Development of enhanced PRA model for Shimane Unit 2 internal event at-power for reflecting new findings including current plant states

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. 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, 2013) 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. An external expert review was conducted using (NEI, 2019) and (ASME/ANS, 2013) in the end of Phase II. 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 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 Facts and Observations (F&Os) raised in the external expert review conducted in the Phase II. This paper reports the processes and the details of improvements of the internal event at-power PRA for Shimane Unit 2 implemented in the As-is Phase.