[Federal Register Volume 60, Number 19 (Monday, January 30, 1995)]
[Notices]
[Pages 5734-5737]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 95-2168]
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NUCLEAR REGULATORY COMMISSION
Proposed Generic Communication Supplement 5 to Generic Letter 88-
20, ``Individual Plant Examination of External Events (IPEEE) for
Severe Accident Vulnerabilities''
AGENCY: Nuclear Regulatory Commission.
ACTION: Notice of opportunity for public comment.
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SUMMARY: The Nuclear Regulatory Commission (NRC) is proposing to issue
Supplement 5 to Generic Letter 88-20, ``Individual Plant Examination of
External Events (IPEEE) for Severe Accident Vulnerabilities.'' This
draft generic letter supplement (1) notifies addressees about proposed
modifications in the seismic IPEEE scope for the focused-scope and
full-scope plants and (2) provides guidance to licensees who wish to
voluntarily modify their previously committed seismic IPEEE programs.
NRC is seeking comment from interested parties regarding both the
technical and regulatory aspects of this proposed generic letter
supplement, which is presented under the Supplementary Information
heading. This proposed generic letter supplement and supporting
documentation were discussed in meeting number 267 of the Committee to
Review Generic Requirements (CRGR) on December 13, 1994. The relevant
information that was sent to CRGR to support their review of the
proposed generic letter is available in the Public Document Rooms under
accession number 9412290183. NRC will consider comments received from
interested parties in the final evaluation of the proposed generic
letter supplement. The final evaluation by NRC will include a review of
the technical position and, when appropriate, an analysis of the value/
impact on licensees. Should this generic letter supplement be issued by
NRC, it will become available for public inspection in the Public
Document Rooms.
DATES: Comment period expires March 1, 1995. Comments submitted after
this date will be considered if it is practical to do so, but assurance
of consideration cannot be given except for comments received on or
before this date.
ADDRESSES: Submit written comments to Chief, Rules Review and
Directives Branch, U.S. Nuclear Regulatory Commission, Washington, DC
20555. Written comments may also be delivered to 11545 Rockville Pike,
Rockville, Maryland, from 7:30 am to 4:15 pm, Federal workdays. Copies
of written comments received may be examined at the NRC Public Document
Room, 2120 L Street, NW. (Lower Level), Washington, DC.
FOR FURTHER INFORMATION CONTACT: John T. Chen, (301) 415-6549.
SUPPLEMENTARY INFORMATION:
NRC Generic Letter 88-20, Supplement 5: Individual Plant
Examination of External Events for Severe Accident Vulnerabilities
Addressees
All holders of operating licenses or construction permits for
nuclear power reactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this
generic letter to (1) notify addressees about proposed modifications in
the recommended scope of seismic reviews which are performed as part of
an individual plant examination of external events (IPEEEs) for the
focused-scope and full-scope plants and (2) give guidance to licensees
who wish to voluntarily modify their previously committed seismic IPEEE
programs.
Background
On June 28, 1991, NRC issued Generic Letter 88-20, Supplement 4,
``Individual Plant Examination of External Events (IPEEE) for Severe
Accident Vulnerabilities,'' (Reference 1), and NUREG-1407F,
``Procedural and Submittal Guidance for the Individual Plant
Examination of External Events (IPEEE) for Severe Accident
Vulnerabilities: Final Report,'' (Reference 2). The generic letter
requested all licensees to perform an IPEEE to find plant-specific
vulnerabilities to severe accidents caused by external events and
report the results to NRC. Section 4.1 of Reference 1 and Chapter 3 of
Reference 2 address the seismic portion of the IPEEE. The lists of
review level earthquakes (RLEs) and review scope defined by the staff
for all U.S. sites are presented in Appendix 3 of Reference 1. Plants
in the central and eastern U.S. have been assigned to appropriate
review categories (plant bins) primarily according to comparison of
available seismic hazard results. The hazard results used in the
binning process included those published in 1989 by Lawrence Livermore
National Laboratory (LLNL) (Reference 3) and the Electric Power
Research Institute (EPRI) (Reference 4). NRC established relative
groups because of the large inherent uncertainties in the probabilistic
estimation of seismic hazard (Appendix A to Reference 2). Using this
approach, the staff compared the relative seismic hazard of the 69
central and eastern U.S. plant sites, and assigned each plant into one
of four categories for the seismic margins method (Reduced-Scope, 0.3g
Focused-Scope, 0.3g Full-Scope, and 0.5g bin). Two plants in the 0.5g
bin [[Page 5735]] have committed to perform a seismic probabilistic
risk assessment and have performed that assessment.
