DEPENDENT FAILURE ANALYSIS OF NPP DATA-BASES

A technical approach for analyzing plant-specific data bases for vulne rabilities to dependent failures has been developed and applied. Since the focus of this work is to aid in the formulation of defenses to de pendent failures, rather than to quantify dependent failure probabilit ies, the approach of this analysis is critically different. For instan ce, the determination of component failure dependencies has been based upon identical failure mechanisms related to component piecepart fail ures, rather than failure modes. Also, component failures involving al l types of component function loss (e.g., catastrophic, degraded, inci pient) are equally important to the predictive purposes of dependent f ailure defense development. Consequently, dependent component failures are identified with a different dependent failure definition which us es a component failure mechanism categorization scheme in this study. In this context, clusters of component failures which satisfy the revi sed dependent failure definition are termed common failure mechanism ( CFM) events. Motor-operated valves (MOVs) in two nuclear power plant d ata bases have been analyzed with this approach. The analysis results include seven different failure mechanism categories; identified poten tial CFM events; an assessment of the risk-significance of the potenti al CFM events using existing probabilistic risk assessments (PRAs); an d postulated defenses to the identified potential CFM events.