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 19, 2019
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:

FRAMEWORK AND INFRASTRUCTURE FOR SUPPORTING
INDUSTRY READINESS FOR ROP IMPLEMENTATION

Dear Dr. Apostolakis:
The Japan Nuclear Regulation Authority (NRA) plans to implement a risk-informed
Reactor Oversight Process (ROP) to collect information about licensees'
performance, assess the information for its safety significance, and provide forappropriate licensee and NRA response. We were informed that this ROP is similar
to that deployed by the United States Nuclear Regulatory Commission (NRC).
The U.S. NRC's ROP contains many elements, one of which is a risk-informed scale
for assessing the safety significance of inspection findings. The intent of the scaling
is to establish appropriate regulatory response to different levels of licensee
performance. The basis for establishing thresholds related to these levels ofperformance was the metrics that have been adopted in Regulatory Guide 1.174 forthe characterization of risk; namely core damage frequency (CDF) and large early
release frequency (LERF). The Significance Determination Process (SDP) for afinding is typically performed in three phases, with a progressively increasing level ofcomplexity and sophistication of the risk assessment that is needed to ensure
realistic characterization of an event or a finding. In this phased framework, thereasonableness of the Phase 2 pre-populated risk characterization tables isimportant to avoid unnecessary entry to Phase 3. Additionally, the Phase 3 SDP istypically complicated and requires a significant level of interactions between thelicensee and the regulator. Therefore, the U.S. experience shows that it is beneficial
to have guidance available for consistently and realistically developing Phase 2 SDP
tables and for conducting the Phase 3 risk assessments.
During our 12th meeting, November 11-15, 2019, we met with representatives of theRisk-Informed Decision Making (RIDM) Promotion Team to discuss a number ofactivities which included support of the ROP, as well as the industry's action plan for
implementation of other risk-informed processes and programs. The objectives ofthese discussions were to allow us to develop a better understanding of these
initiatives and to provide our insights regarding potential areas of improvements.

-1-The NRRC is actively assisting the Japanese nuclear industry's development ofstate-of-the-practice PRA methodologies to be used for creating risk assessment
models and tools in support of risk-informed decision-making programs, with aparticular near-term focus being support for deployment of the ROP. However, weare not aware of any tools or guidance that may have been developed to facilitate
effective and efficient implementation of key elements of the ROP, such as the SDP.
We are also not aware of the risk scaling that is used to assess the severity of aninspection finding and the basis for this risk ranking scale. Knowledge about these
aspects of the ROP and understanding of their technical bases are important toenable us to provide our assessment of the NRRC’s research activities in support ofROP implementation and related future RIDM initiatives.
During our May 2020 meeting, we request a briefing from the organization that isresponsible for the industry’s readiness for ROP implementation. In particular, wewould appreciate a detailed presentation on the risk-informed elements of theJapanese ROP, including the risk scaling of findings, the basis for this risk scaling,
and the expected process for the execution of risk ranking.
Sincerely,John W. Stetkar
Chairman
REFERENCES
1.

U.S. Nuclear Regulatory Commission, "An Approach for Using Probabilistic
Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes tothe Licensing Basis," Regulatory Guide 1.174, Revision 3, January 2018.

-2-