Development of a Probabilistic Model for Passive System Reliability Quantification

In this study a methodology to quantify the reliability of Passive Safety Systems (PSS) proposed for use in advanced reactor design is developed. The reliability of a PSS refers to the ability of the system to carry out function under the prevailing condition when required, trusting upon physical principles as free convection, conduction, gravity, etc. The present research activity is finalized at the reliability estimation of passive B Systems, i.e. implementing moving working fluids, cf. IAEA: for this purpose a generic system relying upon natural circulation for decay heat removal is is evaluated. From a methodical view it is more appropriate to base a reliability methodology on passive functions instead of passive systems. The matter treatment considers the introduction of the functional reliability concept, i.e. the probability of mission failure, through the R-S (Resistance-Stress) model taken from structural reliability, where, in the present functional model, R and S are coined as expressions of functional Requirement and system State). Water mass flow circulating through the system is accounted as physical quantity defining the PSS performance and probability distribution functions (pdf) are assigned to both R and S quantities in the functional model; thus the mission of the passive system defines which parameter values are considered a failure by comparing the corresponding pdfs according to a defined safety criteria.