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
February 25, 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:RESULTS OF IKATA SSHAC LEVEL 3 PROJECT AND PLAN FOR
SSHAC-BASED PSHA DEVELOPMENT IN JAPANDear Dr. Apostolakis:
This letter report provides our observations, conclusions, and recommendations fromour review of the results of the Ikata Senior Seismic Hazard Analysis Committee(SSHAC) Level 3 Project and plans for SSHAC-based Probabilistic Seismic Hazard
Analysis (PSHA) development in Japan.
Due to the ongoing COVID pandemic, we could not meet with your team in our usual
format to exchange information and discuss our comments and questions on these
topics. In early October 2021, the Nuclear Risk Research Center (NRRC) seismic
research team sent us the following materials:•

English translations of the Ikata SSHAC Level 3 Final Report and Executive
Summary•

Review comments from Dr. Kevin Coppersmith (Special Advisor to the project)and Dr. Martin McCann (member of the Participatory Peer Review Panel)•

Presentations that summarize the technical content of the Ikata study and the
NRRC's preliminary plans for future SSHAC-related activities

We reviewed these materials and prepared our individual comments and questions,as we would normally do before our meeting. We sent several of those individual
member comments and questions to you for preliminary consideration by the
SSHAC project team and the NRRC staff. We then held a 2-hour video conferencein December 2021 to discuss selected comments and questions. The SSHAC teamalso provided detailed and thoughtful written responses to supplement our oral
discussions. We deliberated on those discussions and the team's responses, andwe developed the Committee's consensus observations, conclusions, andrecommendations that are provided in this letter report.

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As discussed later, completion of the Ikata SSHAC project and activities related touse of the SSHAC approach in Japan are some of the NRRC's most criticalundertakings in its mission to expand the use of risk-informed decision-making(RIDM). The purpose of our review was to provide comments and observations on
the Ikata SSHAC report, NRRC's plans to develop guidance for the use of SSHAC
approaches by other utilities, plans for the development of regional SSHAC studies,and plans for specific studies to characterize local site response.
As we have often stated in our letter reports during the last two difficult years, ourexperience from this effort continues to reinforce the vital importance of the dynamic
interactions during our face-to-face meetings. This is worth emphasizing again. We
sincerely hope that we can return to our normal meetings in 2022.
CONCLUSIONS AND RECOMMENDATIONS
1. We congratulate and commend Shikoku Electric Power Company, the NRRC,
and the entire SSHAC team for undertaking a complex, but essential, project andsuccessfully completing it in accordance with the SSHAC guidance. This is agroundbreaking project that should help Japanese utilities to conduct
comprehensive seismic risk evaluations, safety analyses, and RIDM applications.
2. The NRRC's planned activities for fiscal years 2022 through 2024 are timely tosupport the utilities' near-term needs. The NRRC should also evaluate the needfor other focused research and studies to further support the utilities' PSHA plans.
3. We understand that NRRC plans to issue a PSHA Implementation Guide inMarch 2022, based on the Ikata SSHAC experience. We request a detailed
briefing on the scope and technical contents of the Guide at the earliest
opportunity. The Discussion section of this report outlines a few preliminary
recommendations for the Guide, based on the available summary material andour discussions to date.
4. We understand that development of a Regional PSHA Study is planned for fiscal
years 2022 through 2024. We request early and periodic briefings on the scopeand specific technical activities for that study when the comprehensive planningis completed.
5. The NRRC research team should develop guidance and an example application
for a SSHAC-based evaluation of local site response for a soft rock site. That
guidance and application should be integrated with the Regional PSHA Study
activities, and it should be completed before utilities begin to perform their
updated site-specific PSHAs.
6. We understand that site-specific PSHA studies are planned to begin in fiscal year
2024 or 2025, using the information from the regional studies. We request abriefing on the scope and technical elements of those site-specific PSHA studies
when the technical interface with the regional studies is determined.

