Quantitative risk trends deriving from PSA-based event analyses; Analysis of results from U.S.NRC's accident sequence precursor program

Watanabe, Norio

The U. S. Nuclear Regulatory Commission has been carrying out the Accident Sequence Precursor (ASP) Program to identify and categorize precursors to potential severe core damage accident sequences using the PSA technique. The ASP Program has identified a lot of risk significant events as precursors that occurred at U. S. nuclear power plants. The results from the ASP Program include valuable information that could be useful for obtaining risk significant insights and for monitoring risk trends in nuclear power industry. The present study examines and discusses quantitative risk trends for the industry level, using two indicators, that is, the occurrence frequency of precursors and the annual core damage probability, deriving from the ASP analysis results. It is shown that the core damage risk at U.S. nuclear power plants has been lowered and the likelihood of risk significant events has been remarkably decreasing. Also, the present study demonstrates that two risk indicators used here can provide quantitative information useful for monitoring risk trends in nuclear power industry.