Description of Circumstances
In 1994, based on a re-elicitation of LLNL ground-motion and
seismicity experts, the staff published revised seismic hazard results
in NUREG-1488 (Reference 5). The new LLNL mean hazard estimates are
lower than the 1989 LLNL results but higher than the EPRI estimates.
The Nuclear Energy Institute (NEI), based on these revised hazard
estimates, advocated that most focused-scope plants should instead
perform reduced-scope studies as part of the seismic IPEEE (Reference
6). NEI also stated that each licensee is responsible for proposing the
most cost-effective program to satisfy the seismic IPEEE request
consistent with the level of seismic hazard at the specific site. Seven
licensees have already informed NRC of their intent to revise their
IPEEE commitments.
These developments prompted NRC to revisit systematically the
seismic IPEEE program rather than dealing with each licensee
individually. The staff stated its intent, to review LLNL's revised
seismic hazard estimates and to determine if it is appropriate to
revise the seismic IPEEE scope, in Information Notice 94-32, ``Revised
Seismic Hazard Estimate,'' (Reference 7). The staff also stated in
Reference 7 that licensees who have not completed the seismic portion
of the IPEEE may continue with their program and submit their completed
IPEEE based on References 1 and 2.
NRC contracted Energy Research, Inc. (ERI) to do the seismic
revisit study to determine whether consideration of the new LLNL
seismic hazard estimates (1) would significantly change the original
binning results, and (2) warranted adjusting the seismic scope and
guidelines of the seismic IPEEE review. The latter effort would also
require a determination of how the scope should be modified and the
technical justification for such modifications. ERI completed the study
and submitted two reports in September 1994 (References 8 and 9). The
staff held a public workshop on October 21 to discuss these reports,
present a peer review group's comments, determine issues to be
addressed, and solicit public input for developing the staff position
on the seismic scope modification. The transcript of the workshop is
available in Reference 10.
Discussion
The staff evaluated the ERI re-assessment reports, the peer review
group's comments, the NEI white paper (Reference 6), and comments
received at and after the workshop. The staff concludes that the scope
of the seismic IPEEE can be modified for all focused-scope and full-
scope plants, by eliminating the need to calculate the capacity of
certain generally rugged components or certain site effects that would
not be significant sources or contributors to seismic severe-accident
risk or would not result in cost-beneficial improvements. The
justification for this reduction in the seismic review scope is that
the perceived seismic hazard estimates and associated risks have
decreased. However, the examination process for the modified seismic
IPEEE remains the same process described in Supplement 4 to Generic
Letter 88-20 and NUREG-1407. The most significant comments and concerns
with respect to reducing the scope of the IPEEE seismic review which
were raised at and after the workshop and the associated resolutions
are summarized in Attachment 1.
However, certain utilities represented at the public workshop
expressed concern that GL 88-20, Supplement 4, and guidance in NUREG-
1407 could be interpreted as precluding the use of the expert judgement
or the use of the most efficient approach to do the seismic portion of
IPEEE. For instance, certain utilities interpreted NUREG-1407 to
require a minimum number of margin capacity calculations (i.e., high
confidence of low probability of failure). The NRC staff wants to
reemphasize that the guidance in the generic letter or NUREG-1407 does
not preclude the use of well-based expert judgement and efficient
approaches to minimize the effort to do an IPEEE. In GL 88-20, the
staff stated:
``The application of the above approaches involves considerable
judgment with regards to the requested scope and depth of the study,
level of analytical sophistication, and level of effort to be
expended.''
The detailed guidelines presented in NUREG-1407 do not preclude use
of this type of judgment. The use of judgment is further recognized in
NUREG-1407 in connection with the importance of the peer review.
Discussions at the workshop indicated that some utilities did use such
judgment, within the framework of the current guidance as discussed, to
reduce the cost of an IPEEE.
Modified Scope of Seismic Examination
The methods originally described and guidelines described in NUREG-
1407 fulfill Supplement 4 to GL 88-20. However, the results of the
revised LLNL seismic estimates, indicate that the perceived seismic
risk has been reduced for most plant sites in the central and eastern
U.S. Accordingly, NRC proposed reducing the scope of the seismic IPEEE
programs for licensees of the focused-scope and full-scope plants. The
proposed scope change follows.
(1) Focused-Scope Plants
The seismic capacities for reactor internals and soil-related
failures need not be evaluated for the seismic IPEEE (Attachment 1).
