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
December 30, 2021
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:PROPOSED NRRC RESEARCH PLAN FOR FISCAL YEAR 2022

Dear Dr. Apostolakis:
In 2021, the ongoing COVID pandemic continued to present significant challenges to
the Nuclear Risk Research Center (NRRC) research teams and the TechnicalAdvisory Committee (TAC). We commend the entire NRRC staff for their efforts tocontinue 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 2022. In late
October, the NRRC research teams sent us presentations that summarize theprojects in each major research area. We reviewed that material and prepared our
individual comments and questions on specific topics, as we would normally do
before our meeting. We sent several of those individual member comments andquestions to you for preliminary consideration by each research team. We then held
two 2-hour video conferences to discuss our comments and questions on each major
research area. The research teams also provided detailed and thoughtful written
responses to supplement our oral discussions. We deliberated on those discussions
and the teams' responses, and we 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
the research plan and its relevance for supporting the NRRC's current mission.
Our experience from this effort continues to reinforce the vital importance of the
dynamic interactions during our face-to-face meetings. While the approach we used
for this review 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 remote video conferences or written questions and answers.
We sincerely hope that we can return to our normal meetings in 2022.

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CONCLUSIONS AND RECOMMENDATIONS
1. We did not identify any major gaps in the overall technical research plan for fiscal
year 2022.
2. We fully support the start of a new research project for Multi-Hazard Probabilistic
Risk Assessment (MHPRA). The project addresses methods and models forevaluation of the risk from seismically-induced tsunamis. This is a complex topic,
with very little international PRA experience. The NRRC research will be an
important contribution to advance the state-of-the-practice for a comprehensive
analysis of the risk from external natural events.
3. During our review, we identified a few individual research activities that meritadditional attention in the plans for fiscal year 2022 and subsequent years. Our
recommendations for those activities are summarized in the Discussion section ofthis report.
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
management.
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 research plan focusedon the objectives of each research project and its supporting tasks, the technicalrelationships 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 understand how those
activities are integrated throughout the plan. We will comment separately on thetechnical elements of individual research projects in our future detailed reviews of
those projects.

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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 2022. The discussion of Individual Research
Activities contains our recommendations for specific elements of four research
projects.
Research Applications
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 our report on the research plan for fiscal year 2021, we explained
the technical reasons why the good quality PRAs for Ikata Unit 3 and KashiwazakiKariwa Unit 7 should be used for those demonstrations. From our purely technicalpoint of view, we continue to strongly recommend that the research teams should
use those PRAs instead of the "model plant" approach. The "model plant" studies
use diverse PRAs that are selected from a variety of plant sites and are currently
developed to varying degrees of technical quality. The "model plant" analyses do
not provide NRRC researchers or utility engineers with practical experience for thedevelopment of an integrated full-scope PRA. Furthermore, the results from those
diverse studies do not demonstrate how insights about the overall plant risk and its
contributors can be used for effective risk-informed decision-making.
The use of other "model plant" PRAs should be reserved only to demonstrate
analysis methods and models for hazards that are not relevant for the Ikata andKashiwazaki-Kariwa sites or the specific features of those plant designs.
Research Extensions
In our report on the research plan for fiscal year 2021, we noted that several
research teams are now proposing further refinements and extensions for a varietyof analytical methods and models. We also noted a distinct trend toward thedevelopment and use of increasingly detailed, computationally-intensive models toevaluate specific hazards and damage mechanisms. We remain concerned thatthose detailed models may lead to a belief in numerical precision that is not justified
for an evaluation of the risk from phenomena that have inherently large uncertainties.
Before more detailed analytical tools are developed further, each research teamshould describe and document how important sources of aleatory and epistemic
uncertainty will be identified, characterized, and quantified as an integral part of theapplied methods and models.
Experience has also shown that utility PRA engineers may have important technicaland resource constraints which substantially limit their use of some very complex
methods and tools. Simpler models which facilitate a careful treatment of theuncertainties often provide more meaningful and realistic support for practical PRA
applications than the implied precision of complex computations. Therefore, we
encourage the research teams to first develop simplified enhancements that provide
an integrated evaluation of the technical issue and its associated uncertainties. The-3-