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BACKGROUND
The fundamental goal of the SSHAC process is to produce a probabilistic hazard
analysis that captures the center, body, and range of technically defensible
interpretations from the scientific community of seismic experts.
The Ikata SSHAC Level 3 PSHA study was sponsored by Shikoku Electric Power
Company. The NRRC provided project management and technical support. The
project Technical Integration teams were comprised of recognized experts fromthroughout the Japanese seismic technical community, including universities andprivate companies. Dr. Kevin Coppersmith, an internationally renowned expert fromthe United States, served as a Special Advisor to the project. Reviews wereconducted by a separate Participatory Peer Review Panel, which included Japaneseexperts and Dr. Martin McCann from the United States. The study was conductedaccording to a SSHAC Level 3 assessment process, following contemporary
guidance from the United States Nuclear Regulatory Commission, as implemented innumerous international SSHAC-based PSHA projects.
The completion of the SSHAC Level 3 project for the Ikata site is a major
accomplishment, considering the importance of seismic issues in nuclear safety andpublic perception of the seismic risk. It is important to demonstrate and explicitly
account for a credible and systematic consideration of uncertainties in the evaluationof a natural hazard that is inherently highly uncertain. The SSHAC methodology
represents an internationally accepted practice that has been utilized for many years.
Shikoku Electric Power Company and the NRRC are commended for carrying out
this project for a complex seismotectonic environment, as well as in a different
cultural and a different regulatory environment and in a reasonable schedule.
We recommended implementation of the SSHAC project for Ikata in our January 24,2015 letter report. Since then, we have been briefed on the progress of the SSHAC
project at almost every meeting. During our November 2016 meeting, NRRC also
started to describe its plans for PSHA enhancements in Japan, based on the Ikata
SSHAC project. We provided our recommendations on this aspect of the project in
our November 27, 2016 letter report. The NRRC plans for SSHAC-based PSHA
development in Japan are critical to gain acceptance by the Japanese utilities and tocreate a practical methodology suitable for the seismotectonic environment andbusiness practices of Japan. The lessons learned from the Ikata project should
provide important inputs to both technical and programmatic elements.
DISCUSSION
Completion of the Ikata SSHAC project is a very significant milestone. The next
steps are to extend the use of SSHAC approaches by other utilities to provide arobust, state-of-practice analysis of the seismic hazard at each site. Thosecomprehensive hazard analyses will facilitate improved evaluations of the risk fromseismic events, regulatory safety assessments, and site-specific RIDM applications.

