[Federal Register Volume 73, Number 228 (Tuesday, November 25, 2008)]
[Proposed Rules]
[Pages 71564-71569]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: E8-27938]
=======================================================================
-----------------------------------------------------------------------
NUCLEAR REGULATORY COMMISSION
10 CFR Part 50
[Docket No. PRM-50-84; NRC-2007-0013]
Mark Edward Leyse; Consideration of Petition in Rulemaking
Process
AGENCY: Nuclear Regulatory Commission.
ACTION: Resolution of petition for rulemaking and closure of petition
docket.
-----------------------------------------------------------------------
SUMMARY: The Nuclear Regulatory Commission (NRC) will consider the
issues raised in a petition for rulemaking (PRM) submitted by Mark
Edward Leyse in the NRC's rulemaking process. The petition was dated
March 15, 2007, and was docketed as PRM-50-84. The petitioner requests
that the NRC amend its regulations to require that nuclear power
reactors be operated in a manner to limit the thickness of crud layers
and/or the thickness of oxide layers on fuel rod cladding surfaces to
ensure that the facilities operate in compliance with the emergency
core cooling system (ECCS) acceptance criteria. The petitioner also
requests that the requirements pertaining to ECCS evaluation models be
amended to explicitly require that the steady-state temperature
distribution and stored energy in reactor fuel at the onset of a
postulated loss-of-coolant accident (LOCA) be calculated by factoring
in the role that the thermal resistance of crud and/or oxide layers on
fuel cladding plays in increasing the stored energy of the fuel.
Lastly, the petitioner requests that the acceptance criteria for
analyses of ECCS cooling performance for light-water nuclear power
reactors be amended to stipulate a maximum allowable percentage of
hydrogen content in the cladding of fuel rods. The NRC will consider
the petitioner's first two requests in PRM-50-84 because the underlying
technical considerations regarding the effects of crud and oxide growth
on ECCS analyses noted by the petitioner are sufficiently related to an
ongoing NRC rulemaking activity on ECCS analysis acceptance criteria.
The NRC will consider the petitioner's third request because the NRC
has already initiated rulemaking activities that will address the
petitioner's underlying technical concerns on fuel cladding
embrittlement.
While the NRC will consider the issues raised in the petition in
its rulemaking process, the petitioner's concerns may not be addressed
exactly as the petitioner has requested. During the rulemaking process,
the NRC will solicit comments from the public and will consider all
comments before issuing a final rule.
DATES: The docket for the petition for rulemaking PRM-50-84 is closed
on November 25, 2008.
ADDRESSES: You can access publicly available documents related to this
petition for rulemaking using the following methods:
Federal e-Rulemaking Portal: Documents related to the evaluation of
this petition are assigned to rulemaking docket ID: NRC-2006-0013.
Further NRC action on the issues raised by this petition will be
considered in the rulemaking to establish Performance-based ECCS
Cladding Acceptance Criteria, (RIN 3150-AH42) which has been assigned
rulemaking docket ID: NRC-2008-0332. Information on this petition and
the related rulemaking can be accessed at the Federal rulemaking
portal, http://www.regulations.gov; search on rulemaking docket ID:
NRC-2007-0013 and NRC-2008-0332. The NRC also tracks all rulemaking
actions in the ``NRC Regulatory Agenda: Semiannual Report (NUREG-
0936).''
NRC's Public Document Room (PDR): The public may examine and have
copied for a fee, publicly available documents at the NRC's PDR, Public
File Area, Room O1-F21, One White Flint North, 11555 Rockville Pike,
Rockville, Maryland.
NRC's Agencywide Document Access and Management System (ADAMS):
Publicly available documents created or received at the NRC are
available electronically at the NRC's Electronic Reading Room at http://www.nrc.gov/NRC/reading-rm/adams.html. From this page, the public can
gain entry into ADAMS, which provides text and image files of NRC's
public documents. If you do not have access to ADAMS or if there are
any problems in accessing the documents located in ADAMS, contact the
NRC PDR Reference staff at 1-800-397-4209, 301-415-4737 or by e-mail to
[email protected].
