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 27, 2022
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 2023

Dear Dr. Apostolakis:
During the 17th meeting of the Technical Advisory Committee (TAC) of the Nuclear
Risk Research Center (NRRC), November 14-18, 2022, we met with the NRRC staff
to review the proposed research plan for fiscal year 2023. 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 research for fiscal year 2023 and the technical objectives of
the individual projects within each major research area remain consistent with theNRRC short-, intermediate-, and long-term goals.
2. During our review, we identified a few individual research activities that meritadditional attention in the plans for fiscal year 2023 and subsequent years. Our
recommendations for those activities are summarized in the Discussion section ofthis 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 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 thoseactivities are integrated throughout the plan. We will comment separately on the-1-

technical elements of individual research projects in our future detailed reviews of
those projects.
DISCUSSION
During this review, we were briefed on all 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 2023 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 research plans and our discussions with the researchteams, we offer the following recommendations for further assessments of three
individual research activities. We also recommend extensions of the planned
research to include two new projects. The new projects should be integrated into theoverall research program for fiscal year 2023 and the plans for subsequent years.
Research Assessments
The following items summarize our recommendations for re-examinations of threeplanned research activities(1) Level 2 PRA Model Plant Study
One planned research activity involves the development of a Level 2 probabilistic
risk assessment (PRA) study of accidents that evolve from internal initiating events
which occur at a model plant that is based on characteristics of Hamaoka Unit 4.
The scope of the Ikata Unit 3 and Kashiwazaki-Kariwa Unit 7 pilot plant PRAs
includes integrated evaluations of core damage (Level 1 PRA), severe accident
progression, and containment failure (Level 1.5 PRA) for internal initiating events
that occur during full-power operation. Those studies are at an advanced stage ofdevelopment. Their technical quality has benefited substantially from on-going
reviews by teams of international PRA experts. Utilities are using the experiencefrom those studies as examples of good quality state-of-the-practice methods tosupport updates and improvements to their current PRAs.
Kashiwazaki-Kariwa Unit 7 is an advanced boiling water reactor (ABWR). HamaokaUnit 4 is a BWR5 with an advanced Mark I containment. Details of the progressionof specific event scenarios, severe accident phenomena, and containment failuremodes will be different for each of these BWR designs. An integrated Level 1 andLevel 2 PRA study of severe accidents that are initiated by tsunamis has been
completed for a model plant that is based on Hamaoka Unit 4. Our June 3, 2022
letter report on "Outcome of Hamaoka Model Plant Level 2 Tsunami PRA Project"contains our comments, conclusions, and recommendations for the Hamaoka study.
An independent peer review of the study is planned for fiscal year 2023. The
Hamaoka study has enhanced the knowledge and understanding of how to evaluate-2-severe accident progression and containment performance in a Level 2 PRA for thatplant design.
Substantial experience has been gained from the two pilot plant PRAs for internal
initiating events and the model plant PRA study for tsunamis. It is not apparent thatthe planned additional model plant study will provide significant further technicalknowledge or practical modeling insights that are needed to support the NRRCobjective of developing methods and guidance that can be used to perform plantspecific Level 2 PRAs throughout Japan. We recommend that the NRRC should reexamine the need for this additional model plant study.
(2) Seismic PRA Model Plant Study
The NRRC research team has completed Phase 1 of a Level 1 PRA study of the risk
from seismic events which occur during full-power operation for a model plant that isbased on characteristics of Kashiwazaki-Kariwa Unit 6, which is an ABWR. That
project has provided important experience with integration of the seismic hazard andconsequential failures of structures and equipment in the PRA models. It has also
provided practical insights and enhancements in techniques for evaluating sitespecific features such as deformation of the foundation ground substrata andcollapse of adjacent slopes. Phase 2 of the study began in fiscal year 2022, and it
extends through fiscal year 2024. It will apply improved modeling techniques andrefinements to the analyses of selected issues that were identified from the Phase 1
effort.
During our discussions, the research team indicated that it is necessary to conduct a
companion study for a pressurized water reactor (PWR) model plant before ageneral methodology and guidance for evaluating the risk from seismic events canbe developed. That study has not yet started, and it is not included in the researchplan for fiscal year 2023.
It is not apparent that a second study for a PWR will provide substantial additional
knowledge or practical experience that are needed to support the NRRC objective ofdeveloping methods and guidance that can be used to perform plant-specificanalyses of the risk from seismic events. International experience from numerous
PRAs and the experience from the in-progress NRRC research study have shownthat the risk from seismic events depends very strongly on specific features of thesite and design details of each reactor unit at that site. From that perspective, there
is nothing inherent in the design of a PWR that would require fundamentally different
seismic analysis methods or PRA modeling techniques, compared to those appliedin the current research study. Of course, there would be differences in the sitespecific seismic hazard, differences in the local ground foundations and slopes, anddifferences in the plant-specific structures and equipment that may be damaged
during the event. However, those types of detailed differences apply for every site inJapan, regardless of whether the site contains a BWR or a PWR. The NRRC
methodologies and guidance should provide practical analysis techniques that canbe applied consistently to develop high-quality evaluations of the seismic risk at any
site, accounting for the wide variety of site-specific and plant-specific differences thatwill be encountered in practice.