potential benefits that may be achieved from applying those improved methods,
models, and tools should then be examined by demonstrating them in an integrated
good quality PRA. Practical experience and insights from those initial applications
should guide decisions about the needed scope and level of detail for the proposed
refinements. More detailed methods and models should be pursued only if theysignificantly improve the understanding of overall plant risk and its contributors, andfacilitate practical risk-informed decision-making by plant engineers and utility
management.
In summary, the priorities and resources for further enhancements to specificanalytical methods, models, and tools should be determined from an integrated riskinformed perspective. That perspective should consider how the specific technicalissue and the proposed enhancements affect an understanding of Level 1 and Level
2 risk from all hazards and all plant operating modes, including the associateduncertainties. Utility engineers and managers should clearly understand how each
proposed enhancement will improve their use of the PRA for practical risk-informed
applications.
Individual Research Activities
The following items summarize our recommendations for re-examination of a few
individual research activities.
(1) Low Power and Shutdown Modes
We were informed that the Japanese utilities have not identified any short-term
priorities for research on generally-applicable methods or models for evaluating therisk from events that occur during low power and shutdown (LPSD) operating modes.
Individual utilities are working to improve their plant-specific LPSD analyses.
Therefore, the NRRC research plan for fiscal years 2022 through 2024 does notcontain any specific activities in this area.
International experience has shown that the core damage frequency from events that
occur during LPSD can be comparable to that during full power operation.
Furthermore, the conditional probability of offsite releases may be much higher, due
to the varying status of containment isolation and containment heat removal systems
throughout an outage. Therefore, for a complete assessment and understanding of
the risk at each nuclear power plant, it is essential that Japanese utilities should
perform comprehensive and consistent Level 1 and Level 2 PRA assessments of therisk during LPSD.
To better understand the current status of LPSD risk assessment methods andpractices, we recommend that the NRRC research plan for fiscal year 2022 should
include a formal survey and structured high-level technical assessment of eachutility's LPSD PRA.
The survey should first determine the scope and definitions of the LPSD plant
operating states (POSs) that are explicitly modeled in the PRA. For each POS, thesurvey should then determine how the PRA models treat each of the following major
issues:

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•
•
•
•
•
•
•

Normal system operating alignments
Simultaneous (correlated) unavailability of multiple components or system trains
due to inspections, maintenance, modifications, etc.
Status of containment isolation
Identification of initiating events caused by equipment failures
Identification of initiating events caused by human errors during operations,
testing, maintenance, and modifications
Identification of initiating events caused by internal flooding, internal fires, andexternal events
Definitions of success criteria and time windows for operator actions to prevent
core damage and offsite releases

The results from this survey may conclusively confirm that no significant technicalimprovements are needed. That information will provide important confidence in theJapanese industry's LPSD risk assessment methods, and it will help us to resolve
many questions that we have raised previously regarding this key element of a fullscope PRA. On the other hand, if the survey identifies possible knowledge gaps,
sources of incompleteness, or inconsistency, that information will help the NRRC toformulate specific research or develop practical guidance.
(2) Spent Fuel
The NRRC spent fuel risk research is currently focused on refinements todeterministic analyses for specific issues such as possible recriticality conditions,modeling of fuel cladding behavior, and the effectiveness of alternative spray coolingand natural convection cooling. We understand that a planned activity for fiscal year
2022 includes a review of the potential risk importance of these issues, based oninternational PRA experience and insights. The research team will then use that
information to help determine priorities for further detailed evaluations, if needed.
We fully endorse that risk-informed perspective to guide the scope and level of detailfor these focused activities.
International experience has shown that an integrated assessment of the risk fromevents that may damage stored spent fuel is an important and potentially challenging
element of a full-scope PRA. The PRA must consistently account for events that
affect spent fuel cooling during all plant operating modes. For example, many PRA
initiating events such as losses of offsite power, support system failures, internal
floods, internal fires, earthquakes, tsunamis, high winds, etc. may simultaneously
affect cooling for the reactor core and the spent fuel. An integrated assessment of
the resulting event scenarios must carefully evaluate the available mitigation options,and it must account for coordinated operator actions that are needed to prevent
damage to fuel in the reactor, damage to the spent fuel, and possible offsite releases.
The PRA must also account for other possible causes for spent fuel damage, suchas reactivity effects due to boron dilution and mishaps that may occur during fuelmovements for reactor refueling, reconfiguring the fuel pool storage pattern, loadingof shipping containers or dry storage casks, etc.
As part of their reviews of international spent fuel risk assessment experience infiscal year 2022, the research team should also examine methods and practices that-5-are used to integrate the analyses of spent fuel risk with the PRA models for fullpower, low power, and shutdown modes. The objective of this effort should be thedevelopment of NRRC guidance for an integrated assessment of spent fuel risk,
supported by a practical PRA demonstration.
(3) Superposed Hazards
We fully support the new research project on Multi-Hazard PRA. This projectaddresses methods and models for evaluation of the risk from seismically-induced
tsunamis. This is a complex topic, with very little international PRA experience. The
NRRC research will be an important contribution to advance the state-of-the-practicefor a comprehensive analysis of the risk from external natural events.
The scope of this research should first focus on the two most difficult elements ofthese analyses: (1) derivation of the composite site-specific hazard for seismic
events and tsunamis, and (2) adaptation of the PRA models to quantify the risk fromthese events.
It is very important to demonstrate how an integrated PRA will consistently andcompletely account for the risk from all of the following types of events:•

