Precursors to potential severe core damage accidents, 1986: A status report: Main report and Appendixes A,B, and C

As reported in licensee event reports (LERs), 63 operational events that occurred at commercial light-water reactors during 1985 are considered to be precursors to potential severe core damage. These are described along with associated significance estimates, categorization, and subsequent analyses. This study is a continuation of the work presented in NUREG/CR-2497 and NUREG/CR-3591, which evaluated the 1969 to 1981 events. The current study incorporates improvements that evolved since issue of the earlier reports, primarily in the areas of plant-class modeling and the review of reactor-trip-related events, in the assessment of the operational events that occurred during 1985. The report sequentially discusses (1) the general rationale for this study, (2) the program methods for LER review and documentation, (3) the development of event-tree-based core damage models, (4) the use of the conditional probability of subsequent severe core damage estimates to rank precursor events, and (5) the application of program results to current concerns pertaining to nuclear power plant risk assessment.