FOR FURTHER INFORMATION CONTACT: Richard Dudley, Mail Stop O12-D3,
Office of Nuclear Reactor Regulation, United States Nuclear Regulatory
Commission, Washington, DC 20555-0001; telephone (301) 415-1116, or e-
mail [email protected].
SUPPLEMENTARY INFORMATION:
The Petition
The NRC received a petition for rulemaking (ADAMS Accession No.
ML070871368) from Mark Edward Leyse (the petitioner) dated March 15,
2007, which was docketed as PRM-50-84. The petitioner requested that
all holders of operating licenses for nuclear power plants be required
to operate such plants at operating conditions (e.g., levels of power
production, and light-water coolant chemistries) necessary to
effectively limit the thickness of crud and/or oxide layers on fuel rod
cladding surfaces. On May 23, 2007, the NRC published a notice of
receipt for this petition in the Federal Register (72 FR 28902) and
requested public comment. The public comment period ended on August 6,
2007.
NRC Evaluation
The NRC review of this petition and evaluation of public comments
are based upon NRC's understanding of several terms used by the
petitioner:
1. Crud is any foreign substance which may become deposited on the
surface of fuel cladding. This layer can impede the transfer of heat.
The NRC believes that the word ``crud'' originated as an acronym for
``Chalk River
[[Page 71565]]
Unidentified Deposit'', based upon deposits on early test fuels
observed at Chalk River Laboratories in Canada. Crud most frequently
refers to deposits of tiny iron or nickel metallic particles eroded
from pipe and valve surfaces. These particles of stable isotopes may
become ``activated'' or irradiated and transform into radioactive
isotopes, such as cobalt-60. In fouling technology today, the term
``crud'' is generally applied to solid deposits on fuel element heat
transfer surfaces (cladding). The NRC staff makes a clear distinction
between crud and pure zirconium oxidation layers. Although both
materials contain metal oxides, crud does not originate at the fuel
rod, while zirconium oxide forms on fuel when the cladding material
reacts with oxygen.
2. Oxide is a product of the reaction of oxygen with the zirconium
cladding material itself. Zirconia, or zirconium dioxide
(ZrO2) is one oxidation product which may be found on the
exterior surface (and sometimes the interior surface) of zirconium fuel
cladding. Although it may be an additional surface layer, formation of
oxides also consumes some cladding base material, thereby decreasing
metal cladding thickness. Compared to the original metal cladding
material, metal oxides usually are more brittle and conduct heat less
effectively. In this discussion, the terms ``corrosion'' and
``oxidation'' are considered one and the same.
3. Hydrogen in a nuclear reactor may be produced by the breakup of
coolant water molecules during the oxidation process described
previously. Hydrogen may not only be present in the reactor coolant,
but may also diffuse into the fuel cladding. It may then either remain
in solution or be precipitated as a zirconium hydride. Hydrogen in
either form has been found to alter both the material properties and
behavior of the cladding material. Formation of zirconium hydrides,
such as ZrH2, has been found to cause embrittlement of zirconium fuel
cladding.
The NRC understands the petitioner as requesting the NRC to conduct
rulemaking in three specific areas:
1. Establish regulations that require licensees to operate light
water power reactors under conditions that are effective in limiting
the thickness of crud and/or oxide layers on zirconium-clad fuel in
order to ensure compliance with 10 CFR 50.46(b) ECCS acceptance
criteria;
2. Amend current regulations in Appendix K to 10 CFR Part 50 to
explicitly require that the steady-state temperature distribution and
stored energy in the reactor fuel at the onset of a postulated LOCA be
calculated by factoring in the role that the thermal resistance of crud
deposits and/or oxide layers plays in increasing the stored energy in
the fuel (these requirements also need to apply to any NRC-approved,
best-estimate ECCS evaluation models used in lieu of Appendix K
calculations); and
3. Amend Sec. 50.46 to specify a maximum allowable percentage of
hydrogen content in cladding.
