Reassessment philosophy for the seismic safety of NPPs in Japan

After the JCO accident that occurred at the uranium processing facility in Tokai-mura, the Japanese Nuclear Safety Commission (NSC) decided to do review and reassessment in the subsequence regulation to improve the regulatory effectiveness. The basic cause of the JCO accident was the unsafe work procedures along with violation of regulations. This accident also led to criticism of the overseeing authorities, for their not noticing the incorrect activities of the operator over such a long period of time. Safety design, construction and operation should be performed without any degrading the safety performance from the plant-specific safety assurance defined in the siting permit. To confirm plant safety into the future, reassessment of the plant safety performance is essential and the results should be reflected in the subsequence regulation to help safety management. Seismic Probabilistic Safety Assessment (PSA) is a suitable method for reassessment of seismic safety performance. Seismic PSA should be conducted following a three-phase procedure according to the status of plant design and operation conditions. In the first phase, the seismic safety assessment scenario is confirmed from the seismic design specification in the safety design assessment report submitted for siting license application. In the second phase, the seismic PSA is performed with the detailed design and construction data before the operation permit. In the third phase, the seismic PSA should be performed based on a walk-down inspection of the plant operation. Development activities of seismic PSA methodology are on-going in Japan, to be utilize in a risk-informed safety management not only concerning prevention of severe accident but also concerning shutdown management, on-line maintenance, in-service inspection, and in-service testing. The NSC is still discussing of the reassessment methodology and how to reflect the reassessment results in the regulation. This paper present a philosophy of the reassessment of the seismic safety performance of NPPs using seismic PSA, to be reflected in the subsequence regulation. (author)