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 Japan
February 16, 2021
Dr. George Apostolakis
Head, Nuclear Risk Research Center
Central Research Institute of Electric Power Industry
1-6-1 Otemachi, Chiyoda-ku
Tokyo, 100-8126 JapanSUBJECT:PROPOSED NRRC RESEARCH PLAN FOR FISCAL YEAR 2021

Dear Dr. Apostolakis:
We will remember the year 2020 as no other. As did individuals, families, andorganizations worldwide, the Nuclear Risk Research Center (NRRC) and its
Technical Advisory Committee (TAC) developed creative ways to adapt to the everchanging challenges and uncertainties. We commend the entire NRRC staff for theirefforts to continue progress on their important research during these very difficult
conditions.
Since we could not meet with your team in our usual format, we used an alternative
approach to conduct our review of the research plan for fiscal year 2021. In
November, the NRRC research teams provided us with presentations thatsummarize the projects in each major research area. We reviewed that material andprepared our individual comments and questions on specific topics, as we would
normally do before our meeting. In lieu of the active meeting discussions that clarify
our understanding of each topic, we sent those individual member comments andquestions to you for consideration by each research team. The researchers
provided detailed and thoughtful responses to each question. We then deliberated
on those responses and developed the Committee's consensus observations,conclusions, and recommendations that are provided in this letter report. The
purpose of our review was to provide comments on the technical merit of theresearch plan and its relevance for supporting NRRC's current mission.
Our experience from this effort has reinforced the vital importance of the dynamic
interactions during our face-to-face meetings. While the approach we used for thisreview achieved our basic objectives, the in-person technical exchanges provide
clarification and understanding that benefit each of us in ways that cannot be
accomplished through written questions and answers. We sincerely hope that we
can return to our normal meetings in 2021.

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CONCLUSIONS AND RECOMMENDATIONS
1. We did not identify any major gaps in the overall technical research plan for fiscal
year 2021.
2. The development of increasingly detailed computational models for specifichazards and damage mechanisms may lead to a belief in the numerical precisionof the results that is not justified for an evaluation of the risk from phenomena thathave inherently large uncertainties. Before more detailed analytical tools aredeveloped further, each research team should describe and document how
important sources of aleatory and epistemic uncertainty will be identified,characterized, and quantified as an integral part of the applied methods andmodels.
3. Most of the NRRC research activities have achieved a level of maturity thatallows practical demonstrations of how they are integrated into a plant-specificprobabilistic risk assessment (PRA). A fully-integrated PRA will demonstrate theimportance of a balanced assessment and understanding of risk and its
contributors. That integrated perspective provides practical insights for plantspecific risk-informed decision-making and risk management that cannot berealized through stand-alone examinations of individual issues. It also provides arisk-informed framework to organize priorities for focused research to further
improve specific methods and models. The currently-available methods andmodels for each NRRC research topic should now be included in the pilot plant
PRAs that are in progress for Ikata Unit 3 and Kashiwazaki-Kariwa Unit 7.
4. During our review, we identified a few individual research activities that meritadditional attention in the plans for fiscal year 2021 and subsequent years. Our
recommendations for specific activities are summarized in the Discussion sectionof this report. We have also issued a separate companion letter report on"Proposed NRRC Research on Selected Seismic Issues for Fiscal Year 2021."
That report contains more detailed discussions and recommendations for two
specific areas of the seismic research program.
BACKGROUND
Since 2014, the NRRC research has made important advances in the scientific andengineering state of knowledge about events, phenomena, and accident scenariosthat contribute to the risk from a nuclear power plant. That knowledge significantly
improves realism in the methods and models that are used to evaluate risk. It also
improves our understanding of risk and its contributors, and it supports confidence inthe scientific basis for each utility's risk management activities. Those benefits areachieved through implementation of the research in the framework of an integrated
probabilistic risk assessment (PRA) that provides a comprehensive and balanced
evaluation of the risk from all internal events, internal hazards, and external events.
The PRA is a vital tool to support effective risk-informed decision-making (RIDM)
programs and practices that focus on the most important sources of risk at eachnuclear power plant site. Therefore, it is essential that the scope and details of eachresearch project are carefully tailored to meet the utilities' needs for integrated risk