Modifying the seismic IPEEE for focused-scope plants in this manner
will make these evaluations equivalent to those for the reduced-scope
plants, with additional evaluations of a few known weaker, but
critical, components or items.
(2) Full-Scope Plants
The seismic IPEEE need not include an evaluation of seismic
capacities for reactor internals. Soil-related failures should still be
evaluated, but only for safety-related supporting systems and equipment
that are founded on soil such that their function might be affected by
liquefaction or general instability of the soil. The licensee may also
need to evaluate the potential for such postulated soil failures or the
consequences resulting from them. Reference 11 contains guidance for
such evaluations; a review of appropriate design and construction
records is adequate.
The staff is aware of recent observations of cracks associated with
reactor internals at some plants. The issue is not yet resolved and is
being evaluated separately both as an operating issue (i.e., within
design basis) (Ref. 12) and with respect to severe accident
implications (i.e., beyond design basis) (Ref. 13), therefore,
eliminating this item will not detract from the IPEEE. The remaining
scope is the same as that outlined in Supplement 4 to GL 88-20 and
NUREG-1407. The staff reviewed discussions at the workshop and other
information and has taken the position that using appropriate judgment
as allowed in the generic letter and NUREG-1407 and eliminating
detailed evaluations for soil-related failures and reactor internals
that may not lead to cost beneficial improvements will maintain the
integrity of the IPEEE process while reducing cost. However, a careful
and thorough seismic walkdown remains the key element to examining
seismic vulnerability regardless of the category assigned the plant.
Requested Information
Licensees of focused-scope and full-scope plants who voluntarily
choose to [[Page 5736]] do seismic IPEEEs using the modified procedures
described above must inform NRC in writing of their intent to do so. If
the revised submittal schedule differs from previously committed
schedules, then the new proposed schedule must be included in the
response. NRC will schedule meetings with the licensee, if requested,
during the examinations to discuss subjects raised by licensees and to
give necessary clarifications.
Licensees who do not modify their seismic IPEEEs are not expected
to submit any response to this generic letter.
Required Response
Within 60 days from the date of this generic letter, all addressees
who voluntarily choose to perform seismic IPEEEs using the modified
procedures described above are required to submit a response containing
the information requested above.
Address the required written reports to the U.S. Nuclear Regulatory
Commission, ATTN: Document Control Desk, Washington, DC 20555, under
oath or affirmation under the provisions of Section 182a, Atomic Energy
Act of 1954, as amended, and Section 50.54(f) of Title 10 of the Code
of Federal Regulations (10 CFR 50.54(f)).
Backfit Discussion
This generic letter only requests information under the provisions
of 10 CFR 50.54(f) from addressees who voluntarily choose to do seismic
IPEEEs using the modified procedures described above. Therefore, the
staff has not performed a backfit analysis. The information requested
is needed to evaluate voluntary changes to the seismic portions of
IPEEE in response to the information in this generic letter.
The evaluation required by 10 CFR 50.54(f) to justify this
information request is included in the preceding discussion.
Attachments:
1. Comments and Resolution
2. References
Attachment 1--Comments and Resolution
All significant comments and concerns raised at and after the
workshop, together with staff's response, are summarized below.
(1) Candidates plant sites for seismic scope reduction: The
industry suggested that candidate sites should not be limited to
focused-scope plants.
Response: In addition to modifying the scope for focused-scope
plants, the staff also reduced the scope of review for full-scope
plants by eliminating the evaluation of reactor internals.
(2) Use of absolute hazard or risk criteria for rebinning or sub-
binning candidate sites: The comments indicated that the absolute risk
criterion should play a significant role in the seismic rebinning.
Response: The staff considered absolute seismic hazard and risk
criteria when it reconsidered seismic rebinning. However, the inherent
uncertainty in the absolute number would affect decision making, in
that small variations in the CDF threshold or in the approximately
calculated CDFs of candidate plants would significantly affect the
binning for many plants. No consensus was reached on the specific risk
criterion that should be selected for the rebinning process. Therefore,
the staff did not recommend using an absolute risk criterion when
determining whether to reduce the seismic scope. However, licensees may
use numerical values in determining which plant-specific improvements
should be implemented.
(3) Overall reduction of seismic scope for all candidate sites: The
suggested reduction as presented in the ERI report, with the exception
of reactor internals, would not reduce the scope of seismic review.
Response: Past experience demonstrated that certain weaker
components need to be retained in the IPEEE. Attachment 1 describes the
rationale for retaining the evaluations of those critical components
and items.
