ML18011A396
| ML18011A396 | |
| Person / Time | |
|---|---|
| Issue date: | 01/12/2018 |
| From: | Gilbertson A F NRC/RES/DRA |
| To: | Houman Rasouli Office of Nuclear Regulatory Research |
| A. Gilbertson 415-1541 | |
| References | |
| Download: ML18011A396 (5) | |
Text
UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 January 12, 2018
MEMORANDUM TO: Houman Rasouli, Branch Chief Performance and Reliability Branch Division of Risk Analysis Office of Nuclear Regulatory Research
FROM: Anders Gilbertson, Reliability and Risk Analyst
/RA/ Performance and Reliability Branch Division of Risk Analysis Office of Nuclear Regulatory Research
SUBJECT:
PUBLIC MEETING ON PROPOSED SCREENING CRITERIA AND DEFINITIONS OF RISK-SIGNIFICANCE FOR THE NEXT EDITION OF THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS AND AMERICAN NUCLEAR SOCIETY LEVEL 1/LARGE EARLY RELEASE FREQUENCY PROBABILISTIC RISK ASSESSMENT STANDARD On November 17, 2017, the Nuclear Regulatory Commission (NRC) staff held a public meeting to discuss the proposed screening criteria and definitions of risk significance for the next edition of the American Society of Mechanical Engineers (ASME) and American Nuclear Society (ANS)
Level 1/Large Early Release Frequency (LERF) probabilistic risk assessment (PRA) standard. External stakeholder participants included representat ives from the ASME/ANS Joint Committee on Nuclear Risk Management (JCNRM), the Pre ssurized-Water Reactor Owners Group, the Nuclear Energy Institute, Arizona Public Service, NuScale Power, Dominion Energy, and Scientech. The meeting announcement and agenda was made publicly available on November 9, 2017, in the NRC's Agencywide Document Access and Management System (ADAMS) under accession number ML17320B104. The following is a summary of the presentations and discussions that occurred.
Members of the ASME/ANS JCNRM gave a presentation to provide contextual information on the development of a screening criteria table and revised definitions of terms related to risk-significance that are proposed for the next in edition of the ASME/ANS Level 1/LERF PRA
standard. The NRC staff provided the following points of feedback.
Regarding the Proposed Screening Criteria Table
- Additional clarification should be provided regarding the application of the cumulative screening criterion. As written, the cumulative screening criterion can be interpreted as applying only to the total risk of screened items for a specific type of risk contributor. However, the cumulative screening criterion value should be applied to the total combined risk of all types of screened PRA items (i.e., hazard groups, initiating events, CONTACT: Anders Gilbertson (301) 415-1541 accident sequences, etc.). The logical scheme for applying the screening criteria within the context of screening a given PRA item and across the entire PRA screening analyses as a whole should be explicitly described both in the narrative text preceding the screening criteria table and in the table itself.
- It is important for users of the PRA standard to consider that some PRA items screened out of the base PRA may need to be included in the PRA model for a specific risk-informed regulatory activity. Additionally, the suitability of the screening criteria needs to be assessed for all applications. For example, the suitability of the accident sequence screening criteria should be assessed as applied in self-approval applications related to National Fire Protection Association (NFPA) standard NFPA 805.
- Consistency between the screening values across the different types of PRA items should be confirmed to ensure that screening under one type of PRA item is not substantially more permissive than another.
- A clear objective basis for determining whet her an analysis is demonstrably conservative should be provided in the standard given the pervasive use of that qualifier and the reliance on that characterization to screen out PRA items.
- Regarding the accident sequence screening, Column (c) includes the phrase "exceeds this screening criterion;" however, the screening criterion that the pronoun "this" is referring to needs to be clarified. Additionally, it should be made clear how one would determine that group of sequences has similar characteristics.
- A non-mandatory appendix would be very helpful for providing the thinking behind and context for the proposed screening criteria framework. Regarding the Revised Definitions of Terms Related to Risk-Significance
- The revised definitions should be supplemented with additional discussion as they are currently highly subjective and could preclude identification of potentially risk significant equipment or actions. For example, objective definitions of the terms "realistic" and "insight" should be provided to give a more complete understanding of whether a structure, system, or component or action is potentially risk significant. As another example, when considering two cut sets that provide the same risk insight but have different frequencies, the lower frequency cut set may be considered not to be risk-significant when it otherwise should be.
- The revised definitions of risk-significance are based largely on the level of analysis that has been performed for a given contributor (i.e., a realistic representation), which could cause risk-significant PRA items not to be ident ified. For example, a contributor that is intentionally represented in the PRA in a conservative manner may not be characterized as risk-significant by virtue of the fact that the contributor was not realistically represented.
- In general, risk-significance should be assessed quantitatively to provide a more objective basis for potential screening of PRA items. For example, the revised definitions could focus more on using different quantitative values of risk-importance measures (e.g., Fussell-Vesely, risk achievement worth) based on the characteristics of the overall risk profile (e.g., peaky versus flat profile).
- Given the subjective nature of the definitions related risk-significance, supporting requirements may need to be revised to include requirements on providing justification for PRA items that are judged not to be risk-significant.
Enclosures:
- 1. List of Meeting Attendees
- 2. Meeting Presentation (ADAMS Accession No. ML17320A207)
- 3. Handout on Screening Criteria (ADAMS Accession No. ML17320A208)
ML18011A396 OFFICE RES/DRA/PRB RES/DRA/PRB NAME A. Gilbertson H. Rasouli (A. Gilbertson for)DATE 01/12/18 01/12/18 LIST OF ATTENDEES Public Meeting to Discuss Proposed Screening Criteria and Definitions of Risk-Significance for the Next Edition of the ASME/ANS Level 1/LERF PRA Standard November 17 th, 2017 9:00 a.m. - 12:00 p.m. Two White Flint North, Room 06A01 11545 Rockville Pike, Rockville, MD Name Organization Mary Drouin NRC/RES/DRA Eryk Tunshi NRC/RES/DRA Hiroki Watanabe NRC/RES/DRA Kevin Coyne NRC/RES/DRA Anders Gilbertson NRC/RES/DRA Michelle Hayes NRC/NRO/DSRA Tony Nakanishi NRC/NRO/DSRA Courtney St. Peters NRC/NRR/DRA Sara Lyons NRC/NRR/DRA JS Hyslop NRC/NRR/DRA Shilp Vasavada NRC/NRR/DRA Mehdi Reisi Fard NRC/NRR/DRA Tom Hook Arizona Public Service Paul Amico ASME/ANS JCNRM Raymond Schneider ASME/ANS JCNRM Robert Budnitz ASME/ANS JCNRM Allen Moldenhauer Dominion Energy Victoria Anderson Nuclear Energy Institute Kevan Griffith NuScale Power Cindy Williams NuScale Power Roy Linthicum PWROG Jana Bergman Scientech
ASME = American Society of Mechanical Engineers ANS = American Nuclear Society DRA = Division of Risk Analysis (RES)/Division of Risk Assessment (NRR)
DSRA = Division of Safety Systems, Risk Assessment, and Advanced Reactors JCNMR = Joint Committee on Nuclear Risk Management NRO = Office of New Reactors NRR = Office of Nuclear Reactor Regulation PWROG = Pressurized-Water Reactor Owners Group RES = Office of Nuclear Regulatory Research