The US NRC accident sequence precursor program: Present methods and findings

Abstract History, methods and current results of the US NRC Accident Sequence Precursor program are described. The ASP program identifies and categorizes the risk significance of light-water reactor events involving core damage initiators, failures in safety-related systems, and degradations in multiple safety-related systems. Four hundred twenty-two precursors from 1969–81 and 1984–87 have been identified. While a higher incidence of such events continues to occur in early years of plant operation, an overall reduction in the number of precursors and in precursor significance was observed in 1984–87 compared to 1969–81, an indication of improved plant performance in the later period.