The NRC will address each of the petitioner's requests below. The
NRC will first address the petitioner's third request because the logic
used to evaluate the other requests can be more easily understood.
Proposal 3--Amendment of 10 CFR 50.46, Acceptance Criteria for
Emergency Core Cooling Systems for Light-water Nuclear Power Reactors,
to include a limit on maximum hydrogen content in cladding.
The petitioner states that an increase in hydrogen content in
cladding contributes to cladding embrittlement. The petitioner cites an
April 4, 2001, NRC Advisory Committee on Reactor Safeguards (ACRS)
subcommittee meeting on reactor fuels during which an expert from
Argonne National Laboratory stated that a reduction of ductility occurs
when hydrogen levels reach about 600 to 700 parts-per-million (ppm) in
Zircaloy cladding. According to the petitioner, another expert from the
Atomic Energy Research Institute stated that a threshold for a
reduction of ductility in Zircaloy cladding occurs at even a lower
hydrogen level of about 150 to 200 ppm. The petitioner also references
an event at Three Mile Island, Unit 1 (TMI-1) during refueling Cycle 10
that involved hydrogen absorption in fuel cladding. The petitioner
notes that hydrogen content in the cladding of a rod that did not fail
measured 700 ppm at TMI-1 and that this level of hydrogen content in
one-cycle cladding is similar to the 800 ppm level measured in fuel
cladding at the H.B. Robinson, Unit 2 facility, a pressurized water
reactor (PWR). The petitioner states that some of the cladding in TMI-1
Cycle 10 contained levels of hydrogen that Argonne National Laboratory
found would have caused a loss of cladding ductility in addition to the
embrittlement resulting from excessive oxide levels.
The petitioner also states that absorption of hydrogen would
contribute to a loss of cladding ductility during a LOCA along with
cladding degradation and massive oxidation. The petitioner cites a
failed fuel rod from the TMI-1, Cycle 10 event when hydrogen absorption
caused hydrided material to break away from the outer portions of the
cladding. The petitioner believes that the effects of increased stored
energy due to a heavy crud layer in the fuel and the severity of
cladding oxidation, embrittlement, and resulting fuel degradation
during an actual event would be substantially greater than in an ECCS
calculation based on clean cladding.
In 2003, the Commissioners directed the NRC staff to undertake
rulemaking to amend 10 CFR 50.46 to provide for a more performance-
based approach to meeting the ECCS acceptance criteria in Sec.
50.46(b). Technical work to finalize the technical basis for this
rulemaking is currently proceeding and includes a study (Research
Information Letter 0801, ``Technical Basis for Revision of
Embrittlement Criteria in 10 CFR 50.46,'' May 30, 2008, ADAMS accession
no. ML081350225; NUREG/CR-6967, ``Cladding Embrittlement During
Postulated Loss-of-Coolant Accidents,'' July 2008, ADAMS accession no.
ML082130389) of the effects on cladding embrittlement caused by
cladding oxidation and hydrogen. Because the NRC is already
investigating the need to amend Sec. 50.46 to address hydrogen effects
on cladding, the petitioner's request in Proposal 3 will be considered
during the current rulemaking. This rulemaking is designated as RIN
3150-AH42 in the ``NRC Regulatory Agenda: Semiannual Report (NUREG-
0936).'' Documents associated with this rule are posted under docket
ID: NRC-2008-0332 on the Regulations.gov Web site. Rulemaking will
begin when a consensus is reached on the technical basis for the
amendments.
Proposal 1--Establish regulations that require licensees to operate
light water power reactors under conditions that effectively limit the
thickness of crud and oxide layers on zirconium-clad fuel to ensure
compliance with 10 CFR 50.46(b) ECCS acceptance criteria.
To support the rulemaking request in Proposal 1 of the petition,
the petitioner lists sources, such as the Electric Power Research
Institute (EPRI) reports, ACRS transcripts, and several journal
articles to show that the thermal conductivities of the crud and oxide
layers are lower than the thermal conductivity of zirconium metal
cladding. The petitioner asserts that because of these lower heat
transfer rates, the stored energy within the fuel and the time to
transfer stored energy will increase. The petitioner cites several
operating instances to support the contention that safety issues can
arise from the thermal resistance of crud and oxide layers on fuel
cladding. Finally, the petitioner
[[Page 71566]]
lists several examples to show that incidents of fuel failures have
increased in recent years.
