proportion of the stochastic risk resulting from
irradiation of the tissue or organ to the total risk when the whole
body is irradiated uniformly and HT is the equivalent
dose in the tissue or organ, T, in the equation:
E = [Sigma] wT [sdot] HT.
Table A.2.--Tissue Weighting Factors, wT
------------------------------------------------------------------------
Tissue or organ wT value
------------------------------------------------------------------------
Gonads..................................................... 0.20
Bone marrow (red).......................................... 0.12
Colon...................................................... 0.12
Lung....................................................... 0.12
Stomach.................................................... 0.12
Bladder.................................................... 0.05
Breast..................................................... 0.05
Liver...................................................... 0.05
Esophagus.................................................. 0.05
Thyroid.................................................... 0.05
Skin....................................................... 0.01
Bone surface............................................... 0.01
Remainder.................................................. a,b 0.05
------------------------------------------------------------------------
a Remainder is composed of the following tissues: adrenals, brain,
extrathoracic airways, small intestine, kidneys, muscle, pancreas,
spleen, thymus, and uterus.
b The value 0.05 is applied to the mass-weighted average dose to the
Remainder tissues group, except when the following ``splitting rule''
applies: If a tissue of Remainder receives a dose in excess of that
received by any of the 12 tissues for which weighting factors are
specified, a weighting factor of 0.025 (half of Remainder) is applied
to that tissue or organ and 0.025 to the mass-averaged committed
equivalent dose equivalent in the rest of the Remainder tissues.
III. Annual Committed Tissue or Organ Equivalent Dose
For internal irradiation from incorporated radionuclides, the
total absorbed dose will be spread out in time, being gradually
delivered as the radionuclide decays. The time distribution of the
absorbed dose rate will vary with the radionuclide, its form, the
mode of intake and the tissue within which it is incorporated. To
take account of this distribution the quantity committed equivalent
dose, HT([tau]) where [tau] is the integration time in
years following an intake over any particular year, is used and is
the integral over time of the equivalent dose rate in a particular
tissue or organ that will be received by an individual following an
intake of radioactive material into the body:
[GRAPHIC] [TIFF OMITTED] TP22AU05.000
for a single intake of activity at time t0 where
HT(t) is the relevant equivalent-dose rate in a tissue or
organ at time t. For the purposes of this rule, the previously
mentioned single intake may be considered to be an annual intake.
[[Page 49065]]
IV. Annual Committed Effective Dose Equivalent
If the committed equivalent doses to the individual tissues or
organs resulting from an annual intake are multiplied by the
appropriate weighting factors, wT, from table A.2, and
then summed, the result will be the annual committed effective dose
equivalent, E([tau]):
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[FR Doc. 05-16193 Filed 8-19-05; 8:45 am]
BILLING CODE 6560-50-P