Passive systems analysis for decay heat removal

A systems analysis is deemed the initial step to be accomplished as support for the development of a methodology aimed at the reliability assessment of passive safety systems, providing important input to further more detailed quantitative studies (e.g. uncertainty and sensitivity analysis), in order to qualify and endorse the methodology itself (cf. RMPS project). 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 (i.e. the Isolation Condenser System) and consists of two parts: the qualitative safety and systems analysis and the reliability analysis. The former one adopts a hazard identification used qualitative method, as FMEA (Failure Mode and Effect Analysis), to assess the potential failure modes and their consequences associated with the passive system operation. Main purpose of this analysis, in the present context, is the identification and evaluation, on a qualitative basis, of the sources of uncertainties related to passive system performance, in terms of parameters judged critical for the natural circulation performance/stability and which are characteristic of the recognised failure modes: in addition a qualitative viewpoint importance and sensitivity analysis is presented. The latter part focuses on the reliability study about the Isolation Condenser System, performed applying the standard reliability tools (i.e. fault trees) on a simplified model, where the natural circulation, upon which the system relies, failure probability is estimated in terms of failures of systems/components designed to assure the best conditions and mechanisms for natural circulation stability and passive operation: the result provides a preliminary appraisal of the whole system unavailability, included therefore also the natural circulation together with the relative weight. (author)