ML19128A145

Ans_Psa_Paper_Lessons

ML19128A145
Person / Time
Issue date:	05/10/2019
From:	Reisi-Fard M, Shilp Vasavada NRC/NRR/DRA/APLB
To:
Wayne Davis, OIS
References
Download: ML19128A145 (5)

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LESSONS LEARNED FROM RECENT SEISMIC RISK EVALUATIONS INCLUDING PROBABILISTIC RISK ASSESSMENTS TO SUPPORT REGULATORY ACTIONS Dr. Shilp Vasavada, Dr. Mehdi Reisi-Fard1,2 United States Nuclear Regulatory Commission 11555 Rockville Pike, Rockville, MD 20852 Shilp.Vasavada@nrc.gov; Mehdi.ReisiFard@nrc.gov The U.S. Nuclear Regulatory Commission (NRC) nuclear power plant. However, since the original design licensees may need to quantitatively address the risk of many NPPs, various efforts to assess the impact of associated with seismic events for implementing certain seismic events on these plants, including the quantitative regulatory actions. The NRC staff has recently completed risk and risk insights, have been undertaken, including:

review of several submittals, which include information related to acceptability of seismic probabilistic risk Unresolved Safety Issue A-46, Seismic assessments (SPRAs) that support regulatory actions. Qualification of Equipment in Operating Plants, Examples of those actions include adopting the program 1980 (Ref. 2),

to risk inform categorization and treatment of structures, Individual Plant Examination of External Events systems and components, adopting risk-informed (IPEEE), 1991 (Ref. 3),

completion time program, and responding to Near-Term Generic Issue 199, Implications of Updated Task Force (NTTF) Recommendation 2.1 "Seismic." This Probabilistic Seismic Hazard Estimates in paper presents some lessons learned from the NRC staffs Central and Eastern United States, 2005 (Ref.

review of the technical acceptability of those SPRAs. 4), and These lessons are related to the acceptability of PRA Fukushima Dai-ichi Response, Near Term Task models that are used as the base for the SPRAs, use of Force Recommendation (NTTF) 2.1, 2012 (Ref.

acceptable peer-review processes, and disposition of key 5).

assumptions and sources of uncertainty in SPRA.

Although these efforts have resulted in an improved I. INTRODUCTION understanding of seismic risk at U.S. NPPs and a number of safety improvements, the seismic risk assessment The NRC staff has reviewed several licensing and other methodologies used in response to these programs have regulatory actions that are supported by information varied. The methodology and the assessments used were related to acceptability of SPRAs. The staffs review of not necessarily maintained beyond initial completion of the technical acceptability of SPRAs has resulted in the effort or program.

several lessons learnt including the acceptability of PRA models that are used as the base for the SPRAs, use of Seismic probabilistic risk assessments (SPRAs) are acceptable peer-review processes for SPRAs, and an approach to identify insights and quantify the metrics disposition of key assumptions and sources of uncertainty related to the seismic risk at NPPs. The SPRAs can also in SPRA. These lessons learnt are discussed in this provide realistic representation of the seismic risk profile manuscript. at NPPs. SPRAs have evolved over the last few decades and are garnering increased safety and regulatory use as II. SEISMIC RISK CONSIDERATIONS FOR U.S. the technology matures.

NUCLEAR POWER PLANT Some U.S. NPPs have developed SPRAs in response The guidance in General Design Criterion (GDC) 2 to recent regulatory actions stemming from the (Ref. 1) necessitates nuclear power plants (NPPs) in the Fukushima Dai-ichi response (specifically, NTTF United States to be designed to withstand credible natural recommendation 2.1). These SPRAs have been developed and manmade hazards, including seismic events (i.e., and peer-reviewed against the 2013 version of the earthquakes). In addition, certain SSCs have to be American Society of Mechanical Engineers designed such that they can operate and remain (ASME)/American Nuclear Society (ANS) PRA Standard operational at the Safe Shutdown Earthquake (SSE), (also known as Addendum B). Those SPRAs are used to which represents the design basis earthquake for each provide information about the seismic risk at U.S. NPPs

1. The views expressed herein are those of the authors and do not represent an official position of the U.S. NRC.

2. The views expressed herein are those of the authors and do not represent an official position of the U.S. NRC.

PSA 2019, Charleston, SC, April 28-May 3, 2019 364

due to the re-evaluated seismic hazard and to determine if from peer-reviews (i.e., peer-reviews of the IEPRA). The additional regulatory action is necessary. In addition, U.S. resolution of the finding level F&Os for the IEPRA can NPPs have also utilized their SPRAs for licensing impact the SPRA in the following ways:

activities such as (i) supporting the categorization of structures, systems, and components (SSCs) in the Certain finding level F&Os may not have been implementation of Part 50.69 to Title 10 of the Code of resolved to meet the basis of the finding but can Federal Regulations, Risk-informed categorization and be shown to not impact certain applications of treatment of structures, systems, and components for the IEPRA model. However, the same nuclear power reactors, (10 CFR 50.69; Ref. 6), and (ii) conclusion may not be reached for the SPRA.

