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
November 19, 2019
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 2020

Dear Dr. Apostolakis:
During the 12th meeting of the Technical Advisory Committee (TAC) of the Nuclear
Risk Research Center (NRRC), November 11-15, 2019, we met with the NRRC staff
to review the proposed research plan for fiscal year 2020. The purpose of ourreview was to provide comments on the technical merit of the research plan and its
relevance for supporting NRRC's current mission.
CONCLUSIONS AND RECOMMENDATIONS
1. We did not identify any major gaps in the overall research plan for fiscal year2020.
2. A number of research activities will soon achieve a level of maturity that allows
practical demonstrations of how they are used to accomplish NRRC's goal tosupport the development of good quality probabilistic risk assessments (PRAs)
that evaluate the risk during all plant operating modes from internal events,
internal hazards such as fires and floods, and external events. We are very
encouraged by this progress.
3. During our review and discussions with the NRRC research teams, we identified
a number of individual research activities that merit additional attention in theplans for fiscal year 2020 and subsequent years. Our recommendations for
specific activities are summarized in the Discussion 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, current state of knowledge,
and potential contributions and significance of the research to the goals of the center.

-1-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 detailed reviews of those projects.
DISCUSSION
The NRRC staff provided a summary of the scope of a strategic plan and a phased
approach for introduction of risk-informed decision-making (RIDM) in the Japanese
nuclear industry. Comprehensive plant-specific PRAs of high technical quality
provide the risk information and engineering insights that are an essential input for
the RIDM process. Therefore, NRRC is conducting research on improved analyticalmethods, models, and data for the performance of good quality PRAs (i.e., state-ofpractice fully-integrated Level 1 and Level 2 PRAs that evaluate plant-specific risk
during all plant operating modes, with extensions to limited-scope Level 3 PRAs).
The scope of those research activities covers a wide range of technical issues suchas collection and analysis of plant operating experience and data, human reliability
analysis, methods for analyzing internal fires and floods, improved modeling ofsevere accident phenomena, and evaluation of the risk from external hazards suchas earthquakes, tsunamis, severe winds, and volcanic eruptions. In addition to
supporting the development of good quality PRAs, the NRRC research team is also
developing guidance for the use of PRA as a tool to support the RIDM process.
During this review, we were briefed on several important research projects, the major
technical tasks in each project, the current status of each task, known or potential
problem issues, and the estimated schedule for completion of each task. The project
timelines and schedules were also useful to illustrate how each activity fits into theoverall research scheme and the context of the NRRC short-, intermediate-, andlong-term goals.
We did not identify any major gaps in the overall research plan for fiscal year 2020.
Furthermore, a number of research activities will soon achieve a level of maturity thatallows practical demonstrations of how they are used to accomplish NRRC's goal tosupport the development of good quality PRAs that are consistent with theinternational state-of-practice. Those activities include:•

Use of the Senior Seismic Hazard Analysis Committee (SSHAC) methodology toperform a probabilistic seismic hazard analysis (PSHA) for the Ikata plant site.

•

Development of a Fire PRA Guide.

•

Development of a Human Reliability Analysis (HRA) Guide.

We are very encouraged by this progress.

-2-Based on our review of the NRRC research plans and discussions with the researchteams, we offer the following recommendations for extensions, applications, andfurther assessments of selected individual research activities. They should be
integrated into the overall research program for fiscal year 2020 and the plans for
subsequent years.
Research Extensions
The following items summarize our recommendations for extensions of the current
research program and specific research activities.
(1) Methods and Guidance for Evaluating Risk during Low Power andShutdown Modes
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.
Each pilot project has benefited substantially from interactive reviews that are being
conducted by teams of international PRA experts. During this meeting, we received
a summary briefing on a recent expert review of the Ikata Unit 3 PRA models for low
power and shutdown (LPSD) modes. We were also informed that a similar review of
the LPSD models for Kashiwazaki-Kariwa Unit 7 will be conducted in December ofthis year.
International experience has shown that events which occur during shutdown modes
can be important contributors to the overall plant risk profile. That experience has
also shown that the risk during shutdown and its contributors can be very plantspecific. Furthermore, the methods, models, data, and analysis techniques that areneeded to evaluate that risk can present challenges that are different from those
associated with the development of PRA models for full-power operation.
The NRRC research program does not currently include any distinct activities thatare focused on development of methods, models, or guidance for evaluating risk
during low power and shutdown modes. However, the scope of any research
activities in this area should be tailored to the technical needs of the current
Japanese analysis practices. We recommend that the research program for fiscal
year 2020 should include an activity to interview the PRA analysts and examine theexpert reviews of the Ikata Unit 3 and Kashiwazaki-Kariwa Unit 7 LPSD PRAs,
identify research or guidance that may be needed to support the development ofanalytical methods and modeling techniques that are consistent with the international
state-of-practice, and integrate those needs into the NRRC research program
beginning in fiscal year 2021.

