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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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.

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) licensees may need to quantitatively address the risk associated with seismic events for implementing certain regulatory actions. The NRC staff has recently completed review of several submittals, which include information related to acceptability of seismic probabilistic risk assessments (SPRAs) that support regulatory actions.

Examples of those actions include adopting the program to risk inform categorization and treatment of structures, systems and components, adopting risk-informed completion time program, and responding to Near-Term Task Force (NTTF) Recommendation 2.1 "Seismic." This paper presents some lessons learned from the NRC staffs review of the technical acceptability of those SPRAs.

These lessons are related to the acceptability of PRA models that are used as the base for the SPRAs, use of acceptable peer-review processes, and disposition of key assumptions and sources of uncertainty in SPRA.

I. INTRODUCTION The NRC staff has reviewed several licensing and other regulatory actions that are supported by information related to acceptability of SPRAs. The staffs review of the technical acceptability of SPRAs has resulted in several lessons learnt including the acceptability of PRA models that are used as the base for the SPRAs, use of acceptable peer-review processes for SPRAs, and disposition of key assumptions and sources of uncertainty in SPRA. These lessons learnt are discussed in this manuscript.

II. SEISMIC RISK CONSIDERATIONS FOR U.S.

NUCLEAR POWER PLANT The guidance in General Design Criterion (GDC) 2 (Ref. 1) necessitates nuclear power plants (NPPs) in the United States to be designed to withstand credible natural and manmade hazards, including seismic events (i.e.,

earthquakes). In addition, certain SSCs have to be designed such that they can operate and remain operational at the Safe Shutdown Earthquake (SSE),

which represents the design basis earthquake for each nuclear power plant. However, since the original design of many NPPs, various efforts to assess the impact of seismic events on these plants, including the quantitative risk and risk insights, have been undertaken, including:

Unresolved Safety Issue A-46, Seismic Qualification of Equipment in Operating Plants, 1980 (Ref. 2),

Individual Plant Examination of External Events (IPEEE), 1991 (Ref. 3),

Generic Issue 199, Implications of Updated Probabilistic Seismic Hazard Estimates in Central and Eastern United States, 2005 (Ref.

4), and

Fukushima Dai-ichi Response, Near Term Task Force Recommendation (NTTF) 2.1, 2012 (Ref.

5).

Although these efforts have resulted in an improved understanding of seismic risk at U.S. NPPs and a number of safety improvements, the seismic risk assessment methodologies used in response to these programs have varied. The methodology and the assessments used were not necessarily maintained beyond initial completion of the effort or program.

Seismic probabilistic risk assessments (SPRAs) are an approach to identify insights and quantify the metrics related to the seismic risk at NPPs. The SPRAs can also provide realistic representation of the seismic risk profile at NPPs. SPRAs have evolved over the last few decades and are garnering increased safety and regulatory use as the technology matures.

Some U.S. NPPs have developed SPRAs in response to recent regulatory actions stemming from the Fukushima Dai-ichi response (specifically, NTTF recommendation 2.1). These SPRAs have been developed and peer-reviewed against the 2013 version of the American Society of Mechanical Engineers (ASME)/American Nuclear Society (ANS) PRA Standard (also known as Addendum B). Those SPRAs are used to provide information about the seismic risk at U.S. NPPs PSA 2019, Charleston, SC, April 28-May 3, 2019 364

due to the re-evaluated seismic hazard and to determine if additional regulatory action is necessary. In addition, U.S.

NPPs have also utilized their SPRAs for licensing activities such as (i) supporting the categorization of structures, systems, and components (SSCs) in the implementation of Part 50.69 to Title 10 of the Code of Federal Regulations, Risk-informed categorization and treatment of structures, systems, and components for nuclear power reactors, (10 CFR 50.69; Ref. 6), and (ii) determining risk-informed completion times (RICTs; Ref.

7). Some NPPs that do not possess a detailed quantitative approach, for determining the seismic risk, such as a SPRA, have proposed conservative approaches for determining the seismic risk for use in determining RICTs.

The NRC staff has reviewed the technical acceptability of SPRAs in the context of various regulatory actions. Several lessons have been learned from such reviews which have been and will continue to be utilized by the staff for current as well as future reviews.

III. KEY LESSONS LEARNED FROM TECHNICAL ACCEPTABILITY REVIEWS OF SPRAs FOR REGULATORY ACTIONS This section will discuss the key lessons learned from the review of the technical acceptability of SPRAs supporting various regulatory actions.