determining risk-informed completion times (RICTs; Ref. Finding level F&Os may have been resolved to 7). Some NPPs that do not possess a detailed quantitative meet the basis of the finding but, based on the approach, for determining the seismic risk, such as a PRA development, the resolutions may not have SPRA, have proposed conservative approaches for been propagated to the SPRA.

determining the seismic risk for use in determining Finding level F&Os may have been resolved to RICTs. meet the basis of the finding but, based on the PRA development, the resolutions may be The NRC staff has reviewed the technical different from those that were propagated to the acceptability of SPRAs in the context of various SPRA at the time of its development.

regulatory actions. Several lessons have been learned from such reviews which have been and will continue to Based on the above considerations, it is important to be utilized by the staff for current as well as future ensure the technical acceptability of the IEPRA that is reviews. used as the base for SPRAs. The staffs reviews of SPRAs have found that available guidance on the peer-review of III. KEY LESSONS LEARNED FROM TECHNICAL SPRAs contained in Nuclear Energy Institute (NEI) 12-ACCEPTABILITY REVIEWS OF SPRAs FOR 13, External Hazards PRA Peer Review Process REGULATORY ACTIONS Guidelines (Ref. 10) does not clearly address determination of the impact of the disposition of IEPRA This section will discuss the key lessons learned from finding level F&Os on the SPRA. The NRC staff has used the review of the technical acceptability of SPRAs publicly available information submitted by U.S. NPPs to supporting various regulatory actions. the NRC as well as established regulatory processes to determine that the above considerations are appropriately III.A. Acceptability of Base Internal Events PRA factored into the development as well as the technical Model for Development of SPRA acceptability of SPRAs. Future refinements to the PRA Standard as well as the peer-review guidance is expected SPRAs are usually built using the internal events to result in clear and consistent review of the IEPRA PRA (IEPRA) as the base. The ASME/ANS PRA technical acceptability for use as the base for SPRAs by Standard, including the 2009 version endorsed by the the U.S. NPPs as well as the peer-reviewers.

NRC in RG 1.200, Revision 2 (Ref. 8), allows for the use of an ad-hoc SPRA developed from scratch. However, Part 5 of the ASME/ANS PRA Standard requires the NRC staff has not encountered such ad-hoc SPRAs to SPRAs to explicitly consider and, if necessary, include date because of (i) the availability of peer-reviewed seismically-induced fires and floods. It is common IEPRAs which provides a technically defensible practice to use the internal fire and internal flood PRAs, foundation as well as resource savings for developing respectively, to support and inform the consideration of SPRAs, and (ii) expectation for the use of IEPRAs as the seismically-induced fires and floods in SPRAs. Therefore, base for SPRAs in the Electric Power Research Institute depending on the level of information derived from the (EPRI) report 1025287, Screening, Prioritization and internal fire and internal flood PRAs, it can be important Implementation Details (SPID) for the Resolution of to determine the technical acceptability as well as impact Fukushima Near-Term Task Force Recommendation 2.1: of finding level F&Os for those PRAs on the SPRA.

Seismic (SPID; Ref. 9). The SPID was endorsed by the NRC as acceptable guidance for use in developing SPRAs III.B. Use of Addendum B of the PRA Standard for to respond to the 10 CFR 50.54(f) letter. The response to SPRA Development the 10 CFR 50.54(f) letter is the primary driver for the recent development of high-quality SPRAs by U.S. NPPs. As noted previously, the response to the 10 CFR 50.54(f) letter is the primary driver for the recent IEPRAs used as the base for SPRA development development of SPRAs by U.S. NPPs. The SPID was often have finding level Facts and Observations (F&Os) endorsed by the NRC as acceptable guidance for use in PSA 2019, Charleston, SC, April 28-May 3, 2019 365

developing SPRAs to respond to the 10 CFR 50.54(f) One of the key comments provided by the staff as letter. The SPID cites Part 5 (the portion related to part of the acceptance of NEI 12-13 is the need to SPRA) of the 2013 version of the ASME/ANS PRA determine the technical acceptability of the IEPRA that is Standard (also known as Addendum B to the PRA used as the base for developing the SPRA as well as other Standard). RG 1.200, Revision 2, endorses the 2009 hazard PRAs (e.g., internal fire and internal flood) used to version of ASME/ANS PRA Standard (also known as support the SPRA development. The discussion of lessons Addendum A to the PRA Standard). Further, Addendum learned on this topic are discussed in Section III.A of this B, including Part 5 of that Addendum, has not been manuscript.

endorsed by the NRC for use in licensing activities (e.g.,

adoption of 10 CFR 50.69 and RICTs). The NRC also Another key comment provided by the staff as part of documented its reservations on the use of Addendum B the acceptance of NEI 12-13 is the explicit documentation (Ref. 11). Therefore, a gap exists when SPRAs that are of method(s) used in the SPRA development that are not developed to respond to the 10 CFR 50.54(f) letter are state-of-practice methods and are new to the industry (i.e.,

submitted to support licensing actions. SPRA practitioner community). Prior to the NRC staffs comments, there was no guidance on documenting the An approach that the staff has found to be effective in review of method(s) that were new to the industry.

