Nuclear Risk Research Center (NRRC)
Central Research Institute of the Electric Power Industry (CRIEPI)
1-6-1 Otemachi, Chiyoda-ku, Tokyo, 100-8126, Japan
Dr. George Apostolakis
HeadMarch 24, 2021
Mr. J.W. Stetkar, Chairman
Technical Advisory Committee
Subject:

TAC Report titled “PROPOSED NRRC RESEARCH PLAN FOR FISCAL YEAR
2021” dated February 16, 2021

Dear Chairman Stetkar:
We appreciated the interactions with TAC throughout the review of our research plan and theCommittee’s insights. The NRRC reply to the TAC recommendations is as follows.
1. Research plan for fiscal year 2021
In the subject report, the Committee stated: “We did not identify any major gaps in the overall
technical research plan for fiscal year 2021.”

We are gratified to receive this comment.

2.Treatment of uncertainties prior to the development of detailed analytical tools
In developing risk assessment methods, we will identify and evaluate aleatory and epistemic
uncertainties of the hazards and phenomena which are the source of the risk in advance. Then,
important factors will be dealt with in the risk assessment method.
3. Inclusion of currently-available methods and models in the pilot plant PRAs
There is no doubt that building an integrated PRA model that supports all hazards and allmodes for one plant is an ideal situation that NRRC should aim for. However, regarding anatural external hazard, it may be more appropriate to use a plant different from Ikata and/or
KK depending on the geographical conditions. In addition, conducting research on multiple
hazards at the same plant imposes a heavy burden on the owner power company, and there isa risk that the research activities will interfere with each other and take much time. We will

make a research plan based on these practical requirements and considering the TAC
recommendation.
4. Recommendations for specific activities in the Discussion section
With regard to four recommendations for specific research activities, we will reconsider theplans for fiscal year 2021 and beyond for those research activities. Details are as follows:(1) Scope of Multi-Unit PRA Research
From JFY 2021, we will carry out a review of the literature and make a research plan on
multi-unit Level 2 PRA. And, from JFY 2022, we will start research on the construction of
MUPRA methods, including the development of Level 2 PRA. In addition, we have been
conducting research related to the seismic Level 1 MUPRA model so far. Studies todemonstrate the applicability of this method will be continued so that it can be utilized atLevel 2.
(2) Methods and Models for Evaluating the Risk from Seismically-Caused Tsunamis
NRRC has conducted studies which aim at developing methods of hazard and fragility
evaluations in a case where combinations of earthquake and seismic-induced tsunamis
impacts are considered.
NRRC is planning a model plant research beginning in fiscal year 2021 to develop an integrated
methodology of Level 1 and Level 2 PRA considering combinations of earthquake andseismic-induced tsunamis.
(3) Evaluation of Volcanic Ash-Fall Contribution to Loss of Offsite PowerWe follow the recommendation and first develop the volcanic ash-fall PRA model on nuclear
power plant systems. We may subsequently add sub-models to improve the analysis.

We

consider the probability for the loss of offsite power to be a candidate for additional
sub-model in the future.
(4) PRA Peer Review Guidance and Implementation
Currently, we are requesting reviews from overseas experts only, but we expect that domestic
PRA engineers will eventually join the reviewers. We recognize that this requires training fordomestic PRA engineers. In order to make plans for training, we are investigating the statusof PRA engineers (number of people, experience, etc.) of utilities, engineering subsidiaries, andplant vendor involved in the development of good PRA models, review the plans of each plant(estimated implementation time) assumed by each utility, and so on. Based on these results,

we will consider how to develop a specific peer review training program.
Sincerely,

George Apostolakis