III.A. Acceptability of Base Internal Events PRA Model for Development of SPRA SPRAs are usually built using the internal events PRA (IEPRA) as the base. The ASME/ANS PRA Standard, including the 2009 version endorsed by the NRC in RG 1.200, Revision 2 (Ref. 8), allows for the use of an ad-hoc SPRA developed from scratch. However, the NRC staff has not encountered such ad-hoc SPRAs to date because of (i) the availability of peer-reviewed IEPRAs which provides a

technically defensible foundation as well as resource savings for developing SPRAs, and (ii) expectation for the use of IEPRAs as the base for SPRAs in the Electric Power Research Institute (EPRI) report 1025287, Screening, Prioritization and Implementation Details (SPID) for the Resolution of Fukushima Near-Term Task Force Recommendation 2.1:

Seismic (SPID; Ref. 9). The SPID was endorsed by the NRC as acceptable guidance for use in developing SPRAs to respond to the 10 CFR 50.54(f) letter. The response to the 10 CFR 50.54(f) letter is the primary driver for the recent development of high-quality SPRAs by U.S. NPPs.

IEPRAs used as the base for SPRA development often have finding level Facts and Observations (F&Os) from peer-reviews (i.e., peer-reviews of the IEPRA). The resolution of the finding level F&Os for the IEPRA can impact the SPRA in the following ways:

Certain finding level F&Os may not have been resolved to meet the basis of the finding but can be shown to not impact certain applications of the IEPRA model.

However, the same conclusion may not be reached for the SPRA.

Finding level F&Os may have been resolved to meet the basis of the finding but, based on the PRA development, the resolutions may not have been propagated to the SPRA.

Finding level F&Os may have been resolved to meet the basis of the finding but, based on the PRA development, the resolutions may be different from those that were propagated to the SPRA at the time of its development.

Based on the above considerations, it is important to ensure the technical acceptability of the IEPRA that is used as the base for SPRAs. The staffs reviews of SPRAs have found that available guidance on the peer-review of SPRAs contained in Nuclear Energy Institute (NEI) 12-13, External Hazards PRA Peer Review Process Guidelines (Ref.

10) does not clearly address determination of the impact of the disposition of IEPRA finding level F&Os on the SPRA. The NRC staff has used publicly available information submitted by U.S. NPPs to the NRC as well as established regulatory processes to determine that the above considerations are appropriately factored into the development as well as the technical acceptability of SPRAs. Future refinements to the PRA Standard as well as the peer-review guidance is expected to result in clear and consistent review of the IEPRA technical acceptability for use as the base for SPRAs by the U.S. NPPs as well as the peer-reviewers.

Part 5 of the ASME/ANS PRA Standard requires SPRAs to explicitly consider and, if necessary, include seismically-induced fires and floods. It is common practice to use the internal fire and internal flood PRAs, respectively, to support and inform the consideration of seismically-induced fires and floods in SPRAs. Therefore, depending on the level of information derived from the internal fire and internal flood PRAs, it can be important to determine the technical acceptability as well as impact of finding level F&Os for those PRAs on the SPRA.

III.B. Use of Addendum B of the PRA Standard for SPRA Development As noted previously, the response to the 10 CFR 50.54(f) letter is the primary driver for the recent development of SPRAs by U.S. NPPs. The SPID was 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) letter. The SPID cites Part 5 (the portion related to SPRA) of the 2013 version of the ASME/ANS PRA Standard (also known as Addendum B to the PRA Standard). RG 1.200, Revision 2, endorses the 2009 version of ASME/ANS PRA Standard (also known as Addendum A to the PRA Standard). Further, Addendum B, including Part 5 of that Addendum, has not been endorsed by the NRC for use in licensing activities (e.g.,

adoption of 10 CFR 50.69 and RICTs). The NRC also documented its reservations on the use of Addendum B (Ref. 11). Therefore, a gap exists when SPRAs that are developed to respond to the 10 CFR 50.54(f) letter are submitted to support licensing actions.

An approach that the staff has found to be effective in resolving the gap is an assessment of the differences between the SPRA supporting requirements (SRs) (i.e.,

Part 5) between Addenda A and B including a discussion of how use of Addendum B for each SR meets the intent and technical basis in Addendum A (see Ref. 12 for an example). An assessment of the differences has shown that for the majority of the SRs in Part 5, Addendum B either encompasses or is similar to the corresponding requirements in Addendum A. However, the assessment of the differences in the requirements related to certain SRs in Part 5 of the two Addenda cannot be performed on a generic basis and requires a SPRA-specific assessment.

Guidance in the SPID on certain aspects of SPRA development results in more refined approaches to meeting the SRs in Addendum B as compared to Addendum A.

The NRC also accepted for use the Code Case to Part 5 of the ASME/ANS PRA Standard with clarifications and qualifications. The acceptance of the Code Case is not limited to the response to the 10 CFR 50.54(f) letter.

Therefore, the Code Case, with the corresponding NRC comments, provides an alternative for development as well as peer-review of SPRAs that would not require a separate assessment against the SRs in Addendum A.

III.C. SPRA Peer-Review Process and Guidance The currently available guidance for performing a peer-review of a SPRA is contained in NEI 12-13. NRC staff provided certain key comments on that document in a letter (Ref. 13). Subsequently, the NRC staff accepted the use of NEI 12-13, with clarifications and qualifications (Ref.