-3-Based on the summary information provided for our reviews and our discussions
with the NRRC staff, we understand that the NRRC's plans for PSHA enhancement
consist of the following three steps:
Step 1: Prepare a PSHA Implementation Guide based on the Ikata SSHAC study (tobe completed in March 2022).
Step 2: Prepare a Regional PSHA Study and supporting technical evaluations which
are broadly applicable to one or more regions in Japan (planned for fiscal
years 2022 through 2024).
Step 3: Conduct site-specific PSHA evaluations, based on information from theregional studies (to begin in fiscal year 2024 or 2025).
We fully support these progressive activities to develop a comprehensive
probabilistic assessment of the seismic hazard at each Japanese nuclear plant site.
The NRRC staff described some of the challenges that were encountered during the
Ikata study and how they may affect the use of SSHAC approaches by other utilities.
Two of the challenges include: (1) budgetary and time resources required for a sitespecific study and (2) availability of expert resources, considering the academic andbusiness environment in Japan. The NRRC's proposed activities are intended toaddress several elements of these challenges.
The following sections summarize our preliminary observations, comments, andrecommendations for each major step of the NRRC plan, based on the informationthat we have received to date.
PSHA Implementation Guide
This near-term "guide" can also be viewed as a lessons-learned report from the Ikata
SSHAC activities. We understand NRRC's desire to issue this guidance as soon as
possible. It is very useful for all Japanese utilities to share the knowledge andexperience from this landmark study. We understand that the working group that is
preparing the Guide consists of subject matter experts, Technical Integration teammembers, a Participatory Peer Review Panel member, and members of the NRRC
team who participated in the Ikata project. The lessons learned from the SSHAC
project can be grouped into the following three general categories:
1. Procedural issues related uniquely to the Japanese academic and businessenvironment.
2. Technical issues. These technical issues can be further divided into generaltechnical issues that are common to all PSHA analyses and issues that are
specific to an application of the SSHAC methodology.
3. Logistics and implementation issues.
Some of the adaptations and adjustments made by the Ikata project team provide
valuable practical experience for ways to cope with procedural and logistical-4-challenges in implementing the SSHAC process in Japan. Consistent with ourtechnical advisory role to the NRRC, we will focus more on the technical issues.
The Ikata SSHAC project team conducted comprehensive studies on several
concepts that are different from the PSHAs that Japanese utilities have implemented
so far. Examples include evaluations of a variety of international analysis methods
and data, selection of a wide range of Ground Motion Prediction Equations (GMPEs),
detailed study of site correction factors, and adoption of specific fault rupture models.
As the NRRC research team noted during our discussions, these are important
issues that should be brought to the attention of utilities before they update their
PSHAs.
We understand that the Guide will address the need for training on the SSHAC
methodology and process. We also understand that the participants in each SSHAC
project will receive individual training when the project is implemented. However, it
would be very useful for the Guide to provide a baseline training plan which can bereferenced and adapted as necessary to suit the needs of an individual project. This
will be a good way to transmit the experience and lessons learned from the Ikata
project, and it will provide a consistent knowledge base when new experts implement
the SSHAC process for the planned regional studies and site-specific analyses.
The Guide should also address some other broader generic issues that areimportant to understanding the technical basis and limitations of the PSHA results.
Examples of these issues include insights into data collection, insights into sourcesof uncertainties, identification of dominating seismic sources, and additional
investigations that could be undertaken for better characterization. This could leadto the development of joint programs by the utilities to further examine these issues
that are common to all PSHAs.
The systematic and transparent consideration of uncertainties is one of the mostimportant elements of the SSHAC process. To enhance the value of the Ikata
experience and develop insights for the next phase, it is suggested that the Guide
should provide a more detailed discussion of lessons learned with respect touncertainties and issues encountered. It should also contain guidance for how toachieve an appropriate, technically-defensible balance between (1) the fundamental
need for a comprehensive, integrated evaluation of epistemic and aleatory
uncertainties in all elements of the analyses, and (2) the performance of resourceintensive, detailed analyses using specific analytical methods and models.
Determination of the site-specific seismic hazard is fundamental, but it is not the final
step. The PSHA results are relevant only when they are used for integrated riskinformed decision-making.
For example, in deterministic safety analyses, acomparison of the design-basis seismic spectra with the probabilistic hazard,
including uncertainty, can be used to evaluate the robustness of structures, systems,
and components against seismic loads over a range of earthquake recurrence
intervals and to decide whether equipment modifications or operating procedureenhancements are needed. The PSHA results are also used in an integrated
probabilistic risk assessment (PRA) that evaluates the risk of damage to the reactor
core and spent fuel (Level 1) and consequential offsite releases (Level 2) from theentire spectrum of earthquake hazards, including explicit treatment of the uncertainty-5-

in the risk. Results and insights from the PRA are key elements in site-specific andindustry-wide risk-informed decision-making applications. It is very important that
the PSHA should be conducted in a manner which produces results that can be
integrated with the desired uses. Therefore, the Guide should include information
about how the analyses are structured to produce results that can be used in RIDM
applications.
Unfortunately, it is not practically feasible for us to review the draft PSHA
Implementation Guide and to exchange technical information, comments, questions,and specific recommendations with the NRRC research team before the planned
March 2022 issuance date. We request a detailed briefing on the scope andtechnical contents of the Guide at the earliest opportunity.
Regional PSHA Study
The NRRC's objective for a Regional PSHA Study is to develop common informationand models that will support eventual performance of site-specific analyses. Thisapproach is intended to improve technical consistency and efficiency, and overcome
some of the challenges associated with completely independent site-specific studies.
This is a practice that has been carried out in other countries, such as the UnitedStates and Spain. The Regional Study also provides a consistent treatment ofcommon elements, such as seismic sources and GMPEs, which may be applicable
to several sites. Based on our preliminary discussions, we understand that, in
addition to GMPEs and treatment of regional seismic sources (e.g., subduction typeand other common areal sources), the study will also evaluate other generally
applicable issues, such as validation of fault rupture models.
We understand that the SSHAC approach for the use of GMPEs andcharacterization of the associated uncertainties differs from the current Japanesepractice, which relies primarily on GMPEs proposed by individual researchers using
different databases. Along with the planned Regional Study, the NRRC isundertaking additional studies of some specific issues, such as treatment of GMPEs
using standardized databases similar to that done in recent projects (e.g., for theCentral and Eastern United States) and development of enhanced site correction
factors (e.g., for site-specific corrections of GMPEs and local amplification). We
agree that these two very important elements should be addressed soon.
We understand that there are other elements of the integrated methodology (e.g.,fault rupture modeling) that can benefit from additional studies, but cannot be
included in the 2022 - 2024 time frame. We suggest that the NRRC carefully
evaluate these issues in the seismic research plans, considering the utilities' needs
and schedules for their next PSHA updates. Of particular importance are thoseelements of the analyses which can have a systematic impact on characterization of
uncertainties. Examples include development of earthquake catalogs utilizing shortrecorded history, incorporation of applicable alternate models rather than focusing on
fewer models with emphasis on precise calculations, and use of nominal applied
uncertainty distributions in lieu of direct quantification of uncertainties through therespective analytical models. Activities related to these issues and planned studies
will require interactions with the utilities and external experts, and a dialogue withthem should be initiated early.