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research activities with the utilities' RIDM programs in a separate letter report.
One of the most important objectives of the research plan is to present the technicalcontext of the research needs, including the rationale, current state of knowledge,
and potential contributions and significance of the research to the goals of the center.
Our review of the research plan focused on the objectives of each research projectand its supporting tasks, the technical relationships and relative priorities among
those activities, and any major needs for additional research. We did not review thetechnical details of individual research activities or their completion milestones,
except as needed to understand how those activities are integrated throughout theplan. We will comment separately on the technical elements of individual research
projects in our future detailed reviews of those projects.
DISCUSSION
The overall scope of research and the technical objectives of the individual projects
within each major research area remain consistent with the NRRC short-,
intermediate-, and long-term goals. We did not identify any major gaps in the overall
technical research plan for fiscal year 2021. The discussion of Individual Research
Activities contains our recommendations for specific elements of a few research
projects.
Research Extensions
Several research projects are now focusing on refinements and extensions ofanalytical methods and models that have been developed during the six-year historyof the NRRC, often continuing from earlier research programs. We have noted a
distinct trend toward the development and use of increasingly detailed finite-element
models, thermal-hydraulic models, fire simulation models, etc. to evaluate specifichazards and damage mechanisms. Based on our own experience and our
discussions with the research teams, we understand that those tools typicallyprovide only "point-estimate" results that are determined by the input parameters andthe selected models for physical phenomena and functional interrelationships.
Because those results are derived from very detailed and computationally-intensive
calculations, they may lead to a belief in numerical precision that is not justified foran evaluation of the risk from phenomena that have inherently large uncertainties.
Explicit identification, characterization, and quantification of uncertainty is afundamental element of the risk assessment process. It is integral to understanding
the overall level of risk from a facility, its contributors, and how that risk can be
managed most effectively.
We have questioned the research teams about how the proposed analytical methods
quantify the effects from inherent uncertainties in the controlling parameters anduncertainties in the applied models, and how the results account for those
uncertainties. We have also noted that it is not appropriate to simply retrofit anominal uncertainty distribution around the "point-estimate" result. To appropriately
account for complex physical and functional interrelationships, the uncertainties must
be quantified as an integral part of the computation process. Thus, models and-3-analyses that do not provide a fully-integrated quantification of uncertainty are oflimited value to support modern full-scope risk assessments. In practice, simpler
models that facilitate a rigorous treatment of the uncertainties often provide moremeaningful and realistic support for risk assessment than the implied precision ofcomplex computations. Before more detailed analytical tools are developed further,each research team should describe and document how important sources ofaleatory and epistemic uncertainty will be identified, characterized, and quantified as
an integral part of the applied methods and models.
Research Applications
The Japanese industry is actively supporting the development of good quality PRAs
for two pilot plants: Ikata Unit 3 and Kashiwazaki-Kariwa Unit 7. These PRAs arevery important to the overall goals of the NRRC and the industry. They demonstrate
how current state-of-the-practice methods and models are implemented to achieve acomprehensive assessment of the plant-specific risk and its contributors. They also
provide important experience and lessons for PRA practitioners at all Japaneseutilities, as they update and extend their current models and analyses to achieve thedesired level of quality. The scope of each pilot project is currently focused primarilyon the development of Level 1 and Level 1.5 PRA models to evaluate the risk frominternal events that occur during full-power operations. We understand that the pilot
plants have also developed preliminary PRA models that evaluate the risk frominternal events that occur during low power and shutdown modes, but excluding therisk from damage to stored spent fuel.
Most of the NRRC research activities have now achieved a level of maturity thatallows practical demonstrations of how the methods and models are integrated into afull-scope PRA. In several cases, the developed methods and models arefunctionally consistent with the international state-of-practice. In some cases, theresearch is still at a relatively preliminary stage, retaining some inherent sources ofconservatism and rather large uncertainties. However, even those preliminary
methods and models are sufficiently well-understood to be examined in the context
of a full-scope PRA.
The current research program includes plans to demonstrate each analyticaltechnique through the use of separate "model plant" PRAs which are selected from avariety of plant sites and are currently developed to varying degrees of technicalquality. In our previous reviews, we have strongly recommended that practicaldemonstrations of the methods and models for each research project should use thebest available Japanese PRAs; i.e., the current versions of the good quality PRAs for
Ikata Unit 3 and Kashiwazaki-Kariwa Unit 7. We continue to strongly recommendthat practice instead of the "model plant" approach.
We certainly understand and are sensitive to the resource constraints of the NRRC,
the Japanese nuclear industry, and the individual utilities that are supporting theseimportant initiatives. However, we are also obligated to explain the technicalrationale for our recommendation.
Risk assessment is not a disjointed mathematical exercise. The power of risk
assessment is realized through a comprehensive evaluation of a broad spectrum of-4-

