Methodology for the reliability evaluation of a passive system and its integration into a Probabilistic Safety Assessment

A methodology has been developed to evaluate the reliability of passive systems characterised by a moving fluid and whose operation is based on thermal-hydraulic (T-H) principles. The methodology includes: - 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 900MWe Pressurized Water Reactor