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The Regional PSHA Study and the development of companion guidance for
integrated SSHAC-based site response analyses are very important.
Weunderstand that our current information is very preliminary and that morecomprehensive planning is ongoing. We request early and periodic briefings on thedetailed plans for the Regional Study, how the Regional Study will be utilized in thenext phase of the analyses, and the results of outreach and communications withutility stakeholders.
Site-Specific PSHA Evaluations
We understand that site-specific PSHA evaluations based on the Regional Study will
begin in approximately fiscal year 2024 or 2025 and continue beyond. The studies
will include elements such as characterization of site-specific seismic sources, site
corrections of regional GMPEs, site amplification, logic tree modeling, etc.
The three major elements of a site-specific PHSA include:(1) Characterization of seismic sources,(2) Characterization of ground motion (transmission of motion from each source tothe bedrock, or reference rock, at the site), and(3) Characterization of local site response (transmission of motion from thereference bedrock to the site structural foundations).
The local site conditions can have an important effect on the spectral frequencies
and magnitudes of the accelerations that are transmitted to the plant structures. The
uncertainties in the local site response are as significant as uncertainties in the otherelements of the PSHA. Furthermore, the uncertainties in the local site responsemodels must be evaluated consistently and integrated carefully with the uncertainties
in the ground motion models that link the seismic sources to the site. For example,
the abstract in a recent U.S. Nuclear Regulatory Commission Research Information
Letter (RIL 2021-15) notes the following:"…The SSHAC process has been consistently applied to the seismic source
characterization (SSC) and ground motion characterization (GMC) componentsof PSHAs performed for critical facilities for more than 15 years. However,
because site response analyses (SRA) have often been conducted outside of the
SSHAC process it has not benefitted from the systematic evaluation of alternative
data, models, and methods within a structured and logical framework….This
report documents work sponsored by the NRC that applied the SSHAC process
for systematically identifying and propagating epistemic uncertainties in the SRA
as has been previously applied to the SSC and GMC analyses. The process wastested at two example sites, the resulting epistemic uncertainty in elements of the
SRA at both sites were found to be as large or larger than those of the SSC and
GMC models. This finding supports the rationale for implementing a structured
process such as SSHAC to capture and document the uncertainties in the SRA."-7-