The petitioner's request in Proposal 1 is founded on the potential
impact of crud and oxide on ECCS performance evaluations. The NRC
generally agrees with the petitioner that crud and oxide formation can
impact the thermal response of the fuel system. Hydrogen embrittlement
is also an issue in the ongoing rulemaking to revise the ECCS
acceptance criteria discussed in Proposal 3 above. The need for any
operational restrictions, as requested by the petitioner, would
presumably be determined (in part) from these considerations. The NRC
believes that the petitioner's Proposal 1 is sufficiently relevant to
the ongoing cladding embrittlement rulemaking to warrant consideration
in that proceeding. The NRC is accepting the petitioner's Proposal 1
for consideration during the current rulemaking to revise Sec.
50.46(b). In deciding to consider the petitioner's concern in the Sec.
50.46(b) rulemaking, the NRC expresses no position on the specific
merits of the petitioner's request and underlying bases. These issues
will be addressed separately as part of the rulemaking.
Proposal 2--Amendment of Appendix K to 10 CFR Part 50, ECCS
Evaluation Models I(A)(l), The Initial Stored Energy in the Fuel, to
also require the thermal resistance of crud deposits and/or oxide
layers as factors in calculations of steady-state temperature
distribution and stored energy in the reactor fuel at the onset of a
postulated LOCA.
In this proposal, the petitioner requested that Appendix K to 10
CFR Part 50 be amended to include explicit instructions on how to
perform the ECCS performance calculations mentioned above. Also, in
lieu of Appendix K calculations, the petitioner requested establishment
of a regulation stating that these requirements must also apply to any
NRC-approved, best-estimate ECCS evaluation model, as described in NRC
Regulatory Guide 1.157. The petitioner states that because layers of
crud and/or oxide increase the quantity of stored energy in the fuel,
Appendix K to Part 50 should explicitly require that the thermal
conductivity of layers of crud and/or oxide be factored into
calculations of the stored energy in the fuel. In support of the
petition, several references are cited. For example, the petitioner
quotes from a letter to the NRC from James F. Klapproth, Manager,
Engineering and Technology at General Electric Nuclear Energy (April 8,
2002, ADAMS accession no. ML021020383): ``The primary effects of [a]
heavy crud layer during a postulated LOCA would be an increase in the
fuel stored energy at the onset of the event, and a delay in the
transfer of that stored energy to the coolant during the blowdown phase
of the event.''
Proposal 2 requests that Appendix K explicitly require
consideration of crud and/or oxide layers in the calculation of stored
energy used in ECCS performance calculations required by Sec. 50.46.
Appendix K provides requirements for one acceptable methodology for
performing Sec. 50.46 ECCS performance calculations that must meet the
acceptance criteria in Sec. 50.46(b). Similar to Proposal 1 above, the
petitioner's request in Proposal 2 is founded on the potential impact
of crud and oxide on ECCS performance evaluations. Because the NRC
agrees with the petitioner that crud and oxide formation can change the
thermal response of the fuel system, it is possible that crud and
oxidation layers could also have an impact on cladding hydrogen
concentration. Also, because hydrogen uptake and concentration are
being considered in the ongoing rulemaking to establish new
performance-based ECCS acceptance criteria, consideration of crud and
oxidation in that context is appropriate. Thus, the NRC concludes that
Proposal 2 is likewise sufficiently relevant to the ongoing rulemaking
to warrant consideration in that proceeding. As in the case of the
petitioner's Proposal 1, the NRC expresses no position on the specific
merits of the petitioner's Proposal 2 and its underlying bases. These
issues will be addressed separately as part of the Sec. 50.46(b)
rulemaking.
