Enhancement of Shimane Unit 2 Internal Event At-Power PRA for Reflecting New Findings Including Current Plant States
Authors: Usui Takahiro , Nakamura Hiroki , Ihara Kenichi , Hirai Yuki , Nojima Hitoshi , Yoneda Satoshi , Kanda Kenichi , Noriyasu Kazunobu , Shiota Daichi , Hirokawa Naoki 
Published: November 2024

After the Fukushima Daiichi nuclear power plants accident, the scope of Probabilistic Risk Assessment (PRA) application was enhanced to meet the new Japanese regulatory standard for nuclear power plants. In addition to that, an importance for risk management has come to be re-recognized in Japan. To use PRA in risk management, Japanese utilities are proceeding the projects for enhancing the quality of PRA model to comply with international standards. One of the risk managements using PRA in Japan is new nuclear regulation inspection process which is based on the Reactor Oversight Process (ROP) of the U.S. Nuclear Regulatory Commission (NRC). Technically adequate PRA is required for such risk-informed inspection. For risk-informed applications, it is important to address findings identified for the PRA, and update PRA continuously and periodically to adequately reflect the current design and operational features.

The Chugoku Electric Power Company is also upgrading the internal event at power PRA model of the Shimane Unit 2 Nuclear Power Plant (Boiling Water Reactor (BWR)) including both the level 1 and level 2 (except the source term analysis) and enhancing PRA quality to reflect international state-of-the-practice approaches and to meet ASME/ANS PRA standard requirements (Capability Category II (CC-II) or higher).

Our PRA upgrading process is divided into 3 phases: “Phase I”, “Phase II” and “As-is”. In the Phase I and Phase II, the PRA model was upgraded. This upgrade includes initiating event refinement, accident sequence re-evaluation, detailed fault tree (FT) analyses, modeling severe accident (SA) measures, updating human reliability analysis (HRA), etc. An external expert review was conducted using NEI 17-07 and ASME/ANS PRA standard in the end of Phase II. The external expert review assessed technical adequacy of each supporting requirement (SR) defined in the ASME/ANS PRA standard. The external expert review judged 228 SRs were met CC-II or higher among 246 applicable SRs. In addition, Facts and Observations (F&Os) were provided to identify technical area in which further improvement is required.

In the As-is Phase, Shimane Unit 2 PRA is further being updated and upgraded. PRA was updated to reflect the as-built and as-operated plant features (e.g., the latest design of the SA measures and the latest procedures) resulted from changes made to the plant design and procedure by licensing activities which were performed in parallel with PRA enhancement in the Phase I and Phase II. In addition, we updated the component failure data newly issued for Japanese nuclear power plant and reflected the findings on PRA for other Japanese plant. We addressed F&Os raised in the external expert review conducted in the Phase II. F&O independent assessment was performed to review closure of some of the F&Os in accordance with Appendix E of NEI 17-07.

This paper outlines the processes and the details of improvements of the internal event at-power PRA for Shimane Unit 2 implemented in the As-is Phase.