A methodology, to quantify the reliability of passive safety systems, proposed for use in advanced reactor design, is developed. Passive systems are identified as systems that do not need any external input or energy to operate and rely only upon natural physical laws (e.g., gravity, natural circulation, heat conduction, internally stored energy, etc.) and/or intelligent use of the energy inherently available in the system (e.g., chemical reaction, decay heat, etc.). The reliability of a passive system refers to the ability of the system to carry out the required function under the prevailing condition when required: The passive system may fail its mission, in addition to the classical mechanical failure of its components, for deviation from the expected behavior, due to physical phenomena or to different boundary and initial conditions. The present research activity is finalized at the reliability estimation of passive B systems (i.e., implementing moving working fluids, see IAEA); the selected system is a loop operating in natural circulation including a heat source and a heat sink.
The functional reliability concept, defined as the probability to perform the required mission, is introduced, and the R-S (Resistance-Stress) model taken from fracture mechanics is adopted. R and S are coined as expressions of functional Requirement and system State. Water mass flow circulating through the system is accounted as a parameter defining the passive system performance, and probability distribution functions (pdf's) are assigned to both R and S quantities; 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. The methodology, its application, and results of the analysis are presented and discussed.