(4) Role of the licensee's seismic review team (SRT): Certain
utilities expressed concern that the role of the licensee's SRT in
decision making is not clear.
Response: Although the guidance in NUREG-1407 allows for the use of
judgment and latitude in implementing the IPEEE program, certain
utilities may not have used the most cost-efficient and expedient
approach. The staff wants to emphasize that the SRT has an important
role in determining how to implement the IPEEE program. The importance
and flexibility of the SRT have been stated clearly in the IPEEE
guidance.
(5) Evaluation of the effects of soil-related failures: No simple
or cost-effective improvements may be available for plants.
Response: Although simple or cost-effective improvements may not be
available for low seismic hazard sites to deal with the effects of
soil-related failures, soil-related failures are still considered to be
important for relatively high seismic hazard sites in the seismic
IPEEE. Therefore, the staff concludes that the licensees of focused-
scope plants may eliminate the evaluation of soil-related failures from
their seismic IPEEE programs. However, the full-scope plants should
continue evaluating the effects of soil-related failure, to gain
insights from those evaluations. However, the evaluation effort should
be focused only on safety-related supporting systems and equipment that
are founded on soil such that their function might be affected by soil-
related failures.
(6) Cost savings: The potential cost savings associated with
eliminating certain evaluations described in the NEI white paper
(Reference 6) are high.
Response: The experience gained at certain plants indicated that
the potential cost savings are likely to be substantially lower than
those presented in the NEI paper. Some of the savings cited by the
utility personnel can be achieved without changing scope, since NUREG-
1407 offers flexibility such as in eliminating detailed evaluation of
reactor internals and using an alternate approach to bad actor1
relay assessment.
\1\``Bad actor'' relays, as described in NUREG-1407, are those
low-seismic-ruggedness relays identified by USI A-46 implementation.
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(7) Seismic capacity evaluation of reactor internals: Should the
evaluation of reactor internals be eliminated?
Response: The results of a few seismic PRAs indicated that un-
cracked reactor internals are inherently rugged (having seismic
capacities well beyond the requested earthquake review level of 0.3g)
and do not contribute significantly to the core damage frequency.
However, a significant effort is involved in calculating the fragility
or capacity of the reactor internal components. On the basis of earlier
study results (assuming un-cracked reactor internals) and the perceived
reduction of seismic hazard estimates and associated seismic risk, the
staff concluded that the cost of the evaluation outweighs the risk of
the failure of reactor internal components and proposes to eliminate
them from the examination. However, the staff is aware of recent
observations of cracks associated with reactor internals at some
plants. The issue is not yet resolved and is being evaluated separately
both as an operating issue (i.e., within design basis) and with respect
to severe accident implications (i.e., beyond design basis), therefore,
eliminating this item will not detract from the IPEEE.
(8) Generic seismic fragilities used in seismic rebinning: The
seismic rebinning on the basis of generic seismic fragilities, as was
done in the ERI's [[Page 5737]] study, would result in anomalous
results.
Response: The staff concurs that seismic rebinning solely on the
basis of generic seismic fragilities could result in anomalous results,
since such items as the plant design basis and vintage of the plant
might not be appropriately included. For instance, plants located at
the same site were put in different bins (Salem and Hope Creek), and
the plants in the New Madrid area were placed in the modified-scope
bin. These observations contributed to the staff's decision to
eliminate the use of an absolute risk criterion in the seismic scope
modifications.
(9) Information exchange through a workshop on lessons learned from
IPEEE: An information exchange workshop on IPEEE lessons learned to
discuss the experience gained from practical or more efficient ways of
carrying out the seismic IPEEEs (i.e., relay chatter issue) would
benefit both industry and staff.
Response: The staff will consider such a workshop in the future.
(10) Components and items needing evaluation and bases: Certain
evaluations of a few known weaker and critical components and items
need to be retained in the seismic IPEEE program.
Response: Those components and items identified as needing
evaluation and the bases for the retention are briefly described below:
(a) Relay Chatter Issue
While preparing the original guidance in NUREG-1407, the NRC staff
developed its position on relay chatter issue after thoroughly
discussing the issue with industry and evaluating the results of
previous studies. The staff drastically reduced the scope of relay
chatter evaluation, retaining only the identification of bad actor
relays. Since these relays are of low capacity, their identification is
considered minimum scope for the IPEEE review. The guidance does not
preclude any efficient and expeditious means of identifying these
relays.
