Technical Advisory Committee of the Nuclear Risk Research Center
Central Research Institute of Electric Power Industry
1-6-1 Otemachi, Chiyoda-ku, Tokyo, 100-8126 JapanJune 6, 2024
Dr. George Apostolakis
Director, Nuclear Risk Research Center
Central Research Institute of Electric Power Industry
1-6-1 Otemachi, Chiyoda-ku
Tokyo, 100-8126 JapanSUBJECT:PRELIMINARY NRRC RESEARCH PLAN FOR FISCAL YEAR 2025

Dear Dr. Apostolakis:
During the 20th meeting of the Technical Advisory Committee of the Nuclear Risk
Research Center (NRRC), May 27-31, 2024, we met with the NRRC staff to review
the proposed preliminary research plan for fiscal year 2025. The purpose of ourreview was to provide comments on the technical merits of the research plan and its
relevance for supporting the NRRC's current mission.
CONCLUSIONS AND RECOMMENDATIONS
1. The overall scope of the preliminary research plan for fiscal year 2025 and thetechnical objectives of the individual projects within each major research area
remain consistent with the NRRC short-, intermediate-, and long-term goals.
2. During our review, we identified four individual research activities that meritadditional attention in the preliminary plans for fiscal year 2025 and subsequentyears. Our recommendations for those activities are summarized in theDiscussion section of this report.
BACKGROUND
One of the most important objectives of the research plan is to present the technicalcontext of the research needs, including the rationale for prioritization and scope ofthe research, current state of knowledge, and potential contributions and significanceof the research to the goals of the center. Our review of the preliminary researchplan focused on the objectives of each research project and its supporting tasks, thetechnical relationships and relative priorities among those activities, and any major
needs for additional research. We did not review the technical details of individual
research activities or their completion milestones, except as needed to understandhow those activities are integrated throughout the plan. We will comment separately-1-

on the technical elements of individual research projects in our future detailed
reviews of those projects.
DISCUSSION
During this review, we were briefed on continuing and planned projects in eachresearch area, the major technical tasks in each project, the current status of eachtask, known or potential problem issues, and the estimated schedule for completionof each task. The overall scope of research for fiscal year 2025 and the technicalobjectives of the individual projects within each major research area remain
consistent with the NRRC short-, intermediate-, and long-term goals.
Based on our review of the preliminary research plans and our discussions with theresearch teams, we offer the following recommendations for further assessments offour individual research activities.
(1) Use of Knowledge from Pilot Plant PRA Peer Reviews
A central NRRC research activity involves the development of methods andguidance to support good quality full-scope probabilistic risk assessments (PRAs) at
all Japanese nuclear power plants. Ikata Unit 3 and Kashiwazaki-Kariwa Unit 7 are
the industry's pilot plants to demonstrate the PRA models and methods to evaluate
the risk from internal initiating events. Groups of international experts are conducting
independent peer reviews of both studies to evaluate their consistency with thetechnical capability requirements in the ASME/ANS PRA Standards. The peer
review process provides two important types of knowledge that improve the technicalquality of a PRA and its use to support risk-informed decision-making (RIDM)
applications.
i.

The review identifies areas where the PRA team has not adequately
implemented elements of the analyses according to the established technicalstandards, methods, and guidance. These findings are addressed by theteam through refinements of the PRA models and improvements to theirsupporting analyses.

ii.

The review identifies an area where a risk contribution is not adequately
addressed, due to lack of a consensus methodology or a technical deficiency
in the applied methodology. These findings require improvements in theapplied methods or, in some cases, development of new methods.

The NRRC research teams should improve their use of the knowledge from the pilot
plant PRA peer reviews to help refine the priorities and the focus of specific research
activities. In some cases, the review findings may identify needs for new methods orguidance. In other cases, the findings may help the NRRC teams to identify specificelements of a methodology or guidance which should receive more attention. Bothtypes of enhancements are needed to support the development of high quality PRAs
that provide a better platform for RIDM.

