Failure mode and effect analysis application for the safety and reliability analysis of a thermal-hydraulic passive system

The inclusion of passive safety-related systems within the advanced reactor design claims high system availability and reliability. A detailed system and safety analysis applying the failure mode and effect analysis (FMEA) approach is required as a primary step for the development of a methodology aimed at the reliability assessment of passive systems. The present study concerns thermal-hydraulic passive systems that are designed for decay heat removal and rely on natural circulation that foresee a heat exchanger immersed in a cooling pool. The main purposes of the work are to identify important accident initiators, find out the possible consequences to the plant deriving from component malfunctions, individuate possible causes, identify mitigating features and systems, and classify accident initiators in initiating events of accident sequences. A qualitative overview of accident sequences could be derived from the FMEA tables looking at consequences' description and preventive and corrective actions. Failure probabilistic evaluations are included as well to point out the probabilities and frequencies to have the plant in fault and/or unavailability conditions during passive system operation, therefore ensuring a complete set of initiating events of reactor accident sequences. Finally, important feedback to the design activities will derive from the FMEA study performed for safety assessment purposes. An important lesson elicited from the analysis is that measures against common-cause failures can reduce significantly the probability of failure of the system.