Safety system function trend indicator: Theory and test application

Abstract

Methods for formulation, interpretation, and validation of dynamic risk and reliability indicators are studied. The use of these indicators for monitoring various levels of safety performance in nuclear power plants, as identified by probabilistic risk assessments (PRAs), such as safety system unavailability, safety system failure frequency, and core-damage frequency, are explored. Simplified indicators for detecting trends in the unavailability of safety systems in nuclear power plants not requiring PRA models and extensive data collection effort are being developed for possible NRC use. These indicators, called safety system function trend indicators (SSFT), are designed because they are easy to implement to display trends in system unavailability which can be detected through their visual inspections. The characteristics of these indicators are studied and optimized through simulation studies to assure sufficient capability in displaying a trend. To further substantiate the findings from visual examination of these indicators, statistical trend tests for sparse data are developed to aid the interpretation of these indicators. To evaluate the indicators from a safety point of view (rather than pure statistical sense) methods for comparing these indicators to a given alert level are being explored. Finally, approaches for engineering interpretation of these indicators are being studied by identifying the major contributors and causes for anomalies in indicator behavior. A preliminary pilot application/validation of these indicators, based on data from a sample of eight units, was completed in FY89. The findings are promising and warrant further refinement in the methodology. 5 refs., 3 figs