This paper presents a methodology to evaluate the reliability of passive systems characterized by a moving fluid and whose working is based on thermal-hydraulic (T-H) principles. The methodology has been developed in the framework of a Project called Reliability Methods for Passive Safety Functions (RMPS), performed under the auspices of the European 5th Framework Programme. The methodology deals with following problems: Identification and quantification of the sources of uncertainties and determination of the important variables. Propagation of the uncertainties through T-H models and assessment of T-H passive system unreliability. Introduction of passive system unreliability in the accident sequence analysis. Each step of the methodology is described and commented and a diagram of the methodology is presented. An example of passive system is presented with the aim to illustrate the possibilities of the methodology. This example is the Residual Passive heat Removal system on the Primary circuit (RP2), an innovating system supposed to be implemented on a 900 MWe Pressurized Water Reactor.
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