No abstract

Ten operational events that affected 10 commercial light-water reactors during 1995 and that are considered to be precursors to potential severe core damage are described. All these events had conditional probabilities of subsequent severe core damage greater than or equal to 1.0 x lo6. These events were identified by first computer-screening the 1995 licensee event reports from commercial light-water reactors to identifjr those events that could potentially be precursors. Candidate precursors were selected and evaluated in a process similar to that used in previous assessments. Selected events underwent engineering evaluation that identified, analyzed, and documented the precursors. Other events designated by the Nuclear Regulatory Commission (NRC) also underwent a similar evaluation. Finally, documented precursors were submitted for review by licensees and NRC headquarters and regional offices to ensure the plant design and its response to the precursor were correctly characterized. This study is a continuation of earlier work, which evaluated 1969-1981 and 1984-1994 events. The report discusses the general rationale for this study, the selection and documentation of events as precursors, and the estimation of conditional probabilities of subsequent severe core damage for the events.

was prepared as an account of work sponsored by an agency of the United States Government Neither the United States Government nor any apncy thereof, nor any of their anployas, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, compictentss, or usefuiness of any infomation, apparatus, product, or process disclosed, or nprcscnts that its use would not infringe privately owned rights. Reference hmin to any spcific commercial product, process, or service by trade name, tradematic, manufacturer. or otherwise does not necessarily constitute or imply its endorsement, reammendation, or favoring by the United States -meat or any agency thereof.

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 ...

No abstract