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(2) Methods and Guidance for Evaluating Risk from Events that Affect Stored
Spent FuelInternational PRA models for plant shutdown modes often include evaluations of therisk from events that affect the movement and storage of spent fuel during specificplant operating states (POSs). Some examples of those events include fuel handling
accidents during POSs when fuel is being moved into and out of the pools, possible
draining of water from the pools during fuel transfer operations, boron dilution, andloss of cooling during POSs when pool heat loads are high from fuel that has been
recently removed from the reactor, including full core offloads. Damage fromexternal events and internal hazards such as fires and floods may also affect thestored spent fuel. Some recent international PRAs have further extended theevaluations of spent fuel risk to include full-scope analyses that account for plantconditions and activities that occur throughout the year.
The NRRC research program for fiscal year 2020 includes activities for testing ofspent fuel cladding performance and use of the MAAP code to evaluate the thermalhydraulic progression of an accident that involves complete loss of spent fuel cooling,including the effectiveness of alternative fuel pool sprays. However, the researchplan does not currently include any activities that address development of integrated
PRA models, methods, or guidance for evaluating the risk from events that affect the
movement or storage of spent fuel. We have not been briefed on details of the Ikata
Unit 3 and Kashiwazaki-Kariwa Unit 7 PRA models for plant shutdown modes.
Therefore, we do not know whether, or how, those models may account for the risk
from events that affect spent fuel. As noted in our preceding comments, the scopeof any research activities in this area should be tailored to the technical needs of thecurrent Japanese analysis practices.
As an adjunct to the activity that is discussed in Item (1) above, we recommend thatthe research program for fiscal year 2020 should also include an activity to interview
the PRA analysts and examine the expert reviews of the Ikata Unit 3 andKashiwazaki-Kariwa Unit 7 LPSD PRAs to identify research or guidance that may beneeded to support the development of analytical methods and modeling techniques
for the evaluation of risk from events that affect the movement and storage of spent
fuel. Those research needs should focus first on integration of the analyses of spent
fuel risk with the PRA models for plant shutdown modes, and then extend thosemethods and models to cover all plant operating modes, consistent with theinternational state-of-practice. Identified research activities should be integrated into
the NRRC research program beginning in fiscal year 2021.
(3) Demonstration of PRA Methods and Models for Evaluating the Risk from
Combined Effects of Earthquakes and Tsunamis
One of the Seismic PRA research activities for fiscal year 2020 involves thedevelopment of analysis techniques and methods to evaluate multiple dependent
hazards. In particular, we were informed that this activity is focused primarily on an
evaluation of the combined hazard from seismically-caused tsunamis. This researchis very important for Japan, and it will significantly extend the international state-ofpractice.

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The development of an integrated PRA model for evaluating the risk fromseismically-caused tsunamis presents unique challenges that are not easily adaptedfrom analyses that evaluate only earthquakes or only tsunamis. For example,
special techniques are needed to account for correlated dependencies in thefrequency and severity of the composite hazard, and in the corresponding PRA
models for coincident seismic damage and tsunami damage to plant structures andequipment.
We recommend that this research should be expanded to include an additional
activity beginning in fiscal year 2021 to develop a site-specific demonstration of how
to model and quantify the combined risk from seismically-caused tsunamis.
(4) Scope of Multi-Unit PRA Research
The scope of the multi-unit PRA (MUPRA) research activities is currently limited toonly an evaluation of Level 1 risk.
The current scope may result in unforeseen technical challenges, iteration, andresearch inefficiency, compared to an approach that initially examines the needed
methods, models, and guidance to evaluate Level 2 risk from all internal events,
internal hazards, and external events. For example, as risk assessment technology
and PRA models have evolved over the last 40 years, we have learned that the earlyguidance for developing only Level 1 PRA models would have benefited substantiallyfrom a better appreciation of the models and analyses that are needed to perform an
integrated Level 2 PRA. In many cases, a limited focus on only core damage has
resulted in the need for analytical iteration and inefficient changes to PRA models
that would not have been necessary if the guidance and models had initiallyaccounted for the broader perspective.
We have also learned that the most important contributions to core damage are notalways the most important contributions to offsite releases. Therefore, if a proposed
MUPRA methodology uses only core damage frequency as the primary metric for
decisions to consolidate or truncate complex models, those decisions may
inappropriately overlook or suppress important contributions to multi-unit accident
scenarios that affect offsite releases.
We recommend that the MUPRA research scope should include an integrated
evaluation of Level 1 and Level 2 risk, beginning in fiscal year 2020.
Research Applications
The following items summarize our recommendations for initial applications ofspecific research activities.
(5) Planned Use of the Fire PRA Guide
The current research plan indicates that a revised version of the NRRC Fire PRA
Guide will be completed in fiscal year 2019. It contains contemporary state-ofpractice methods, models, and guidance for evaluation of the risk from internal fires,-5-