internal events, internal hazards, and external events that can challenge plant safety
during all operating modes. The PRA provides the framework for an integrated,
objective, and balanced evaluation of those sources of risk, including their inherent
uncertainties. That integrated perspective provides practical insights for plantspecific risk-informed decision-making and risk management that cannot be realized
through stand-alone examinations of individual issues.
Integration of the evolving methods and models into a good quality full-scope PRA
also provides vital risk-informed feedback to identify research priorities for focused
improvements to specific analytical techniques. In practice, PRA always involves aniterative refinement process that systematically examines the most important
contributors to risk. In some cases, sources of significant conservatism or
uncertainty may be reduced by performing more refined analyses. In other cases,
modifications to plant hardware, operating procedures, or maintenance practices
may provide the most beneficial risk management option.
In the context of an integrated risk perspective, further refinements to potentiallyconservative analyses are not justified or needed if the respective issues are a small
contribution to the overall plant risk. In principle, those analyses might be improved
further. However, in practice, there is no reason to spend additional effort on those
improvements, because they would not have a meaningful effect on theunderstanding of plant risk and its contributors. Thus, for example, even though aspecific damage mechanism might account for 90% of the risk from a particularhazard, further refinements to those models and analyses are not warranted if thatdamage contributes to only 1% of the overall plant risk.
The use of diverse "model plant" studies to demonstrate the application of eachresearch topic and to inform priorities for further research refinements is notconsistent with the Japanese industry and NRRC goals to develop integrated risk
assessment tools and capabilities for balanced risk-informed decision-making. Useof the pilot plant PRAs for Ikata Unit 3 and Kashiwazaki-Kariwa Unit 7 would support
those goals and reinforce the importance of an integrated perspective on plant risk
and risk-informed decision-making.
Individual Research Activities
The following items summarize our recommendations for re-examination of a few
individual research activities.
(1) Scope of Multi-Unit PRA Research
We continue to recommend that the scope of the multi-unit PRA (MUPRA) research
activities should include an integrated evaluation of Level 1 and Level 2 risk,
beginning in fiscal year 2021.
We understand that some Level 2 PRA modeling and analysis methods are notcurrently as well-developed as Level 1 PRA techniques. However, the entire topic ofintegrated multi-unit risk assessment is an evolving discipline in the international
PRA community. The NRRC research teams have acknowledged that it is important
to examine the risk from offsite releases that may occur when multiple units are-5-damaged. Thus, this research should be focused on that objective. Although thecurrent Level 2 PRA models may be improved in the future, this research should now
include explicit consideration of contributions to containment failure and offsite
releases as key metrics of the MUPRA models and analyses. The research canaddress well-understood accident scenarios, failure modes, and phenomena that
contribute to offsite risk, despite limitations in specific Level 2 PRA models.
As we noted in our report on the research plan for fiscal year 2020, a focus on only
Level 1 core damage frequency may result in unforeseen technical challenges,
iteration, and research inefficiency. More importantly, it may subtly influence
decisions about applied analytical methods, techniques to consolidate or truncatecomplex models, and other practices that may inappropriately overlook or suppress
important contributions to multi-unit accident scenarios that affect offsite releases.
To avoid these potential pitfalls, we recommend that the MUPRA research activities
should explicitly evaluate integrated Level 1 and Level 2 risk from fiscal year 2021
forward.
(2) Methods and Models for Evaluating the Risk from Seismically-Caused
Tsunamis
In our report on the research plan for fiscal year 2020, we recommended that theseismic and tsunami research should be expanded to include an additional activity
beginning in fiscal year 2021 to develop a site-specific demonstration of how tomodel and quantify the risk from seismically-caused tsunamis. We explained why
this issue is important for completeness of the analyses of the risk from external
events and why development of coherent models for the frequency andconsequences from these correlated hazards is technically challenging.
We were informed that NRRC is considering inclusion of this topic in the researchplan. We recommend that the external natural events research plan for fiscal year
2021 should include a specific project and schedule to develop a practical hazard
analysis methodology for seismically-caused tsunamis, and to perform a trial PRA
application that evaluates the risk from correlated seismic and tsunami damage.
(3) Evaluation of Volcanic Ash-Fall Contribution to Loss of Offsite PowerThe proposed research plan for volcano risk assessment includes a task to develop
models and analytical methods to evaluate the conditional probability that volcanic
ash deposition on electrical insulators will result in sufficient arcing to cause a loss ofoffsite power at a particular site. We recommend that this research activity should
not be continued.
In practice, there are many reasons why offsite power may be lost during a severe
volcanic event with associated seismic activity and substantial regional or localized
ash-fall. Development of numerical analysis methods to estimate the thickness ofash that may accumulate on insulators and bushings will not provide a meaningful
estimate for the conditional probability that offsite power is lost during these events.
In fact, those analyses might provide only an approximate estimate of the absolute
minimum lower bound for that conditional probability. Furthermore, there issubstantial uncertainty in the models for ash transport, ash deposition, moisture-6-

