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, 2016
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:

INTERIM REPORT ON THE STATUS OF TSUNAMI RISKASSESSMENT RESEARCH

Dear Dr. Apostolakis:
During the sixth meeting of the Technical Advisory Committee of the Nuclear Risk
Research Center (NRRC), November 7-11, 2016, the NRRC staff provided anoverview of the center's proposed research activities related to tsunami hazard andfragility assessments. The two presentations provided a high level description of theresearch strategy and the general methodology. The staff's stated purpose for thesebriefings was to obtain our feedback on potential gaps in the proposed analysis
scope and approach. Our conclusions and recommendations are based only on thematerial provided in the referenced presentations and limited interactions with thestaff during our meeting.
CONCLUSIONS AND RECOMMENDATIONS
1. The project's objective of developing an integrated and holistic understanding of
the potential risk induced by tsunamis through investigating the phenomena thatgenerate tsunamis, estimating the frequency of potentially damaging tsunamis,and evaluating the plant response to direct and indirect impacts of tsunamis isimportant and appropriate.
2. The proposed analysis scope for investigating the reliability of flood protection
barriers (such as seawalls, water-tight doors, etc.) that may have been weakened
by the tsunami-inducing seismic event is desirable. This is an area of researchthat is beyond the current state of practice. Since there is little, if any, previous
work to build on, it is important that this task proceeds with a significant level ofdeliberation and consultation with structural experts to ensure the scope is welldefined, and it provides realistic and technically defensible results.
3. The graded analysis approach, which consists of a series of progressively morerefined analyses that depend on the site-specific tsunami hazard severity, is-1-consistent with contemporary risk assessment practice. However, the teamshould reconsider how the screening criteria for the graded analyses aredeveloped and applied to account for uncertainties in the tsunami hazard.
4. Based on the limited details provided in the presentation material and ourdiscussions with the staff, we have questions about the following issues related topossible site-specific vulnerabilities.
• Whether the failure probability of all potential flood propagation pathways will
be evaluated consistently• Whether indirect consequential impacts from a tsunami event will beconsidered adequately
5. Until there is a better understanding of the potential site-specific risk fromseismically-induced tsunamis, the scope of the initial research work and theprobabilistic risk assessment (PRA) models should not try to account for therelative timing of the seismic and tsunami effects.
6. The team should determine how the combined effects from seismically-induced
tsunamis will be represented and quantified in the PRA models. The methods todevelop the composite hazard and to evaluate the structural and equipmentfragilities for each source of loading should be compatible with the PRA modelingframework.
BACKGROUND
Tsunami waves may affect safety of a nuclear power plant by damaging structures,systems, and components (SSCs) needed for core decay heat removal, containment
functions, and spent fuel cooling. All nuclear power plants that are potentially
exposed to the tsunami hazard have installed barriers and features which aredesigned to protect those SSCs. Assessing whether these protection features
adequately address all potential vulnerabilities and determining that they maintainthe tsunami risk at an acceptable level are key components of a comprehensive risk
management program. The NRRC staff is planning to conduct studies that develop
a technical basis and provide analytical tools for a systematic and probabilistic
assessment of the tsunami hazard.
DISCUSSION
In general, the presented material and our discussions with the NRRC staff indicatedthat the proposed studies provide a solid basis for evaluation of the tsunami risk.
The strategy for developing a tsunami hazard PRA is generally consistent with thestate-of-practice for PRA evaluations of other external hazards. The proposed
methodology for assessing the risk from composite damage caused by a seismicallyinduced tsunami is beyond the state of practice. The following discussion andcorresponding recommendations are provided to enhance the proposed studies.