-3-We recommend that the NRRC should re-examine why another seismic PRA study
for a PWR model plant site is needed as a prerequisite to developing a general
methodology and guidance that can be used evaluate the risk from seismic events.
(3) Detailed Analyses of Structural Failure Modes
The research plan contains an activity for the use of three-dimensional non-linear
finite element analyses to evaluate the seismic fragility of a reactor building,accounting for both horizontal and vertical loading. The research team indicated thatthe objective of these analyses is to distinguish between partial damage and totalcollapse of the structure, as distinct failure modes that affect the conditional
probability of core damage.
The terminology of "partial damage" is not reliable as a damage state, and it may
convey unintended perspectives. In a PRA, the deformation, the local load-bearing
capacity, and the resulting functional effects are the most important considerations.
A detailed and realistic analysis, which will improve an understanding of progressive
structural collapse, should also examine the effects of consequential damages atvarious intermediate loading levels to better understand the impacts on risk.
Depending on its physical characteristics and location, partial damage of the reactor
building structure can have a significant functional impact on the availability ofsystems that are needed to prevent core damage and mitigate offsite releases. For
example, building penetrations that contain critical piping systems, electrical power
cables, and instrumentation and control cables may be displaced or damaged byfalling debris. Debris may also damage other items inside the reactor building and inclose proximity to the building exterior. The combined functional effects from thatdamage may lead directly to core damage, or the damage may significantly increase
the conditional probability that core damage will occur if the remaining mitigation
systems fail. Thus, it is not apparent how a nominal distinction between partial
damage and complete collapse will facilitate a better understanding and improved
models for evaluating the risk from a spectrum of possible structural damage
conditions.
Without better consideration of the possible functional effects from a range of partial
damage conditions in the context of a PRA model, it is not apparent how these finite
element analyses will support a comprehensive and realistic evaluation of the risk
from seismic damage at an actual plant. We recommend that the NRRC should
confirm how the evaluation of partial damage and its consequences will be used in apractical PRA, before extensive finite element models are developed and detailed
analyses are performed.
Research Extensions
The following items summarize our recommendations for two extensions of theNRRC research programs, beginning in fiscal year 2023.

