Dependent failure analysis of NPP data bases

A technical approach for analyzing plant-specific data bases for vulnerabilities to dependent failures has been developed and applied. Since the main purpose of this work, to aid in the formulation of defenses to dependent failures, is different than that for other dependent failure analyses, the approach of this analysis is also critically different. For instance, component failures have been defined to include all types of component function loss (e.g., catastrophic, degraded, incipient). Also, the determination of component failure dependencies has been based upon identical failure mechanisms rather than failure modes. Consequently, in the context of this study, both the definition of component failure and a categorization scheme for component failure mechanisms are used to identify clusters of component failures which are termed common mechanism failure (CMF) events. Motor-operated valves in two nuclear power plant data bases have been analyzed with this approach. The analysis results include seven different failure mechanism categories; identified potential CMF events; an assessment of the risk-significance of the potential CMF events using existing probabilistic risk assessments; and postulated defenses to the identified potential CMF events