Comparison of PRM-50-84 With Previous Similar Petitions
PRM-50-84 is the fifth in a series of petitions for rulemaking
submitted to the NRC regarding the build-up, analysis, and release of
crud on nuclear power plant heat exchange surfaces, and the oxidation
of zirconium fuel cladding. Each of the four previous petitions (PRM-
50-73 and PRM-50-73A (68 FR 41963; July 16, 2003); PRM-50-76 (70 FR
52893; September 9, 2005); and PRM-50-78 (69 FR 56958; September 23,
2004)) have been denied by the Commission. The NRC evaluated each of
the previous petitions and concluded that the requested actions would
not contribute to maintaining the public safety or security, nor would
it improve the regulatory efficiently and effectiveness. The current
petition is being considered because it includes the assertion that the
accumulation of crud and oxide deposits will interfere with effective
heat exchange between the cladding and coolant, increase fuel
temperatures, and thus, lead to safety problems. Additionally, the
NRC's knowledge of the effects of crud, oxidation, and hydrogen content
on cladding integrity has increased in the last few years.
In 2003, the NRC initiated work to develop the technical basis for
new, performance-based ECCS acceptance criteria that would apply to all
zirconium cladding alloys.\1\ Laboratory testing was performed on non-
irradiated and irradiated zirconium alloys with different burnups to
determine what parameters affected cladding embrittlement. On May 30,
2008, the NRC summarized the results of this research effort in a
letter (Research Information Letter 0801, ``Technical Basis for
Revision of Embrittlement Criteria in 10 CFR 50.46,'' May 30, 2008,
ADAMS accession no. ML081350225). The NRC is now evaluating this
information to determine if it provides an adequate basis for
establishing the new, performance-based ECCS acceptance criteria. Two
significant conclusions of this work are that hydrogen content of
cladding is an important factor in causing cladding embrittlement and
that cladding oxidation is a key contributor to cladding hydrogen
content. Because crud and oxide formation can impact the thermal
response of the fuel system, it is possible that crud and oxidation
layers could also have direct or indirect impacts on cladding hydrogen
concentration. Also, because all these factors appear to be
interrelated, the NRC will consider all of the phenomena addressed in
PRM-50-84 (crud, oxidation, and hydrogen content) in the ongoing
rulemaking to establish new performance-based ECCS acceptance criteria
in Sec. 50.46(b).
---------------------------------------------------------------------------
\1\ The acceptance criteria in the current regulations are
specifically applicable to only two cladding alloys, Zircaloy and
Zirlo. Fuel designs with other, more advanced cladding alloys must
be reviewed on a case-by-case basis and require NRC approval of an
exemption to the existing requirements.
---------------------------------------------------------------------------
Analysis of Public Comments
Comments in support of PRM-50-84 were provided by the Union of
Concerned Scientists (UCS), two individuals, and the petitioner. The
Nuclear Energy Institute and Strategic Teaming and Resource Sharing
organization submitted comments in opposition to the petition. A
summary of the comments and the NRC's evaluation of those comments
follow.
Comment: A commenter referenced various technical reports and
[[Page 71567]]
operational events to demonstrate that the accumulated hydrogen content
of zirconium fuel cladding reduces the ductility of the cladding and
increases the possibility that core geometry could change during a LOCA
and reduce fuel cooling. (MEL 7-1)
NRC Response:
The NRC agrees with the commenter that cladding ductility can be
reduced by hydrogen absorption in zirconium cladding. Since 2003, the
NRC has been working to develop the technical basis for a new
regulation on performance-based ECCS acceptance criteria applicable to
the various zirconium cladding alloys. The NRC accepts this aspect of
the petitioner's request and will consider hydrogen embrittlement
issues during the ongoing rulemaking.