which can be an important contribution to the overall plant-specific risk profile. Thisversion of the Guide will benefit from input that has been provided by Japaneseutilities and international fire PRA practitioners.
There are two schools of thought regarding use of this version of the Guide. One
school is based on the U.S. experience from issuance and use of the guidance in
NUREG/CR-6850. The lessons learned from that experience indicate that it is very
important to first use the guidance to perform an integrated plant-specific fire
analysis, identify needs for improvement based on that experience, and then issue
the final guidance for general use. The second school of thought is that the Guide
should now be issued for general use by the Japanese industry, without anintermediate iteration.
Of course, in practice, it is likely that the first users of the Guide will identify areas forpossible improvement in both the technical methods and application guidance.
However, unlike the U.S. experience, the NRRC Fire PRA Guide has alreadyincorporated the lessons learned from use of NUREG/CR-6850 in the U.S. and othercountries, including advances in fire analysis methods, models, and data that have
evolved during the intervening years. Thus, there is a low risk of any significant
technical deficiencies or programmatic problems in this version of the Guide, andthat risk is outweighed by the greater benefits from timely use of the guidance by abroader cross-section of Japanese fire analysts.
We support issuance of the Fire PRA Guide for general use, without an intermediate
trial application and iterative revision. We will comment separately on technicaldetails of the guidance after this version of the Guide is issued.
(6) Seismic PRA Demonstration Project
One of the Seismic PRA research activities that is planned to begin in late fiscal year
2019 or early fiscal year 2020 involves the development of a plant-specific model todemonstrate how to integrate the analyses of seismic events with the Level 1 PRA
models for internal initiating events. During this meeting, the research teamdiscussed their selection of a model plant site and its supporting PRA models for thisdemonstration project.
We recommend that this demonstration project should instead use the pilot plant
PRA models for Ikata Unit 3 or Kashiwazaki-Kariwa Unit 7. Some of the reasons forour position are based on proprietary information that was discussed during ourmeeting. However, in addition to those considerations, the pilot plant applications
will provide the most effective demonstration of how state-of-the-practice methods
and models for evaluating seismic events are used to develop good quality fullyintegrated plant-specific PRAs. Consistent integration of the models for various
internal events, internal plant hazards, and external events to develop acomprehensive risk profile is a fundamental element of the risk assessment processthat cannot be demonstrated effectively by the use of separate models for different
plants.
Despite the fact that good quality PRA models for internal initiating events are being
developed at Ikata Unit 3 and Kashiwazaki-Kariwa Unit 7, we favor use of the Ikata-6-