content, latent insulator contamination, arc development, and the consequencesfrom flashovers of insulators on multiple transmission lines, transformers, and circuit
breakers in a variety of geographic locations, physical arrangements, and electricalconfigurations.
Considering the frequency of severe volcanic eruptions, multiple possible causes for
power failures, and the substantial analytical uncertainties, it seems that areasonable bounding evaluation of the risk from a severe volcanic event could simply
assume that offsite power will be lost for an extended period of time (e.g., longer
than 24 to 48 hours). The PRA models and analyses would then focus on how theash affects plant-specific equipment that is needed to mitigate the resulting event
scenarios (e.g., air supplies for emergency generators, building ventilation, coolingwater systems, etc.). Methods and models for conducting those analyses areappropriately included in the current scope of this research.
Of course, the assumed loss of offsite power might involve considerable
conservatism for a specific eruption, a particular nuclear power plant site, and its
surrounding electrical grid. However, in practice, more complex and detailed
analyses to estimate the conditional probability for loss of offsite power, and its
duration, from all causes that are related to the eruption would be justified only ifthese events are important contributors to overall plant risk.
(4) PRA Peer Review Guidance and Implementation
Comprehensive, independent peer reviews are vital for establishing confidence inthe technical quality of a PRA and its use for risk-informed applications. Thosereviews provide the utility engineers and managers with objective feedback on howeach element of the plant-specific PRA models and analyses conforms with welldefined technical capability criteria. Understanding the capabilities and limitations of
the PRA is essential for effective use of that tool to support day-to-day decisions
about plant operations, maintenance, proposed modifications, and emergent issues.
In the U.S., consistently applied peer reviews also provide regulatory confidence thata licensee's PRA can be used to support proposed risk-informed changes to theplant's licensing basis. Use of standardized peer review guidance, criteria, andassessments for every PRA also enhances regulatory confidence for industrysponsored risk-informed initiatives that apply to all licensees or selected cohorts ofplants.
The NRRC has had a lead role in developing the current draft PRA peer review
guidance. We understand that the Japanese nuclear industry, and ultimately eachnuclear power plant, is responsible for implementing the peer reviews. Experiencefrom the U.S. has demonstrated the importance of technical consistency in thosereviews, performed by trained and experienced PRA experts. The U.S. experiencehas also shown that development of those review capabilities and the necessary
technical expertise requires considerable effort and time.
The NRRC and other organizations currently conduct training sessions that arefocused on expanding the capabilities of Japanese utility engineers in a variety of
PRA technical disciplines and methods. Beginning in fiscal year 2021, werecommend that the NRRC should develop a specific peer review training program-7-

to provide assurance that peer reviews will be conducted according to consistenttechnical practices for every Japanese nuclear power plant PRA.
We look forward to our continuing interactions with the NRRC research team toreview the overall research program and individual research projects, and to help the
NRRC and the Japanese nuclear industry achieve their goals of comprehensive riskinformed decision-making.
Sincerely,

John W. Stetkar
Chairman
REFERENCES
1.

"NRRC Overview: Research Program for FY2021, Risk Assessment,"
Presentation to NRRC Technical Advisory Committee, November 4, 2020,
Proprietary.

2.

"NRRC Overview: Research Program for FY2021, External Natural Events,"
Presentation to NRRC Technical Advisory Committee, November 4, 2020,
Proprietary

3.

"NRRC Overview: Research Program for FY2021, RIDM Promotion,"
Presentation to NRRC Technical Advisory Committee, November 4, 2020,
Proprietary

4.

Technical Advisory Committee individual members' comments and questions
on "NRRC Overview: Research Program for FY2021, Risk Assessment,"
December 4, 2020, Confidential.

5.

Technical Advisory Committee individual members' comments and questions
on "NRRC Overview: Research Program for FY2021, External Natural
Events," December 4, 2020, Confidential.

6.

Technical Advisory Committee individual members' comments and questions
on "NRRC Overview: Research Program for FY2021, RIDM Promotion,"
December 4, 2020, Confidential.

7.

NRRC Research Team responses to comments and questions on "NRRC
Overview: Research Program for FY2021, Risk Assessment," January 30,
2021, Confidential.

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8.

NRRC Research Team responses to comments and questions on "NRRC
Overview: Research Program for FY2021, External Natural Events," January
30, 2021, Confidential.

9.

NRRC Research Team responses to comments and questions on "NRRC
Overview: Research Program for FY2021, RIDM Promotion," January 30,
2021, Confidential.

10.

Technical Advisory Committee of the Nuclear Risk Research Center,"Proposed NRRC Research on Selected Seismic Issues for Fiscal Year
2021," February 16, 2021.

11.

Technical Advisory Committee of the Nuclear Risk Research Center,"Proposed NRRC Research Plan for Fiscal Year 2020," November 19, 2019.

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