THE UNITED-STATES NRC ACCIDENT SEQUENCE PRECURSOR PROGRAM - PRESENT METHODS AND FINDINGS

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. © 1989.