-2-(2) Human Reliability Data Collection
One of the major human reliability analysis (HRA) research activities involves theinvestigation and development of an improved methodology to quantify human error
probabilities (HEPs). It is very important that a consistent HRA methodology is used
for all Japanese nuclear power plant PRAs. This is an essential element of theoverall risk assessment process. Experience has shown that the use of diverse HRA
methods can be a source of technical inconsistency for plant-specific and industrywide risk-informed applications. Furthermore, the use of different methods can also
result in variations in the risk profiles which are not associated with differences infundamental human performance.
We have not yet reviewed details of the proposed HEP quantification methodology.
However, we understand that it relies on the use of human performance data thatare collected from the utilities' simulator training programs. It is very important thatthe supporting data should be a comprehensive compilation of the Japanese training
experience. Furthermore, the scope of the data that are collected at each plant, andthe data format, should be consistent with its intended use in the quantification
methodology. The HRA research team should ensure that the data collection
methods and guidance are clearly understood by the training personnel at each plant.
The team should also audit the data collection activities at each utility to ensure thatthey are being performed in a comprehensive and consistent manner.
(3) Seismic Fragility Correlation Methodology
Experience from Japanese and international PRAs has shown that coincident
failures of similar equipment are often an important contribution to the overall risk
from seismic events. The NRRC research program includes several continuing
activities that are developing methods and guidance for more realistic evaluations ofseismic hazards and fragilities. The research plan also includes an activity todevelop methods and guidance for an evaluation of the risk from events that affectmultiple units at a site. The multi-unit PRA (MUPRA) research is a particularly
challenging activity, due to a general lack of international experience and practicalmethods for these complex analyses.
The effects from intra-unit and inter-unit coincident failures must be evaluated in aconsistent manner to quantify the risk from earthquakes that affect a single-unit site,earthquakes that result in damage to only one unit at a multi-unit site, andearthquakes that damage multiple units. To support these analyses for all Japanesesites, the research teams should ensure that consistent methods are developed toevaluate correlations among the structural and equipment fragilities for applications
in single-unit and multi-unit risk assessments.
(4) Risk from Volcanic Ash-Fall
The research plan includes activities to develop methods and guidance for anevaluation of the risk from volcanic ash-fall. Those activities include a probabilistic
analysis of the ash-fall hazard, an assessment of the plant vulnerabilities toconsequential damage from the deposition or ingestion of airborne ash and the-3-

intake of waterborne pumice, and a demonstration of the PRA methods and models
that are used to evaluate the risk.
Unlike most initiating events that are analyzed in a PRA, volcanic eruptions cancontinue for periods of several days or even weeks, with widespread dispersals oflarge amounts of ash. Volcanic ash can impact plant systems, structures, andcomponents, as well as impacting actions of operators. For example, ash
accumulation can affect the performance of safety systems, such as emergency
diesel generators, cooling water intakes, and ventilation systems. Other possible
impacts from ash-fall include:
Volcanic ash with humidity is electrically conductive and can cause short circuits
and electrical insulation failures.
Ash can cause contamination of plant surfaces and surroundings, potentiallyleading to radiation exposure risks.
Ash-fall incidents can challenge the adequacy of emergency preparedness andresponse plans.
Therefore, it is critical to develop a systematic evaluation of the impact on the plant
initial response, as well as the plant performance following the initial response. Thisevaluation should consider the ash-fall duration and intensity, as well as the plantresponse (both the design features and procedures) to ash-fall. To this end, datacollection and evaluation of these topics are critical for developing a systematicincorporation of ash-fall into the PRA to enable operators to enhance the resilienceof their facilities against volcanic hazards and ensure comprehensive risk
management in the face of this hazard. As part of their development of theprobabilistic ash-fall hazard analyses, the research team should compile data fromJapanese and international historical records regarding the durations of volcanic
eruptions and ash-fall accumulations.
We look forward to our continuing interactions with the NRRC research team toreview the overall research program and individual research projects, and to help theNRRC and the Japanese nuclear industry achieve their goals of comprehensive riskinformed decision-making.
Sincerely,John W. Stetkar
Chairman-4-

REFERENCES
1.

"NRRC Overview: Research Program for FY2025, Risk Informed Decision
Making Promotion Team," Presentation to NRRC Technical Advisory
Committee, May 27, 2024, Proprietary.

2.

"NRRC Overview: Research Program for FY2025, Risk Assessment,"
Presentation to NRRC Technical Advisory Committee, May 27, 2024,
Proprietary.

3.

"NRRC Overview: Research Program for FY2025, External Natural Events,"
Presentation to NRRC Technical Advisory Committee, May 27, 2024,
Proprietary.

4.

ASME/ANS RA-Sb-2013, Addenda to ASME/ANS RA-S-2008, "Standard forLevel 1 / Large Early Release Frequency Probabilistic Risk Assessment for
Nuclear Power Plant Applications," American Society of MechanicalEngineers, September 2013.

5.

ANS/ASME 58.22-2015, "Requirements for Low Power and Shutdown
Probabilistic Risk Assessment", American Society of Mechanical Engineers,
2015 (approved for trial use only).

6.

ASME/ANS RA-S-1.2-2014, "Severe Accident Progression and Radiological
Release (Level 2) PRA Standard for Nuclear Power Plant Applications forLight Water Reactors (LWRs)," American Society of Mechanical Engineers
and American Nuclear Society, January 2015 (version for trial use and pilotapplications).

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