Summary of the 20th Technical Advisory Committee (TAC) Meeting
Date:
Place:

May 27(Monday)-30(Thursday), 2024
Nuclear Risk Research Center (NRRC),
Central Research Institute of Electric Power Industry

Participants:
TAC:
Mr. Stetkar (Chair), Mr. Afzali, Dr. Chokshi, Mr. Miraucourt, Dr. Takada,and Dr. Yamaguchi
NRRC:
Dr. Apostolakis (Director), andResearch staff of the Nuclear Risk Research Center
Proceedings:
In the 20th Technical Advisory Committee meetings, the following issues werereviewed:• “Overview of NRRC Research Plan for FY2025 –Risk-informed Decision-making (RIDM)
Promotion–”• “Overview of NRRC Research Plan for FY2025 –Risk Assessment Research–”• “Overview of NRRC Research Plan for FY2025 –External Natural Event Research–”• “Revision of the Draft Guidelines for Risk-Informed On-Line Maintenance (OLM)” *• “Collection of Japanese Industry Equipment Failure Data and Quantification of GenericEquipment Failure Rates” *• “Methods for Evaluating Correlated Seismic Fragilities” *The following meetings were held closed.
• “Draft Guidelines for Risk-Informed Containment Vessel Leak Rate Testing(CVLRT)” *• “Experience from the Noto Peninsula Earthquake on January 1, 2024” *• “Briefing on Good Practices for Risk-Informed Decision-Making (RIDM)”*
Note: The meetings of the agenda items marked with an asterisk (*) were attended online byelectric power companies.
Monday, May 27, 2024
Topic 1. Overview of NRRC Research Plan for FY2025 –RIDM Promotion–
TAC’s advice and comments are as follows.
• Full-scope PRA
The goal of the Japanese nuclear industry, which was established about 10 years ago, isto develop a good quality, full-scope Level 2 PRA that is consistent with the international
state of practice. Level 1 and 2 PRA for various internal and external hazards need to be
performed in an integrated manner for all plant states. TAC has already issued lettersrecommending the development of full-scope Level 1 and Level 2 PRA methods based on1

the PRA models of Ikata Unit 3 and Kashiwazaki-Kariwa Unit 7.
• Peer reviews and reviewer training
The NRRC needs to train Japanese reviewers capable of strict reviews. The U.S. reviewer
training processes set certification requirements. Similarly, the Japanese peer review
guidance needs to include reviewer certification guidance.
In the early 90s, when PRA peer reviews started in the U.S., peer reviews wereconducted mostly by consultants who were involved in model and method development
at that time. The NRRC may want to develop experience with PRA model systems,
technical skills, and review methods to support "perfect" independent peer reviews;
however, no guidance, methods, or reviewers are perfect. Japan has 40 years of PRA
experience. Consultants and manufacturers could be readily available as experts for thefirst peer reviews, while additional independent experts are being trained.
Topic 2. Overview NRRC Research Plan for FY2025 –Risk Assessment Research–
TAC’s advice and comments are as follows.
• Fire PRA
When using deep learning technology for fire propagation analysis, be careful not tomake up artificial “information” that may be misleading. Fire PRA needs a lot of humanjudgments, which are difficult to teach AI.
• Spent Fuel (SFP) PRA
For the SFP-PRA development, the researchers should clarify how the time difference
from cladding failure until the start of fuel melting affects the PRA models for accident
progression. If the impact is small and the risk-benefit is marginal, the research and tests
on cladding burst phenomena should be given a lower priority.
• Human Reliability Analysis (HRA)
Human reliability data should be collected consistently throughout the industry
irrespective of BWRs and PWRs.
• Multi-hazard PRA
Research on different hazard combinations should be promoted, not only onearthquakes and tsunamis.
Topic 3. Overview NRRC Research Plan for FY2025 –External Natural Event Research–
TAC’s advice and comments are as follows.
•Seismic PRA
It is important to have a well-documented report of the model plant seismic PRA by theend of Phase 2 of the project. A milestone should be set for issuing the report by the endof FY2024 or the beginning of FY2025.
•Tsunami PRA
The development of a tsunami PRA implementation guide should be included in the2roadmap and study implementation items. Issuing the guide should be listed as amilestone around FY2025.
•Tornado and High-Wind PRA
The development plan of a tornado PRA for domestic plants in FY2025 should include amilestone for issuing a guide.
•Volcanic ash-fall PRA
It is necessary for the PRA to examine how effective the filter replacement work isagainst volcanic ash-fall impacts on the air intake systems. The actions to replace orclean filters should be included in the human reliability analyses (HRA). The HRA should
account for the ash-fall density and duration, the number of filters, and the needed
manpower. It is also necessary to assess long-duration volcanic ash-fall hazards because
the ash-fall phenomenon may continue for several weeks.
Tuesday, May 28, 2024
Topic 4. Revision of the Draft Guidelines for On-Line Maintenance (OLM)
TAC’s advice and comments are as follows.
In discussions on OLM with the NRA, it is important to get the NRA’s approval toperform planned maintenance at power. Specifically, it is important to get permission tocarry out maintenance that is not specified in the Technical Specifications using risk
information.
The revised guidelines additionally incorporate multiple OLM outages of individualcomponents and simultaneous OLMs of multiple systems during the same operating
cycle. What brought these additions to the guidelines?
It may be more prudent to first implement the scope of OLM as described in the initial
guidance, and obtain useful industry and NRA experience before the scope is expanded.
TAC will review the revised guidelines for their technical adequacy.
Tuesday, May 28, 2024
Topic 5. Collection of Japanese Industry Equipment Failure Data and Quantification ofGeneric Equipment Failure Rates
TAC’s advice and comments are as follows.
Regarding the screening out rules e and f, let’s assume that a component has two failuremodes, failure to open and failure to close. Some failure mechanisms cause both failuremodes. In this case, screening by failure modes can only collect either one of the modes.
Plant-specific failure rates are obtained from Bayesian updates of the industry genericfailure rates using plant-specific failure data. When the methodology is used to derive
the industry data, you should check if the data from a single "outlier" plant has asignificant effect on the generic failure rates. One or two outliers will not affect theestimated failure rates for a large plant population with many years of data; however,3they can significantly affect the estimation for a small plant population such as those inJapan (27 plants for 7 years).
When the industry pooled data and Jeffreys’ non-informative prior are used in theBayesian update, the difference in the estimated failure rates between zero failures andone failure is a factor of three, and that between zero failures and two failures is a factorof five. If you have a lot of failure data, as observed in the U.S. for 100 units in morethan 25 years, the effect of the Jeffreys’ non-informative prior on the failure rateestimation is reduced.
Wednesday, May 29, 2024
Topic 6. Methods for Evaluating Correlated Seismic Fragilities
TAC’s advice and comments are as follows.
The seismic fragility evaluation methodology considering the seismic correlations, which
was applied to SRVs in this study, should be developed to also apply to other types ofequipment and general PRA applications.
The methodology must be consistent with the multi-unit PRA methodology.
Now that the methodology has been developed for calculating the seismic fragilities
considering correlations, it is important to study how to determine correlation
coefficients.
Thursday, May 30, 2024
Exit Meeting [Closed]4