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
June 03, 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:OUTCOME OF HAMAOKA MODEL PLANT LEVEL 2 TSUNAMI PRA
PROJECT

Dear Dr. Apostolakis:
During the 16th meeting of the Technical Advisory Committee of the Nuclear Risk
Research Center (NRRC), May 23-27, 2022, we had a final briefing on the technicalbases and results from the research project of a Level 1 and Level 2 PRA evaluationof the risk from tsunamis, using a model plant that is based on Hamaoka Unit 4.
CONCLUSIONS AND RECOMMENDATIONS
1. Completion of the Level 1 and Level 2 PRA research project to evaluate thetsunami risk for a model plant is a milestone achievement for the Japanese
nuclear industry, and it has extended the methods and modeling tools that can beused to analyze risk in tsunami-prone areas worldwide.
2. The models, data, and supporting analyses should receive an independent andin-depth technical review.
3. A targeted sensitivity analysis should be performed to quantify the Level 1 andLevel 2 models without the suppression chamber cooling and fission productscrubbing effects from the tsunami flood water in the reactor building
4. The research team should examine how timing considerations are typicallymodeled throughout a practical full-scope PRA before more detailed event
scenario timing models are developed specifically for the tsunami analyses.
5. After resolution of the preceding recommendations, it should be possible toefficiently transform this research model into a technically sound, realistic,
practical Level 1 and Level 2 PRA model for evaluating the risk from tsunamis at-1-Hamaoka Unit 4. The Hamaoka plant-specific PRA could then be used for riskinformed decision-making.
6. The NRRC should develop tsunami risk assessment guidance for use by theJapanese utilities and the standards committee.
BACKGROUND
The primary objective of this research project was to develop a technical basis forevaluating the risk from tsunamis, using enhanced analytical methods and anintegrated Level 1 and Level 2 PRA model. We have received several briefings on
this project throughout its evolution, including research results regarding the tsunami
hazard analyses, fragility analyses, PRA models and accident analyses, and interim
results for the frequencies of core damage and containment failure. Our November
27, 2016 letter report provided our conclusions and recommendations on supporting
research for the preliminary models and analyses. During the present meeting, theNRRC research team reported the results from the Level 2 PRA source term
evaluation and uncertainty analyses.
DISCUSSION
Completion of the Level 1 and Level 2 PRA research project to evaluate the tsunamirisk for a model plant is a milestone achievement for the Japanese nuclear industry,
and it has extended the methods and modeling tools that can be used to analyze risk
in tsunami-prone areas worldwide. The level of analytical details in the model,
including the systematic process used for evaluation of uncertainties and theextensive coverage of tsunami-induced risk scenarios, enables its use as a valuablerisk management tool.
As with all pioneering research, there are important lessons to be learned from thisinitial application and potential enhancements that will further improve its technicaladequacy and capabilities.
Independent Technical Review
To date, the models, data, and supporting analyses have been reviewed primarily by
the project team as part of their normal research, development, and testing practices.
The team was also advised by an expert committee, formed by the Ministry ofEconomics, Trade, and Industry (METI), the project sponsoring organization. We
have provided individual member comments on specific issues during our periodic
briefings, but we have not performed a review of the complete project and its results.
It is essential that a study of this technical complexity and importance should receive
an independent and in-depth technical review. That review is needed to provide
confidence in the technical quality of the methods, models, and results, so that they
may be adapted for use in plant-specific PRAs at a variety of sites in Japan andworldwide.

-2-The review should be comprehensive. However, special attention should be focusedon analyses and models that were developed specifically during this project, such as
the tsunami hazard analyses, fragility analyses for structures and equipment,
selection of tsunami initiating events as a function of wave height at the control point,tsunami scenario event tree, flooding inundation models, integrated human reliability
analyses, containment failure modes and locations, adaptations of MAAP models forsuppression chamber external cooling, MAAP models for fission product releases
and transportation in submerged and dry areas of the reactor building, etc.

Reactor Building Flooding Analyses
The PRA model evaluates multiple possible pathways for the tsunami flood water toenter the reactor building, depending on the wave height and the status of floodbarriers. The consequential depth of water inside the building and the corresponding
damage to equipment depend on the specific event scenario and the appliedinundation models.
One objective of the Level 2 analyses in this study was to examine the following
effects from the water in the reactor building:•External cooling of the suppression chamber•

Scrubbing of fission products that are released into the flooded compartments

The research team has indicated that more detailed models are desirable to improvefidelity in all of the following analyses:•Water flow into and distribution throughout the reactor building•

MAAP models for external cooling of the suppression chamber•

MAAP models for fission product releases into the flooded areas of the reactor
building and transportation throughout the building•

Improved evaluation of decontamination factors for fission product scrubbing
during releases into flooded compartments

The current study results indicate that the most important contributions to the source
term for cesium involve accident scenarios with releases into the dry areas of thereactor building. However, those results account for the cooling and scrubbing
effects that are discussed above. During our meetings with the project team, wecould not determine the extent to which the source term and the overall risk are
mitigated by those effects.
A targeted sensitivity analysis should be performed to quantify the Level 1 and Level
2 models without the suppression chamber cooling and fission product scrubbing
effects from the water in the reactor building. The sensitivity analysis should retain
the effects from equipment damage due to the reactor building flooding. However,
the heat transfer from the suppression chamber into the reactor building water-3-

should be removed (e.g., heat transfer coefficient set to zero 1), and the effects fromfission product scrubbing through the reactor building water should be removed (e.g.,decontamination factor set to zero).
A comparison between the results from the sensitivity analysis and the baseline
study results (e.g., for core damage frequency and the cesium source term) will
provide an estimate of the overall risk benefit that is afforded by the applied reactor
building flooding models. It will also identify whether the most important risk benefits
are from the suppression chamber cooling effect, the fission product scrubbing effect,
or both. The team should then make a risk-informed decision regarding whether
more detailed models are needed.