Unit 3 models for this demonstration project. The seismic hazard for the Ikata site
has been developed through an advanced Level 3 application of the SSHAC
methodology. Thus, use of the Ikata models to demonstrate how the risk fromseismic damage is integrated into the full-scope PRA has the additional advantage toshow how the best available seismic hazard information is used to develop the sitespecific initiating event frequencies and the input to the fragility analyses for plantstructures and equipment.
(7) Level 2 PRA Demonstration Project
One of the Level 2 PRA research activities that is planned to begin in approximately
fiscal year 2021 or fiscal year 2022 involves the development of a plant-specificmodel and analyses to demonstrate how to integrate the Level 2 models for severe
accident progression and containment performance with the Level 1 models for coredamage. We recommend that this demonstration project should use the pilot plant
PRA models for both Ikata Unit 3 and Kashiwazaki-Kariwa Unit 7. Use of both plants
will provide valuable experience and insights for the evaluation of severe accident
phenomena, application of analytical methods, and development of models that arequite different for PWRs and BWRs.
Research Assessments
The following items summarize our recommendations for re-examinations ofindividual research activities.
(8) High Wind Prediction Tool
One of the High Wind PRA research activities for fiscal year 2020 involves thedevelopment of a "high wind risk detection and prediction tool". Based on our
discussions with the research team, we understand that this tool is intended toprovide a capability to predict the severity and arrival time of high winds at a nuclear
power plant site, with a particular emphasis on tornadoes. The team noted thatsome plants may need a warning time before the arrival of high winds to confirm thatportable mitigation equipment is secure and to move potential wind-borne missiles
away from plant buildings.
We do not have sufficient information to fully understand the technical needs for thisresearch and why the NRRC is the most appropriate organization to develop this tool.
We recommend that the NRRC should re-examine this research activity and confirm
that it effectively supports the technical objectives and goals of the overall research
program and the needs of the Japanese utilities.
(9) Phenomenological Relationship Diagram (PRD) Methodology
During this meeting, we were briefed on the basic concepts of the proposed
Phenomenological Relationship Diagram (PRD) methodology, including two
simplified numerical applications. According to the research team, the methodology
is intended to streamline the computational process for analyses of severe accident
progression, physical and thermal-hydraulic phenomena, and containment
performance, and it facilitates more efficient quantification of the associated-7-uncertainties. Based on the information provided during the meeting, it is notapparent that the simplified models and assumptions that are inherent in theexample applications will provide the analytical capabilities that are needed tosupport a state-of-the-practice best-estimate evaluation of risk, including thetreatment of complex phenomenological and timing dependencies.
Although analytical tools such as MELCOR and MAAP are resource-intensive andcomputationally complex, they are accepted internationally as providing a firm
technical foundation for integrated analyses of accident progression and containment
performance. Research projects have also demonstrated how uncertainties in theLevel 2 and Level 3 PRA analyses can be quantified through integrated applicationsof the MELCOR and MACCS codes, albeit with substantial computational burden.
In summary, without better examples and benchmark comparisons with
contemporary best-estimate analytical results, it is difficult for us to understand why
the proposed PRD methodology will provide an appropriate alternative to theestablished modeling tools. We recommend that the NRRC should re-examine thisresearch activity and confirm that it effectively supports the technical objectives andgoals of the overall research program and the needs of the Japanese utilities.
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 FY2020, Risk Assessment,"
Presentation to NRRC Technical Advisory Committee, November 11, 2019,
Proprietary.

2.

"NRRC Overview: Research Program for FY2020, External Natural Events,"
Presentation to NRRC Technical Advisory Committee, November 11, 2019,
Proprietary

3.

"NRRC Overview: Research Program for FY2020, RIDM Promotion,"
Presentation to NRRC Technical Advisory Committee, November 11, 2019,
Proprietary

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4.

"Ikata SSHAC Project – Summary of Final PSHA Results, Lessons Learned
from Ikata SSHAC Project, Including Plans for SSHAC Applications at Other
Sites," Presentation to NRRC Technical Advisory Committee, November 12,
2019, Confidential.

5.

"Improvement of Quantification Technique of Containment Event Trees(CETs) and Source Term with PRD Methodology," Presentation to NRRC
Technical Advisory Committee, November 12, 2019, Proprietary.

6.

"Revised HRA Guide – Current Status, Summary of Changes, Update
Schedule," Presentation to NRRC Technical Advisory Committee, November
12, 2019, Proprietary.

7.

"Fire PRA (FPRA) Guide – Status of the Practical Interpretation by JapaneseIndustries (Draft)," Presentation to NRRC Technical Advisory Committee,
November 12, 2019, Proprietary.

8.

"KK-7 Project Internal Event Operating Level 1 PRA Model (Level 1.5 PRA
Shutdown PRA Model) Sophistication Progress Report," Presentation to
NRRC Technical Advisory Committee, November 13, 2019, Proprietary.

9.

"Ikata Unit 3 Project Status Update," Presentation to NRRC TechnicalAdvisory Committee, November 13, 2019, Proprietary.

10.

"Activities to RIDM Implementation and ROP," Presentation to NRRC
Technical Advisory Committee, November 13, 2019, Proprietary.

11.

"Action Plan for RIDM Process Introduction (Phase 2)," Presentation to NRRC
Technical Advisory Committee, November 13, 2019, Proprietary.

12.

"Summary of RIDM Workshop," Presentation to NRRC Technical Advisory
Committee, November 13, 2019, Proprietary.

13.

U.S. Nuclear Regulatory Commission and Electric Power Research Institute,"EPRI / NRC-RES Fire PRA Methodology for Nuclear Power Facilities,"
NUREG/CR-6850, EPRI 1011989, September 2005.

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