Comment: Several commenters referred to numerous technical reports,
papers, and articles to document the existence of crud and oxidation
layers on light-water reactor fuel cladding and show that the thermal
resistance associated with the crud and oxidation layers significantly
affects fuel temperatures and ECCS performance. (RHL-1, RHL-2, MEL 6-1,
MEL 6-2, MEL 7-1, MEL 7-2, MEL 7-3, RHL 8-2, RHL-10)
NRC Response:
The NRC reviewed the technical information provided or referenced
by the commenters. The NRC agrees with the commenters that formation of
cladding crud and oxide layers is an expected condition at nuclear
power plants. However, the amount of accumulated crud and oxidation
varies from plant to plant and from one fuel cycle to another. The NRC
agrees that crud and/or oxide layers may directly affect the stored
energy in the fuel by their thermal resistance as well as indirectly
affecting the stored energy through an increase in the fuel rod
internal pressure. In addition to the thermal insulating effect of
crud, the NRC notes that a crud layer can also change surface
topography, which has also been shown to affect cladding oxidation. As
part of the ongoing rulemaking on performance-based ECCS analysis
acceptance criteria, the NRC will evaluate the effects of these
phenomena on cladding hydrogen content and embrittlement to determine
their overall significance and if the regulations should be amended in
this area.
Comment: A commenter asserted that the need to implement PRM-50-84
is shown by analysis of the NRC's February 28, 2006 inspection report
on the River Bend Station (ML060600503). The inspection reviewed
activities conducted by the licensee related to the identification and
resolution of problems, including calculated higher cladding
temperatures in fuel Cycle 8 and the formation of tenacious crud on the
fuel rod cladding and fuel rod bowing in River Bend Cycle 11. (RHL-2)
NRC Response:
The NRC agrees with the commenter that the River Bend experience
shows that exceptionally large accumulations of oxide and crud can have
an impact on thermal hydraulic analyses. As part of the ongoing
rulemaking on performance-based ECCS analysis acceptance criteria, the
NRC will evaluate: (i) The effects of these phenomena on cladding
hydrogen content and embrittlement to determine their overall
significance, (ii) if such large accumulations are likely to occur
under current NRC requirements and industry practices, and (iii) if the
NRC's requirements should be amended in this area.
Comment: Thermal-hydraulic analyses of ECCS performance approved by
the NRC are often inadequate because they may not consider or
improperly consider the thermal resistance of accumulated crud and/or
oxidation on fuel cladding. Commenters cited examples of plant-specific
ECCS analyses and asserted that had crud been properly considered, it
is likely that the licensee would not be in compliance with the ECCS
analysis acceptance criteria in Sec. 50.46(b). (RHL-2, MEL 7-1, MEL 7-
2, MEL 7-3)
NRC Response:
Assertions regarding potentially non-compliant ECCS analyses at the
facilities mentioned are issues which are separate from resolving a
petition for rulemaking on the adequacy of existing regulations. These
assertions are not appropriate for consideration in a rulemaking
context and are outside the scope of review of this PRM. This
information has been referred to the Office Allegation Coordinator to
determine the need for additional plant-specific regulatory review.
Comment: A commenter cited Generic Safety Issue No. 191 (GSI-191)
regarding pressurized water reactors (PWRs), ``Assessment of Debris
Accumulation on PWR Sump Performance,'' and a related document, ``Peer
Review of GSI-191 Chemical Effects Research Program'' (NUREG-1861), as
justification for the petitioner's conclusion that the current
regulations in Sec. 50.46 should be amended. The commenter asserts
that these documents discuss the possibilities of incomplete modeling
of crud-related thermal properties of fuel cladding. (UCS 3-4)
NRC Response:
In GSI-191, the NRC is addressing issues involving PWR containment
sump performance and related chemical effects during a loss-of-coolant
accident. The GSI-191 issues are different from the long-term buildup
of crud and oxidation on reactor fuel which typically occurs during
plant operation. The NRC agrees with the commenter that dissolved
solids in post-accident cooling water that impinges on hot fuel
surfaces could be deposited or precipitated out and could impede heat
transfer from the fuel. The evaluation of GSI-191 by the NRC is a
separate issue.