The report also documents the efficacy of alternative methods for incorporating theresults of the SRA into the final PSHA hazard calculations.
The Ikata site is a hard rock site. Therefore, it was not necessary to address theissue of site amplification in the Ikata SSHAC study. In our letter report of November
27, 2016, we suggested that the NRRC consider a SSHAC project at a site withsofter rock conditions. We understand the reasons why this was not practicallyfeasible. We agree that it is important to proceed with development of the PSHA
Implementation Guide and the Regional PSHA Study in a timely manner. However,
we recommend that a study which demonstrates a SSHAC-based site response
methodology and integrated treatment of uncertainties for a soft rock site should beundertaken in coordination with the Regional Study. The methodology, guidance,
and an example application should be completed before utilities begin to performtheir updated site-specific PSHAs.
We fully endorse the planned approach to extend the regional studies to site-specific
PSHAs. However, it is not clear how this will be accomplished. For example, we do
not know if NRRC plans to conduct example studies that demonstrate the general
methods, or whether the plans include a complete PSHA evaluation for a selectedsite (in cooperation with a utility). We request briefings on the scope and technicalelements of the site-specific PSHA studies, including the development of methods to
conduct the local site response analyses, when the technical interface with theregional studies is determined.
The systematic development of site-specific SSHAC-based PHSAs is a criticalelement of an integrated understanding of the seismic risk at each Japanese nuclear
power plant. That knowledge is essential to support a comprehensive process ofrisk-informed decision-making. We look forward to our continuing interactions with
the NRRC seismic research team to review interim plans, draft guidance, andresearch results for these topics in a timely manner.
Sincerely,

John W. Stetkar
Chairman
REFERENCES
1.

Shikoku Electric Power Company, "Ikata SSHAC Level 3 Project Final
Report," October 2021, Proprietary.

2.

Shikoku Electric Power Company, "Ikata SSHAC Level 3 Project Final Report,
Executive Summary" October 2021, Proprietary.

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3.

Review comments from Dr. Kevin J. Coppersmith, Special Advisor to the Ikata
SSHAC Level 3 PSHA Project, various dates, 2016 – 2020, Confidential.

4.

Review comments from Dr. Martin W. McCann, Jr., member of Ikata SSHAC
Level 3 PSHA Participatory Peer Review Panel, various dates, 2016 – 2018,
Confidential.

5.

Nuclear Risk Research Center, "Results of Ikata SSHAC Level 3 Project andPlan of SSHAC-Based PSHA Development in Japan," Presentation to NRRC
Technical Advisory Committee, October 2021, Proprietary.

6.

"Appendix: Outline of Technical Study Contents of the Ikata SSHAC Project,"
Presentation to NRRC Technical Advisory Committee, October 2021,
Proprietary.

7.

Technical Advisory Committee individual members' comments and questions
on "Ikata SSHAC Level 3 Project Final Report," November 23, 2021,
Confidential.

8.

Technical Advisory Committee individual members' comments and questions
on "Ikata SSHAC Level 3 Project Final Report Executive Summary,"
November 23, 2021, Confidential.

9.

Technical Advisory Committee individual members' comments and questions
on "Review of NRRC SSHAC Presentations, Executive Summary, and Final
Reports," November 23, 2021, Confidential.

10.

Technical Advisory Committee individual members' comments and questions
on presentation for "Results of Ikata SSHAC Level 3 Project and Plan of
SSHAC-Based PSHA Development in Japan," November 23, 2021,
Confidential.

11.

U.S. Nuclear Regulatory Commission, "Recommendations for Probabilistic
Seismic Hazard Analysis: Guidance on Uncertainty and the Use of Experts,"
NUREG/CR-6372, April 1997.

12.

U.S. Nuclear Regulatory Commission, "Practical Implementation Guidelines
for SSHAC Level 3 and 4 Hazard Studies," NUREG-2117, Revision 1, April
2012.

13.

U.S. Nuclear Regulatory Commission, "Updated Implementation Guidelines
for SSHAC Hazard Studies," NUREG-2213, October 2018.

14.

Technical Advisory Committee of the Nuclear Risk Research Center, "Seismic
Hazard and Fragility Evaluations at Ikata Unit 3," January 24, 2015.

15.

Technical Advisory Committee of the Nuclear Risk Research Center, "Interim
Report on Probabilistic Seismic Hazard Analysis Enhancements in Japan andFault Displacement Evaluations," November 27, 2016.

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16.

Goulet, C., Y. Bozorgnia, N. Abrahamson, N. Kuehn, L. Al Atik, R. Youngs,
and R. Graves, "Central and Eastern North America Ground-Motion
Characterization," PEER report No. 2018/08, Pacific Earthquake Engineering
Research Center at the University of California, Berkeley, 2018.

17.

U.S. Nuclear Regulatory Commission, "Documentation Report for SSHAC
Level 2: Site Response," Research Information Letter (RIL), RIL 2021-15,
November 2021.

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