The development of a methodology aimed at the reliability assessment of thermal-hydraulic passive safety systems entails, as initial step, a detailed system and safety analysis and Failure Mode and Effect Analysis (FMEA) approach has been chosen to perform the safety analysis at system level. The present study is concerning Passive Systems designed for Decay Heat Removal relying upon natural circulation, which foresee, for the most part, a condenser immersed in a cooling pool. Main purposes of the work are: to identify important accident initiators, to find out the possible consequences on the plant deriving from component failures, individuate possible causes, identify mitigating features and systems and classify accident initiators in initiating events of accident sequences. A qualitative overview on accident sequences could be derived from the FMEA tables looking at consequences description and preventive/corrective actions. Moreover criticality analysis is applied (Failure Mode and Effect and Criticality Analysis) to include estimates of the loss frequencies, through failure probabilistic estimation, in order to point out the probabilities/frequencies to have the plant in fault and/or unavailability conditions during passive system operation, assuring therefore 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.
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