Probabilistic Evaluation of Failure Modes Leading to the Mispositioning of Certain ECCS Motor Operated Valves in WestingHouse NSSS

The recently approved industry standard ANSI-N6581, "Single Failure Criteria for PWR Fluid Systems", expands the definition of active failure to include the spurious action of a powered component but allows exemption for components whose reliable operation can be demonstrated. This report is a summary of the methodologies employed to demonstrate such reliable operation for certain emergency core cooling system (ECCS) motor operated valves. It is intended that this treatment of a single failure issue will increase the awareness of the industry to the potential role of quantitative analysis in determining failure criteria for nuclear power generating stations.