Comment: A commenter identified two distinguishable layers in BWR
fuel cladding deposits: an inner spinel structure and an outer iron
oxide structure. The commenter further described the use of zinc in the
coolant chemistry of some reactors to reduce radiation buildup on out-
of-core surfaces and stated that the potential culprit in cladding
overheating could be the tenacious ferrite deposit. Because the thermal
conductivity of the ferrite is not known, the commenter concluded that
the potential effects of the tenacious layer should be seriously
evaluated. (LIN-4)
NRC Response:
The NRC has considered the comment and agrees with much of the
information provided. The structure and the composition of crud
deposits may be complex. Also, the relationship between crud deposition
and coolant chemistry is difficult to completely characterize. As part
of the ongoing rulemaking on performance-based ECCS analysis acceptance
criteria, the NRC will evaluate the effects of these phenomena on
cladding hydrogen content and embrittlement to determine their overall
significance and if the regulations should be amended in this area.
Comment: A commenter referred to an NRC press release regarding an
order issued to First Energy Nuclear Operating Company. The order
addresses the prompt sharing of information that may be relevant to
regulatory activities. The commenter asserted that a proprietary EPRI
report, ``BWR Fuel Deposit Sample Evaluation, River Bend Cycle 11 Crud
Flakes,'' has information relevant to regulatory activities associated
with PRM-50-84. The commenter implied that the River Bend Station
licensee should be subject to a similar NRC order requiring that it
provide information, such as the EPRI report, to the NRC. (RHL-9)
NRC Response:
The NRC reviewed the information about River Bend Cycle 11 provided
by the petitioner and commenters and the inspection report
(ML060600503)
[[Page 71568]]
prepared by the NRC inspection team that investigated the crud
occurrences in River Bend Cycles 8 and 11. Although the NRC inspection
report referenced the proprietary EPRI report, the NRC staff evaluating
PRM-50-84 did not review the EPRI report. Nevertheless, the NRC agrees
with the commenter that the River Bend experience shows that
exceptionally large accumulations of oxide and crud can have an impact
on thermal hydraulic analyses. As part of the ongoing rulemaking on
performance-based ECCS analysis acceptance criteria, the NRC will
evaluate the effects of these phenomena on cladding hydrogen content
and embrittlement to determine their overall significance and if the
regulations should be amended in this area.
Comment: A commenter opposed granting the petition because the
petition relies heavily on abnormal operating experiences at four
plants: River Bend (1998-1999 and 2001-2003), Three Mile Island 1
(1995), Palo Verde Unit 2 (1997), and Seabrook (1997), when localized
sections of thick crud developed during normal operation. The commenter
stated that NRC guidelines in Section 4.2 of the Standard Review Plan
(NUREG-0800) do not specify a specific limit on the maximum allowable
corrosion thickness, but require the impact of corrosion on the thermal
and mechanical performance to be considered in fuel design analysis
regarding the design stress and strain limits.
The commenter stated that cladding hydrogen content can have an
adverse effect on ductile/brittle behavior of zirconium alloys heated
into the beta phase and quenched (as would occur in a LOCA). The
hydrogen impact on post-quench cladding ductility is a complex function
of the oxidation temperature and pre-quench cooling path. The potential
impact of hydrogen on the Sec. 50.46(b) fuel acceptance criteria has
been recognized for several years. Experimental programs are underway
to assess this impact on current and newer cladding alloys developed to
minimize hydrogen build-up during irradiation. The commenter further
states that, based on these data, the NRC Office of Nuclear Regulatory
Research is developing the technical basis for new performance-based
fuel acceptance criteria in Sec. 50.46(b) that include the effects of
hydrogen.
In summary, the commenter states that the incidents cited by the
petitioner were isolated operational events and would not have been
prevented by imposing specific regulatory limits on crud thickness. The
industry is actively pursuing root cause evaluations and has developed
corrective actions to mitigate further cases of excessive crud
formation. The separate effects of hydrogen on cladding embrittlement
will be addressed in future rulemaking to implement new acceptance
criteria that are already being developed by the NRC. (NEI 5-1, NEI 5-
2, NEI 5-3, NEI 5-4, NEI 5-5, NEI 5-6, NEI 5-7)
NRC Response:
The NRC agrees with a great deal of the technical information
provided by the commenter and with the commenter's view that new
regulations imposing specific regulatory limits on crud thickness would
not necessarily have prevented the occurrences of heavy crud deposits
resulting from the operational events cited by the petitioner.