-2-Graded Approach to Scope of Analyses
The proposed methodology applies a graded approach to the evaluation of tsunami
risk, depending on the frequency and severity of the site-specific tsunami hazard.
The graded approach suggests that the most detailed assessments would beperformed for sites that have the most challenging tsunami hazard, while more
simplified conservative assessments are performed for sites at which the hazard isless severe. This approach is reasonable and consistent with general PRA practices.
The proposed screening criteria for application of this graded approach are based on
the mean occurrence frequency of tsunami waves that may overtop the site seawall.
Since there is substantial uncertainty in the tsunami hazard at any site, the meanvalue of the exceedance frequency may not be adequate for this type of screening
process. This is especially true if the site-specific analyses indicate that there is asignificant probability that damaging waves may occur more frequently than thenominal mean value screening criterion. Thus, more complete consideration of theuncertainty could affect decisions regarding the needed scope of the damage
assessments. These considerations are potentially important for sites that are near
the numerical screening boundaries between the successive evaluation categories.
The team should consider how the screening criteria are developed and applied toaccount for uncertainties in the tsunami hazard. For example, the criteria may more
appropriately account for a certain confidence that the hazard will not exceed thespecified severity, rather than reliance on only the mean exceedance frequency.
Potential Flood Propagation Pathways and Impact Analysis
The tsunami PRA methodology presentation and our discussions with the NRRC
staff did not provide adequate assurance that the failure probability of all potential
flood propagation pathways will be evaluated consistently. For example, floodpropagation paths via failures of waterproof seals (e.g., randomly, or due toincreased tsunami-induced back pressure) seem to have been dismissed without
adequate technical justification. Additionally, it was not clear whether indirectconsequential impacts from a tsunami event, such as plugging of cooling water
intake screens or fouling of in-plant heat exchangers due to the introduction of large
volumes of tsunami-generated debris (e.g., plastic bags, seaweed, oil from nearby
industrial or storage facilities, etc.) will be considered adequately.
Treatment of Tsunami Arrival Time for Seismically-Caused Tsunamis
The proposed research contemplates developing a method and supporting models
to explicitly account for the delay between the time that a seismic event affects a site
and the tsunami waves reach the site. There is uncertainty in the timing of thetsunami arrival at the affected site, depending on the location of the earthquake
source, its magnitude, site-specific bathymetry, etc. On the other hand, the durationof the seismic event is typically relatively short. The substantial effort needed toexplicitly account for these timing considerations may not provide unique additional
insights regarding the most important contributions to risk. In practice, PRA models
for specific risk-significant scenarios are occasionally refined to account for timing
information, if it is very important to their frequency or consequences. However,-3-those refinements are applied only after the scenario-specific risk contributors areidentified and are well understood.
Based on these considerations, the initial research work in this area should not try to
account for the relative timing of the seismic and tsunami effects. It is mostimportant to first develop methods and models that can be used to derive thecomposite hazard, consistently evaluate the site-specific fragilities, and represent thecombined seismic and tsunami damage for quantification in the PRA models.
Integration of Seismic and Tsunami Damage in PRA Models
This is a difficult task, and its solution will extend the current state-of-practice in PRA.
It is a significant challenge to develop consistent methods to analyze the composite
seismic and tsunami hazard, evaluate site-specific fragilities for the combined loads,
and quantify the effects from the overall damage in the PRA models. Experiencehas shown that it is often useful to structure the supporting analyses for thesecomplex problems by first understanding how the results will be used in the PRA.
Therefore, that element of this research should receive early priority.
The team should determine how the combined effects from seismically-induced
tsunamis will be represented and quantified in the PRA models. The methods todevelop the composite seismic and tsunami hazard and to evaluate the structuraland equipment fragilities for each source of loading should be compatible with thePRA modeling framework. During the meeting, we briefly discussed a general
conceptual construct for these hazard and fragility assessments. We look forward tocontinuing our discussions with the NRRC staff as they develop and refine theirmethods and models for this important research topic.
Sincerely,

John W. Stetkar
Chairman
REFERENCES
1.

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

2.

"Development of PRA Methodology for Superposed Seismic and Tsunami
Hazards," NRRC Staff Presentation to NRRC Technical Advisory Committee,
November 7, 2016, Proprietary.

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