Precursors to potential severe core damage accidents: 1992, A status report. Volume 17, Main report and Appendix A

Abstract

This report provides the 1992 results of the Nuclear Regulatory Commission's ongoing Accident Sequence Precursor (ASP) Program. The ASP Pr_ogram provides a safety significance perspective of nuclear plant operational experience. The program uses probabilistic risk assessment techniques to provide estimates of operating event significance in terms of the potential for core damage. The types of events evaluated include initiators, degradations of plant conditions, and safety equipment failures that could increase the probability of postulated accident sequences. The primary objective of the ASP program is to systematically evaluate U.S. nuclear plant operating experience to identify, document, and rank those operating events which were most significant in terms of the potential for inadequate core cooling and core damage. In addition, tile program has tile following secondary objectives: (1) to categorize the precursor events for plant specific and generic implications, (2) to provide a measure which can be used to trend nuclear plant core damage risk, and (3) to provide a partial check on PRA predicted dominant core damage scenarios. In recent years, licensees of U.S. nuclear plants have added safety equipment, and have improved plant and emergency operating procedures. Some of these changes, particularly those involving use of alternate equipment or recovery actions in response to specific accident scenarios, are not currently incorporated in the basic ASP models. Consequently, the ASP estirnates of core damage p_'obabilities could be conservative for certain accident sequences. To address this issue, the 1992 preliminary ASP analyses were transmitted to the pertinent nuclear plant licensees and to the NRC staff for Peer Review. These licensees were requested to review and comment oil tile technical adequacy of the analyses, including the depiction of their plant equipment and equipment capabilities. Each of the Peer Review comments was evaluated for reasonableness and pertinence to the ASP analysis in an attempt to use best-estimate values. All of the preliminary precursor events were reviewed, and the conditional core damage probability calculations were revised where necessary to consider information provided during the review. The objective of the Peel" Review process was to provide as realistic an analysis of the significance of the event as possible. As a result, the 1992 ASP significant precursor conditional core damage probability results are somewhat lower than would have been calculated with the methods used in previous years. Although this will make the year-to-year trending of risk somewhat more difficult, we believe it is an important step towards more realistic identification of significant events and conditions. The most important precursor events of 1992 (with one exception) involved electrical problems, including the reliability of the electrical transmission lines (the grid) serving the plant, and plant electrical problems, such as failure of equipment in the switchyard. One of these precursors ...