-4-(1) Spent Fuel Risk Assessment
The spent fuel risk research plan for fiscal year 2023 currently contains the samemajor activities, technical tasks, and schedules as the plan for fiscal year 2022. In
our letter report for last year's research plan, we explained why it is important toextend this research to include a more comprehensive assessment of thecontributors to spent fuel damage. We also explained why an integrated evaluationof the risk from events that may damage stored spent fuel is an important andpotentially challenging element of a full-scope PRA. In particular, the PRA must
consistently account for the composite effects from conditions that simultaneously
compromise cooling for the fuel in the reactor vessel and cooling for the stored spent
fuel during every combination of reactor and spent fuel pool operating modes.
The development of integrated PRA models that correctly account for the physical,functional, and human dependencies that affect the progression of these complex
event scenarios can be a significant technical challenge. Furthermore, there is very
limited international experience, with few reference PRA examples to demonstrate
how this problem has been solved in practice.
The research plan for fiscal year 2023 should be expanded to include an explicit
activity for development of methods and modeling practices that will be used tointegrate the analyses of spent fuel risk with the PRA models for full power, lowpower, and shutdown modes. The objective of this effort should be the developmentof NRRC guidance for an integrated assessment of reactor core and spent fuel risk,
supported by a practical PRA demonstration.
(2) Risk Integration
One of the fundamental goals of the NRRC is to provide support for the developmentof good quality full-scope PRAs at every nuclear power plant. Those PRAs should
evaluate the frequency of damage to the reactor core and stored spent fuel (Level 1risk) and the frequency of offsite radioactive releases (Level 2 risk) during all plant
operating modes (full power, low power, and shutdown). The PRAs shouldsystematically examine the contributions to that risk from a wide variety of sources:Internal events (plant transients, loss of coolant accidents, losses of offsite power,
support system failures)
Internal hazards (fires, floods, turbine missiles, toxic chemicals)
External natural events (earthquakes, tsunamis, high winds, volcanic eruptions,cooling water intake blockage, other site-specific hazards)
External man-made hazards (aircraft crashes, nearby industrial facility accidents,pipeline explosions, shipping accidents, railway accidents, highway accidents,
other site-specific hazards)

The PRAs for sites that contain multiple units should also account for the risk fromevents that can simultaneously affect two or more units at the site.
Finally, the PRAs must explicitly quantify the uncertainties in the overall risk and itscontributors. For example, experience has shown that uncertainties in the risk frominternal events are often much smaller than uncertainties in the risk from very rare-5-severe external events. Experience has also shown that the magnitudes andsources of those uncertainties can have an important effect on utility management
decisions about how to most effectively manage specific contributions to the overallplant risk.
Since its founding in 2014, the NRRC has made substantial contributions to thedevelopment and demonstration of methods and PRA models to evaluate the risk
from many types of internal and external events. However, that research has beenfocused primarily on detailed analyses of individual hazards and their associatedtechnical issues in the context of isolated PRA models. Aggregation of the resultsfrom those individual hazard assessments to provide a comprehensive quantificationof the overall plant risk and a balanced understanding of its contributors is not a
simple task of addition. For example, to prevent inappropriate conservatism andbias in the results from a full-scope PRA, the combined models and theirquantification process must avoid numerically accounting for the same risk
contributions multiple times. Furthermore, the supporting models must be structured
logically to provide a complete evaluation of all contributing hazards and accident
scenarios. Consistent screening criteria must be used for each analyzed hazard, toensure that the scope of the quantified risk contributors is transparent and wellunderstood, and that specific categories of events are not inadvertently omitted.
Consistent methods must also be used to quantify the sources of uncertainty in eachsupporting analysis.
International experience has shown that systematic and comprehensive treatment ofthese risk integration issues is important, and it must not be left as an afterthought.
As each element of the supporting analyses and models is developed, it should
maintain a fundamental perspective on the need for an integrated assessment ofoverall plant risk. Those integrated assessments should also facilitate the needs forutility managers, regulators, and the Japanese public to understand the overall risk
results, their contributors, and their underlying uncertainties.
The NRRC should initiate a research activity to develop methods and practicalguidance for the integration of all supporting PRA models and results to provide afull-scope assessment of the overall plant risk and its contributors. The guidanceshould also explain how to interpret and cope with various ranges of uncertainty inthe risk results. That activity should start in FY2023.
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-6-REFERENCES
1.

"NRRC Overview: Research Program for FY2023, RIDM Promotion,"
Presentation to NRRC Technical Advisory Committee, November 14, 2022,Proprietary.

2.

"NRRC Overview: Research Program for FY2023, Risk Assessment,"
Presentation to NRRC Technical Advisory Committee, November 14, 2022,
Proprietary.

3.

"NRRC Overview: Research Program for FY2023, External Natural Events,"
Presentation to NRRC Technical Advisory Committee, November 14, 2022,
Proprietary.

4.

Technical Advisory Committee of the Nuclear Risk Research Center,"Outcome of Hamaoka Model Plant Level 2 Tsunami PRA Project," June 3,2022.

5.

Technical Advisory Committee of the Nuclear Risk Research Center,"Proposed NRRC Research Plan for Fiscal Year 2022," December 30, 2021.

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