(b) Masonry and Block Walls
Probabilistic risk assessments and margin studies have demonstrated
that failure of masonry or block walls might be a significant safety
concern in existing nuclear power plants. The earthquake experience
database and analytical evaluations of seismic fragility demonstrate
that masonry and block walls without proper reinforcements are
vulnerable to earthquake motion. Although this type of construction
would not be appropriate for use in the current design of nuclear power
plants, it has been used in several plants. In evaluating these walls,
more lenient criteria were used; thus, the available margins beyond the
safe shutdown earthquake may not be comparable to those of other
components of the plant. Therefore, in doing the seismic IPEEE review,
the licensee needs to identify and evaluate masonry and block walls
where they may affect safety components required for safe plant
operation. The licensee would need to correct, if warranted, any
situation that may present a significant threat to plant safety.
(c) Flat-Bottom Tanks
Earthquake experience data and analytical fragility evaluations
have demonstrated that flat-bottom tanks with poor anchorage are
vulnerable to earthquake ground motion. The typical failure mode of
concern is the buckling at the base of the tank, which could cause the
liquid contents to escape or cause the tank to collapse. If a flat-
bottom tank fails, it could flood surrounding areas in the plant, in
addition to the consequences of loss of function of the tanks. Past
seismic studies of nuclear power plants have designated flat-bottom
tanks as low-capacity components. Such components include the refueling
water storage tank and the condensate storage tank, whose failures
would often significantly affect plant safety. The identification and
evaluation of flat-bottom tanks should, therefore, be included as a
fundamental element of the seismic IPEEE review to correct, if
warranted, any situation that may threaten plant safety.
(d) Other Items
The licensee would also need to consider several other items that
pertain to inadequate anchorage and bracing, adverse physical
interactions, building impact, or pounding. These items include the
weaker components of the diesel generators or pumps. However, the
licensee's seismic review team should determine whether seismic
capacities of those components need to be evaluated in the seismic
review.
Attachment 2--References
[1] U.S. Nuclear Regulatory Commission, Generic Letter 88-20,
Supplement No. 4, ``Individual Plant Examination of External Events
(IPEEE) for Severe Accident Vulnerabilities--10 CFR 50.54(f),'' June
1991.
[2] NRC, NUREG-1407, ``Procedural and Submittal Guidance for the
Individual Plant Examination of External Events (IPEEE) for Severe
Accident Vulnerabilities,'' Final Report, June 1991.
[3] NRC, NUREG/CR-5250, ``Seismic Hazard Characterization of 69
Nuclear Power Plant Sites East of the Rocky Mountains,'' January
1989.
[4] Electric Power Research Institute (EPRI), NP-6395-D,
``Probabilistic Seismic Hazard Evaluation at Nuclear Plant Sites in
the Central and Eastern United States: Resolution of the Charleston
Issue,'' April 1989.
[5] NRC, NUREG-1488, ``Revised Livermore Seismic Hazard Estimates
for 69 Nuclear Power Plant Sites East of the Rocky Mountains,''
April 1994.
[6] Letter from W. Rasin (NEI) to A. Thadani (NRC), ``NEI White
Paper, `Justification for Reduction in IPEEE Program Based on
Revised LLNL Seismic Hazard Results,''' April 5, 1994.
[7] NRC IN 94-32, ``Revised Seismic Hazard Estimate,'' April 29,
1994.
[8] Energy Research, Inc. (ERI) Report (ERI/NRC 94-502), ``A
Proposed Approach to Seismic Scope Re-assessment for Individual
Plant Examination of External Events (IPEEE),'' Final Draft,
September 1994
[9] ERI/NRC 94-504, ``Approaches for Proposed Modifications of
Seismic IPEEE Guidelines for Focused-Scope Plants'', Final Draft,
September 1994.
[10] NRC Transcript, ``Workshop in Seismic IPEEE Revisit,'' October
21, 1994.
[11] EPRI NP-6041, ``A Methodology for Assessment of Nuclear Power
Plant Seismic Margin,'' October 1988.
[12] NRC Generic Letter 94-03, ``Intergranular stress Corrosion
Cracking of Core Shrouds in BWR Reactors,'' July 25, 1994.
[13] NRC memorandum from W. Russell to E. Beckjord, ``NRR User Need
Request for Support of Resolving Problem of Stress Corrosion of
Reactor Vessel Internal Components,'' December 2, 1994.
Dated at Rockville, Maryland, this 20th day of January 1995.
For the Nuclear Regulatory Commission.
Brian K. Grimes,
Director, Division of Project Support, Office of Nuclear Reactor
Regulation.
[FR Doc. 95-2168 Filed 1-27-95; 8:45 am]
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