Treatment of Scenario Timing
The research team has discussed the need for more detailed treatment of the event
scenario timing and dynamics. For example, for tsunamis that are generated byearthquakes, one potential element of the detailed scenario timing involves theresponses of plant systems and personnel during the interval between arrival of thefirst seismic shock and arrival of the first tsunami wave. Another potential element of
the detailed scenario timing involves the development of dynamic inundation models
that determine the times at which specific equipment fails and personnel actions areneeded, with corresponding effects on the thermal-hydraulic models for the event
scenario progression.
Experience from numerous contemporary full-scope PRAs indicates that analysts
typically make many simplifying assumptions which conservatively bound thefunctional effects from detailed timing considerations. For selected risk-significant
scenarios, timing may be evaluated explicitly, but at a rather coarse level that issufficient to account for the most important functional effects (e.g., timing models forloss of offsite power, failures of onsite emergency generators, and recovery of offsitepower). More detailed treatment of event scenario timing for the tsunami analyses isjustified if that treatment would have a significant effect on overall plant risk.
Furthermore, treatment of potential mitigation actions and their timing should accountfor actual operating experience.
The research team should examine how timing considerations are typically modeled
throughout a practical full-scope PRA before more detailed event scenario timing
models are developed specifically for the tsunami analyses.
Transition to a Plant-Specific PRA
During our discussions with the project team, we were informed that the results fromthe model plant evaluation might be used as a reference to identify high-priority
accident scenarios and to support risk-informed decisions for accident management
guidance at Hamaoka Unit 4.
The sensitivity analysis should remove credit for the heat transfer from the suppression chamber into
the water in the reactor building. The analysis should account for the consequential effects fromsuppression chamber heatup on the conditional probability of core damage and scrubbing of fission
products in the suppression chamber.
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After resolution of the preceding recommendations, it should be possible to efficiently
transform this research model into a technically sound, realistic, practical Level 1 andLevel 2 PRA model for evaluating the risk from tsunamis at Hamaoka Unit 4. The
Hamaoka plant-specific PRA could then be used for risk-informed decision-making.
For example, an important initial risk-informed application could be confirmation ofthe effectiveness of the very significant prevention and mitigation measures alreadyin place for tsunamis, as well as the optimization of accident management actions forresponse to severe tsunamis. The models could also be used to evaluate thepotential risk benefits from proposed additional prevention or mitigation measures, orto identify conservatisms leading to overly burdensome constraints.

Guidance Development
Considering the fact that this pioneering work can be the foundation for an advanced
and holistic analysis of the tsunami risk and is a major contribution to theinternational risk assessment community, the NRRC should develop detailed
tsunami risk assessment guidance that can be used by the Japanese utilities and thestandards committee.
Sincerely,John W. Stetkar
Chairman
REFERENCES
1.

"Outcome of Level 2 PRA on Hamaoka Tsunami PRA Project – Source Term
Evaluation and Uncertainty Analysis," NRRC Staff Presentation to NRRC
Technical Advisory Committee, May 25, 2022, Proprietary.

2.

"Tsunami Fragility," NRRC Staff Presentation to NRRC Technical Advisory
Committee, May 31, 2016, Proprietary.

3.

"CRIEPI / NRRC's R&D," NRRC Staff Presentation to NRRC TechnicalAdvisory Committee, May 31, 2016, Proprietary.

4.

"Strategy of Development of Tsunami PRA," NRRC Staff Presentation toNRRC Technical Advisory Committee, November 7, 2016, Proprietary.

5.

"Determination of Scenario Tsunami Conditions for Fragility Assessment
Based on Hazard Deaggregation," NRRC Staff Presentation to NRRC
Technical Advisory Committee, May 22, 2017, Proprietary.

6.

"PTHA and TFA in Tsunami PRA," NRRC Staff Presentation to NRRC
Technical Advisory Committee, November 8, 2017, Proprietary.

-5-7.

"Tsunami PRA Hazard and Fragility (Outdoor) Assessment (As a Study
Applied to Hamaoka NPP)," NRRC Staff Presentation to NRRC TechnicalAdvisory Committee, May 23, 2018, Proprietary.

8.

"Tsunami PRA Level 1 (Accident Sequence Evaluation) and Level 2(Hamaoka Study)," NRRC Staff Presentation to NRRC Technical Advisory
Committee, May 23, 2018, Proprietary.

9.

"Progress of Tsunami PRA Project," NRRC Staff Presentation to NRRC
Technical Advisory Committee, May 22, 2019, Proprietary.

10.

"HRA Application Method in Hamaoka Tsunami PRA and the Effectiveness ofHRA Improvement in PRA Results," NRRC Staff Presentation to NRRC
Technical Advisory Committee, May 22, 2019, Proprietary.

11.

Technical Advisory Committee of the Nuclear Risk Research Center, "Interim
Report on the Status of Tsunami Risk Assessment Research," November 27,
2016.

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