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 PROBABILISTIC SEISMIC HAZARDANALYSIS ENHANCEMENTS IN JAPAN AND FAULTDISPLACEMENT EVALUATION

Dear Dr. Apostolakis:
During the sixth meeting of the Technical Advisory Committee of the Nuclear Risk
Research Center (NRRC), November 7-11, 2016, we met with representatives of theNRRC staff to review the current status of the Ikata Senior Seismic Hazard Analysis
Committee (SSHAC) Level 3 project, its technical findings, and key challenges forthe enhancement of probabilistic seismic hazard analysis (PSHA) in Japan. We also
discussed fault displacement evaluation, associated issues, and the NRRC researchplan to develop methodologies for design and probabilistic risk assessment (PRA)
applications. Our conclusions and recommendations are based only on the material
provided in the referenced presentations and limited interactions with the staff during
our meeting.
CONCLUSIONS AND RECOMMENDATIONS
1. The successful implementation of the SSHAC project for the Ikata site is apioneering and very important initiative for seismic risk assessments, applicationof risk-informed approaches, and the risk-informed decision making process inJapan. Based on the progress made so far, important insights have beendeveloped regarding technical and procedural challenges that need to beaddressed for application of the SSHAC process at other sites in Japan.
2. It is important that adaptations of the SSHAC process at other sites throughout
Japan preserve the fundamental technical elements of the SSHAC process.
3. Before further adaptation of the SSHAC process, the NRRC should consider aSSHAC project for one more site which has a seismo-tectonic environment withsome features that are different from the Ikata site. This will provide more robust-1-insights for developing guidance and pertinent
implementation of the SSHAC process in Japan.

research

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4. As suggested by the Participatory Peer Review Panel, training of the TechnicalIntegration team members for the Ikata project is critical for successful
implementation of the SSHAC process. We also recommend that it be carriedout.
BACKGROUND
In our letter of January 24, 2015 (Ref. 1), we recommended that:"Consistent with the recent international practice, SEPCO should implement
probabilistic seismic hazard analysis (PSHA) using the Senior Seismic Hazard
Analysis Committee (SSHAC) procedures (Refs. 2 and 3). In particular, theSSHAC Level 3 or higher procedure should be used for the Ikata Unit 3 site."
Since then, Shikoku Electric Power Company (SEPCO) and the NRRC have started
the process of implementing the Ikata SSHAC Level 3 project.
At this meeting, we were briefed on the current status of the Ikata SSHAC Level 3
project, and the NRRC staff discussed the overall schedule for the project. The first
SSHAC workshop was held in September 2016, and preliminary findings from thatworkshop were discussed. In addition to the findings coming from the Ikata SSHAC
project, technical and procedural challenges that need to be considered for
implementation of the SSHAC process throughout Japan were discussed. Some ofthese challenges arise from the more complex seismo-tectonic environment in Japanand its higher ground motion levels, compared to the U.S. and other countries wheremost experience has been gained in implementation of the SSHAC process.
We were also briefed on the research for fault displacement evaluation. Because of
the potential influence of faults at some Japanese sites, consideration of
displacements induced by fault movements and their impacts on plant performanceis quite unique to the Japanese situation. Very little state-of-practice experience
exists to address this issue, either deterministically or probabilistically. The
presentation was centered around the NRRC research plan to address the issue.
The plan contains the following three elements:
1. Hazard assessment: quantitative assessment of fault displacement, both
deterministic and probabilistic;
2. Fragility assessment: setting design criteria and fragility evaluation ofstructures and equipment against fault displacement; and3. Accident sequence evaluation related to fault displacement.