Earthquakes that do not produce a tsunami (i.e., only seismic effects)•

Tsunamis that result from submarine landslides or earthquakes which do notdamage any structures or equipment at the site (i.e., only tsunami effects)•

Tsunamis that result from earthquakes which may damage at least one structure,system, or component at the site (i.e., combined seismic and tsunami effects)

The hazard analyses should demonstrate how the exceedance frequencies for these
events are quantified consistently. For example, the correlated hazard curves forseismically-induced tsunamis should evaluate conditional probabilities for a spectrumof tsunami wave heights that may result from tsunamigenic earthquakes, over the fullrange of the site-specific seismic hazard. In practice, this means each seismic
initiating event that is quantified in the PRA should have a correlated probability
distribution for a range of tsunami wave heights that may be produced by submarine
earthquakes which contribute to that initiating event. That is a very complex
assessment.
It must carefully account for the locations, magnitudes, anddisplacements of numerous submarine seismic sources. It must also account foreach source's contribution to the overall seismic hazard, and the range of tsunami
wave heights that can be generated by the corresponding fault motions. A focused
Senior Seismic Hazard Analysis Committee (SSHAC) evaluation may be needed to
examine proposed methods to correlate the site-specific seismic and tsunami
hazards, and to quantify the associated uncertainties in the combined hazard.
The research project should demonstrate how the PRA models are structured toprovide a comprehensive and integrated assessment of the risk from earthquakes
and tsunamis. It should also demonstrate how the models are structured logically to
avoid "double counting" for some risk contributors. To satisfy these key technicalobjectives for a practical risk analysis, the demonstration should not be limited to an-6-

evaluation of only seismically-induced tsunamis. It should show analysts how todevelop PRA models that correctly quantify the overall risk from all earthquakes andtsunamis (i.e., from all three types of events listed above).
(4) Volcanic Ash-Fall
The research plan indicates that a PRA demonstration of the methods and models toevaluate the risk from volcanic ash deposition will start in fiscal year 2022.
That risk analysis should account for the accumulation of ash during a volcanic
eruption that may continue for several days or weeks. For example, the initial effects
may result in an automatic plant trip or a forced manual (i.e., anticipatory) shutdown
before or during the ash deposit. Those effects would be included in the PRA
models for full-power operation. However, continued ash accumulation may affectsystems that are needed to maintain decay heat removal for an extended period oftime after the reactor is shut down (i.e., much longer than the 24-hour mission timethat is typically used in a full-power PRA). Those effects can be included in the PRA
models for plant shutdown modes, which account for the decay heat levels andplant-specific system operating configurations as a function of time after shutdown.
Those models could also account for personnel actions that may be needed to clean
or replace intake air filters (and perhaps cooling water filters) that become clogged
over time. Therefore, the demonstration should use the combined PRA models forfull-power operation and shutdown modes to illustrate how the overall risk from anextended eruption is evaluated.
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 FY2022, Risk Assessment,"
Presentation to NRRC Technical Advisory Committee, November 15, 2021,
Proprietary.

2.

"NRRC Overview: Research Program for FY2022, External Natural Events,"
Presentation to NRRC Technical Advisory Committee, November 18, 2021,
Proprietary.

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3.

"NRRC Overview: Research Program for FY2022, RIDM Promotion,"
Presentation to NRRC Technical Advisory Committee, November 18, 2021,
Proprietary.

4.

Technical Advisory Committee individual members' comments and questions
on "NRRC Overview: Research Program for FY2022, Risk Assessment,"
November 11, 2021, Confidential.

5.

Technical Advisory Committee individual members' comments and questions
on "NRRC Overview: Research Program for FY2022, External Natural
Events," November 11, 2021, Confidential.

6.

Technical Advisory Committee individual members' comments and questions
on "NRRC Overview: Research Program for FY2022, RIDM Promotion,"
November 11, 2021, Confidential.

7.

Video conference on NRRC FY2022 research program for Risk Assessment,
November 15-16, 2021, Confidential.

8.

Video conference on NRRC FY2022 research programs for External Natural
Events and RIDM Promotion, November 18-19, 2021, Confidential.

9.

NRRC Research Team responses to comments and questions on "NRRC
Overview: Research Program for FY2022, Risk Assessment," December 11,
2021, Confidential.

10.

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

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