14).

Several of the staffs clarifications and qualifications of NEI 12-13 are similar to those in Ref. 13. Therefore, it is important that the NRC staffs comments are included in the performance of the peer-review of SPRAs.

One of the key comments provided by the staff as part of the acceptance of NEI 12-13 is the need to determine the technical acceptability of the IEPRA that is used as the base for developing the SPRA as well as other hazard PRAs (e.g., internal fire and internal flood) used to support the SPRA development. The discussion of lessons learned on this topic are discussed in Section III.A of this manuscript.

Another key comment provided by the staff as part of the acceptance of NEI 12-13 is the explicit documentation of method(s) used in the SPRA development that are not state-of-practice methods and are new to the industry (i.e.,

SPRA practitioner community). Prior to the NRC staffs comments, there was no guidance on documenting the review of method(s) that were new to the industry.

Therefore, it is important that U.S. NPPs using the SPRA to support their regulatory actions highlight the use and review of such method(s). The NRC staff uses the results of the peer-review and its established regulatory processes (e.g., regulatory audit process) to identify and determine the acceptability of such method(s). Formal approval of newly-developed methods on a generic basis requires the use established regulatory processes such as the topical report review process. Use of NEI 12-13 along with the clarifications and qualifications from the NRC staffs acceptance is expected to explicitly document the use of method(s) new to the SPRA community by licensee and review of such methods by the peer-review process.

NRC staff included specific comments on the use of the guidance in NEI 12-13 on in-process peer-reviews for SPRAs. The purpose of an in-process peer-review is to review individual technical elements of a SPRA (e.g., the seismic hazard development) separately and prior to the seismic plant response so that technical issues in those elements (identified as finding level F&Os by the peer-reviewers) can be resolved prior to the integration of that technical element in the SPRA. The in-process review was a flexibility sought by the SPRA practitioners to account for the important role played by the seismic hazard development and the fragility development in the SPRA. The comments provided by the staff are intended to ensure that the in-process peer-review is exercised consistent with the endorsed and accepted peer-review process, which has been extensively used to support regulatory activities.

The NRC staffs reviews have encountered a single occurrence of peer-review performed in a manner where the hazard technical element was reviewed separately from the fragility and plant response technical elements.

In that case, as opposed to the in-process type of peer-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 between the different technical elements, the use of distinct peer-reviews resulted in finding level F&O remaining open after the closure review. Based on that experience, it appears that that the in-process or distinct peer-reviews may not result in the level of flexibility originally expected for such reviews.

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 experience has been that plant walkdowns performed by the peer-review team are extremely valuable in confirming important modeling details as well as assumptions used in the SPRA development. In addition, NRC staff observations have also noted the peer-review team identifying technical issues from their walkdown.

NRC staff considers walkdowns performed as part of SPRA peer-reviews to be a crucial element of performing peer-reviews, the results of which could be used in regulatory decision making.

III.D. Key Assumptions and Sources of Uncertainty RG 1.200, Revision 2, states that for each application that calls upon this regulatory guide, the applicant identifies the key assumptions and approximations relevant to that application. This will be used to identify sensitivity studies as input to the decision-making associated with the application. Further, Section 4.2 of RG 1.200 states that [t]hese assessments provide information to the NRC staff in their determination of whether the use of these assumptions and approximations is appropriate for the application, or whether sensitivity studies performed to support the decision are appropriate. RG 1.200, Revision 2, defines the terms key assumption and key source of uncertainty. Therefore, identification and disposition of key assumptions and sources of uncertainty for the SPRA in the context of the application the SPRA is supporting is important.

An effective approach for the identification of key assumptions and sources of uncertainty is to compile all the assumptions used in the development of the SPRA across the different technical elements (i.e., hazard, fragility, and plant response) and use the definitions in RG 1.200, Revision 2, to determine which assumption meets the definition of key assumption. The disposition of the identified key assumptions can then be performed using qualitative or quantitative (i.e., sensitivity studies) means on an application-specific basis. Several sensitivity studies are usually performed as part of the SPRA development and are reviewed by the peer-review process to determine the impact of various modeling assumptions.

However, it is important to recognize that not all of those assumptions, and therefore, sensitivities would meet the definition of key assumption according to RG 1.200, Revision 2. Further, such sensitivities would be performed relative to the base SPRA and may not be directly applicable to the application being supported by the SPRA (e.g., an assumption and relative sensitivity that does not impact the risk metrics but impacts the change in risk).

IV. CONCLUSIONS The NRC staff has recently completed review of several submittals, which include information related to acceptability of SPRAs that support licensing and other regulatory actions. Several lessons learned from such reviews have been presented including lessons related to the acceptability of PRA models that are used as the base for the SPRAs, use of acceptable peer-review processes for SPRAs, and disposition of key assumptions and sources of uncertainty in SPRA.

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