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DISCUSSION
The undertaking of the Ikata SSHAC project by Shikoku Electric Power Company
and the NRRC is a very significant step in development and application of PSHA
technology in Japan, consistent with international practice. Both Shikoku and NRRC
are to be commended for undertaking this complex, challenging, and very important
project. As stated in our January 24, 2015 letter, any probabilistic seismic hazard
analysis has large inherent uncertainties, as well as uncertainties related to limited
data, differing interpretations, and alternate models. The explicit consideration ofuncertainties and differing interpretations is critical in a PSHA to avoid controversies
and enhance credibility. The fundamental goal of a SSHAC process as stated in
Reference 3 is:"…to properly carry out and completely document the activities of evaluation andintegration, defined as:
Evaluation: The consideration of the complete set of data, models, and methods
proposed by the larger technical community that are relevant to the hazard
analysis.
Integration: Representing the center, body, and range of technically defensible
interpretations in light of the evaluation process (i.e., informed by the assessmentof existing data, models, and methods)."
The technical issues that arise from the complex seismo-tectonic environment inJapan and the need for development of methods and models to address theseissues pose significant challenges to carry out Japan-wide implementation of
SSHAC projects. Some specific challenges are discussed later. While thesechallenges may require some adaptation of the SSHAC process, it is important thatthe fundamental goal of the process as stated above is maintained and its integrity
preserved, particularly in light of the importance of seismic events to overall risk andits public perception. This is important for the credibility, transparency, andacceptance of the hazard results and their use in risk assessments, risk-informed
decision-making, and potential acceptance of risk-informed, performance-based
seismic design criteria for Japan.
To gain more robust insights and to provide more effective adaptation of the SSHAC
process for Japan, at least one more SSHAC Level 3 project, similar to the Ikata
project, should be implemented at a site which has seismo-tectonic features that aredifferent in some aspects from the Ikata site. For example, a soft site that requiresconsideration of local subsurface features and site response evaluations may
provide additional insights into technical issues, uncertainties, and development ofdata, methods, and models. This will allow more effective development of theguidance and may identify additional necessary research. As discussed by theNRRC staff, the experience gained from the Ikata SSHAC project has alreadyprovided significant insights about some of the technical issues that are common
elements of any PSHA.
The experience has also identified challenges in
implementation of the SSHAC process itself. Some considerations are offered below
to enhance the development of guidance and activities that may help in dealing withthe challenges.
-3-One technical issue is related to the importance of site-specific sources andassociated ground motion characterization, along with the need for considering fault
ruptures. Appropriate treatment of these sources will require development of sitespecific models. The Ikata experience (and experience from the additional project)
will provide insights into methods and data collection that are needed to characterizedominant local sources and ground motion with their associated uncertainties.
These supporting activities can proceed prior to their use in the formal SSHAC
process.
Another technical issue is that PSHA includes characterization of regional sources
and areal sources along with the ground motion characterization. It may be
worthwhile to consider the feasibility of early development of the necessary regionalmodels, which can then be used by several sites. This may alleviate some concernswith the timeliness and needed resources for each site-specific SSHAC project.
The classification of information and knowledge from the Ikata SSHAC workshops
into site-specific and generic categories is providing initial considerations needed toaddress the above two issues.
As observed by the Participatory Peer Review Panel for the Ikata project, an
important issue for implementation of the SSHAC process is that training of theTechnical Integration (TI) team members is crucial to success of the SSHAC processand will provide necessary insights on conduct of the TI team to fulfill its role. Suchtraining may be considered for other potential TI teams that may be used to carry out
the SSHAC process at other sites. This will enhance the broad understanding of key
concepts and elements of the SSHAC process and will, to some extent, alleviate
concerns related to the availability of expert resources. For example, methods toaccount for uncertainty and the consideration and recognition of cognitive biases are
typically not part of the technical education of many experts involved in the SSHAC
process. Early development and training of the TI teams may help in implementing
the SSHAC process at several sites in parallel.
The NRRC interactions with other SSHAC projects, such as Diablo Canyon andother high-seismicity sites, are valuable both in terms of experience gained in thoseprojects and feedback to guidance and approaches being developed by the NRRC.
As noted in the presentation, the treatment of fault rupture models in the Diablo
Canyon project and differences in current Japanese practice provide insights into
potential research and model development. We agree that Japan-specific researchis needed. In a similar vein, the SSHAC guidance that is currently used in the U.S.
(Ref. 3) is being revised to account for the experience gained from recent hazard
studies. The updated guidance may provide useful insights on some of the issues
faced in Japan, particularly those related to procedural aspects of the SSHAC
process.
The research project related to fault displacement is crucial for both regulatorycompliance and the understanding of risk. We agree with the three basic elementsof the NRRC research plan: hazard, fragility, and plant response. It is important that
this research should emphasize realistic evaluations that include consideration of
pertinent uncertainties and provide inputs that can be used for the risk-informed-4-framework. The deterministic evaluations should also have a clear understanding of
how the methodology accounts for uncertainties and what margins andconservatisms are embedded to address those uncertainties. The experience
gained from the SSHAC process is useful to achieve these goals.
Sincerely,John W. Stetkar
Chairman
REFERENCES
1. "Seismic Hazard and Fragility Evaluations at Ikata Unit 3," Technical Advisory
Committee Letter to Head, Nuclear Risk Research Center, January 24, 2015.
2. NUREG/CR-6372, "Recommendations for Probabilistic Seismic Hazard Analysis:
Guidance on Uncertainty and Use of Experts," Volumes 1 and 2, U.S. Nuclear
Regulatory Commission, April 1997.
3. NUREG-2117, Revision 1, "Practical Implementation Guidelines for SSHAC Level
3 and 4 Hazard Studies," U.S. Nuclear Regulatory Commission, April 2012.
4. "PSHA Enhancements in Japan Based on Ikata SSHAC Level 3 Project," NRRC
Staff Presentation to Technical Advisory Committee, November 7, 2016,
Proprietary.
5. "Fault Displacement Evaluation," NRRC Staff Presentation to Technical Advisory
Committee, November 7, 2016, Proprietary.

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