Nevertheless, formation of cladding crud and oxide layers is an
expected condition at nuclear power plants. The thickness of these
layers varies from plant to plant. The commenter acknowledged that the
hydrogen impact on post-quench cladding ductility is a complex function
of the oxidation temperature and pre-quench cooling path, and that
these effects will be evaluated in the ongoing rulemaking to develop
more performance-based cladding acceptance criteria. Because crud and
oxide considerations also have potential impact on these new criteria,
the NRC has determined that the petitioner's issues are sufficiently
related to the ongoing cladding acceptance criteria rulemaking and
should be considered in that proceeding.
Comment: Commenters stated that industry-funded research has
resulted in chemistry controls, core design constraints, and
operational guidance that reduce the susceptibility to heavy crud
deposition and that many pressurized water reactors, especially those
most susceptible to heavy crud deposition, make extensive use of the
industry guidance. Commenters stated that the requested rulemaking
would not make a significant contribution to safety because existing
regulations and guidance already address consideration of crud-related
parameters for core cooling. A commenter stated that NRC and licensee
efficiency and effectiveness would be decreased by the requested
regulations because significant resources would be required for the NRC
to promulgate the rule, for licensees to generate additional
information as part of the development of their ECCS evaluation models,
and for the NRC to evaluate the licensees' data and analysis. (NEI 5-1,
STARS 11-1, NEI 5-2, STARS 11-2, STARS 11-3)
NRC Response:
The NRC acknowledges that voluntary industry guidance, if properly
implemented by licensees, can be effective in reducing the
susceptibility to heavy crud deposition. However, the NRC has
determined that crud and oxidation layers can have an impact on
cladding hydrogen concentration. Because hydrogen uptake and
concentration are being considered in the ongoing rulemaking to
establish new performance-based ECCS acceptance criteria, consideration
of crud and oxidation in that context is appropriate. If the NRC
decides that additional regulations are needed regarding the
accumulation of crud and oxidation, the NRC will estimate the
additional NRC and licensee burden associated with the proposed changes
and evaluate the overall cost-effectiveness of the requirements.
Late Comment: On September 5, 2008, after the close of the public
comment period on PRM-50-84, the NRC received an additional public
comment from Mr. Mark Leyse. The NRC reviewed the information contained
in the late comment and determined that it provided no additional
information that would affect the NRC's decision to address the issues
raised in PRM-50-84 in the ongoing Sec. 50.46(b) rulemaking.
Resolution of Petition
The NRC will consider the petitioner's requested rulemaking
changes, the underlying issues relevant to the petition, and the
comments submitted on PRM-50-84, in the ongoing rulemaking to revise
Sec. 50.46(b). This rulemaking is directed at establishing
performance-based ECCS acceptance criteria to prevent fuel cladding
embrittlement. The petitioner's requested changes and the underlying
issues address crud, oxidation, and hydrogen content. These parameters
may be factors in hydrogen embrittlement of zirconium cladding, which
is being addressed in the Sec. 50.46(b) rulemaking. After the
conclusion of the NRC's technical evaluation of the factors relevant to
fuel cladding embrittlement, the NRC will determine whether to adopt
the petitioner's requested rulemaking changes in the Sec. 50.46(b)
rule. If the ongoing work to establish the technical basis for this
rulemaking does not support the issuance of a proposed rule, the NRC
will issue a supplemental Federal Register notice that addresses why
the petitioner's requested rulemaking changes were not adopted by the
NRC. With this resolution of the petition, the NRC closes the docket
for PRM-50-84.
[[Page 71569]]
Dated at Rockville, Maryland, this 5th day of November 2008.
For the Nuclear Regulatory Commission.
Martin J. Virgilio,
Acting Executive Director for Operations.
[FR Doc. E8-27938 Filed 11-24-08; 8:45 am]
BILLING CODE 7590-01-P