INVESTIGATIONS OF INTER-SYSTEM COMMON CAUSE FAILURES: AN UPDATE

摘要

Intra-system common-cause failures (CCFs) are widely studied and addressed in existing PSA models, but the information and studies that incorporate the potential for inter-system CCFs are limited. However, the French Safety Authority has requested that EDF investigate the possibility of common-cause failure across system boundaries for Flamanville 3 (an EPR design). Also, the modeling of inter-system CCF, or the determination that their impact is negligible, would satisfy Capability Category III for one of the requirements in the ASME/ANS PRA standard in the U.S. EDF and EPRI have presented at PSA '08 the proposition of a method to assess when it is necessary to take into account inter-system CCF in a PSA model. This method is based both on the likelihood of inter-system CCF and on its demonstrated potential impact on core-damage frequency (CDF). This method had been applied for pumps in different systems using a PSA model for an operating plant. Since that application was completed, the method has been applied to address the potential for failure of motor-operated valves across different systems, using the same PSA model. More recently, this application has been extended to consider the high-voltage circuit breakers in a PSA model of Flamanville 3. This paper describes the results of these last two studies and shows how they helped in refining the methodology. All three studies have shown either that components in different equipment are not susceptible to common causes of failure, or that the potential for inter-system common-cause failure had a negligible impact on the overall risk.