[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]
BILLING CODE 7590-01-P