resolving the gap is an assessment of the differences Therefore, it is important that U.S. NPPs using the SPRA between the SPRA supporting requirements (SRs) (i.e., to support their regulatory actions highlight the use and Part 5) between Addenda A and B including a discussion review of such method(s). The NRC staff uses the results of how use of Addendum B for each SR meets the intent of the peer-review and its established regulatory processes and technical basis in Addendum A (see Ref. 12 for an (e.g., regulatory audit process) to identify and determine example). An assessment of the differences has shown the acceptability of such method(s). Formal approval of that for the majority of the SRs in Part 5, Addendum B newly-developed methods on a generic basis requires the either encompasses or is similar to the corresponding use established regulatory processes such as the topical requirements in Addendum A. However, the assessment report review process. Use of NEI 12-13 along with the of the differences in the requirements related to certain clarifications and qualifications from the NRC staffs SRs in Part 5 of the two Addenda cannot be performed on acceptance is expected to explicitly document the use of a generic basis and requires a SPRA-specific assessment. method(s) new to the SPRA community by licensee and Guidance in the SPID on certain aspects of SPRA review of such methods by the peer-review process.

development results in more refined approaches to meeting the SRs in Addendum B as compared to NRC staff included specific comments on the use of Addendum A. the guidance in NEI 12-13 on in-process peer-reviews for SPRAs. The purpose of an in-process peer-review is to The NRC also accepted for use the Code Case to Part review individual technical elements of a SPRA (e.g., the 5 of the ASME/ANS PRA Standard with clarifications seismic hazard development) separately and prior to the and qualifications. The acceptance of the Code Case is not seismic plant response so that technical issues in those limited to the response to the 10 CFR 50.54(f) letter. elements (identified as finding level F&Os by the peer-Therefore, the Code Case, with the corresponding NRC reviewers) can be resolved prior to the integration of that comments, provides an alternative for development as technical element in the SPRA. The in-process review well as peer-review of SPRAs that would not require a was a flexibility sought by the SPRA practitioners to separate assessment against the SRs in Addendum A. account for the important role played by the seismic hazard development and the fragility development in the III.C. SPRA Peer-Review Process and Guidance SPRA. The comments provided by the staff are intended to ensure that the in-process peer-review is exercised The currently available guidance for performing a consistent with the endorsed and accepted peer-review peer-review of a SPRA is contained in NEI 12-13. NRC process, which has been extensively used to support staff provided certain key comments on that document in regulatory activities.

a letter (Ref. 13). Subsequently, the NRC staff accepted the use of NEI 12-13, with clarifications and The NRC staffs reviews have encountered a single qualifications (Ref. 14). Several of the staffs occurrence of peer-review performed in a manner where clarifications and qualifications of NEI 12-13 are similar the hazard technical element was reviewed separately to those in Ref. 13. Therefore, it is important that the from the fragility and plant response technical elements.

NRC staffs comments are included in the performance of In that case, as opposed to the in-process type of peer-the peer-review of SPRAs. review discussed in Refs. 10 and 11, the peer-review of the hazard technical element was finalized (i.e., not considered interim). Therefore, a final peer-review was PSA 2019, Charleston, SC, April 28-May 3, 2019 366

not deemed necessary. However, due to the inter-play assumptions, and therefore, sensitivities would meet the between the different technical elements, the use of definition of key assumption according to RG 1.200, distinct peer-reviews resulted in finding level F&O Revision 2. Further, such sensitivities would be remaining open after the closure review. Based on that performed relative to the base SPRA and may not be experience, it appears that that the in-process or distinct directly applicable to the application being supported by peer-reviews may not result in the level of flexibility the SPRA (e.g., an assumption and relative sensitivity that originally expected for such reviews. does not impact the risk metrics but impacts the change in risk).

The guidance in NEI 12-13 cites walkdowns performed by the peer-review team to be an important part of the peer-review of SPRAs. The NRC staffs IV. CONCLUSIONS experience has been that plant walkdowns performed by the peer-review team are extremely valuable in The NRC staff has recently completed review of confirming important modeling details as well as several submittals, which include information related to assumptions used in the SPRA development. In addition, acceptability of SPRAs that support licensing and other NRC staff observations have also noted the peer-review regulatory actions. Several lessons learned from such team identifying technical issues from their walkdown. reviews have been presented including lessons related to NRC staff considers walkdowns performed as part of the acceptability of PRA models that are used as the base SPRA peer-reviews to be a crucial element of performing for the SPRAs, use of acceptable peer-review processes peer-reviews, the results of which could be used in for SPRAs, and disposition of key assumptions and regulatory decision making. sources of uncertainty in SPRA.

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