Passive safety systems are incorporated extensively in current SMR designs. Compared to active safety systems, which are driven by external power sources, and must be activated to function, passive safety systems rely more on physical phenomena, like gravity or natural circulation. Because of the nature of the phenomena, such as weak driving forces in natural circulation, the assessment of the failure probability for a passive function is challenging.
The paper presents a reliability estimation method of passive safety systems in order to support the probabilistic risk assessment (PRA) of small modular reactors (SMR). The method has been influenced by the guidelines of EPRI and IAEA, and it can be summarized in two main consecutive steps. First, the scenarios that induce different conditions to the passive safety system are systematically and thoroughly identified, grouped and screened. Second, the conditional failure probability of the passive safety function is estimated using expert judgment in each scenario. Thermal-hydraulic simulations are used to support the expert judgments. Compared to many existing methods, which are often simulation-based, this method avoids the performance of a very large number of deterministic simulations.
In the method, the integration of plant level scenarios and conditions to component level phenomena and estimating their impact to system level reliability supports the very needs of PRA. The method is demonstrated in the paper using the LDR-lite SMR design concept, which is a public research study version of the LDR-50 SMR design created at VTT and now developed and commercialized by Steady Energy. The LDR-lite design, which is a low-temperature, integral, district heating reactor, relies on passive emergency core cooling function that is in the focus of the presented reliability analysis demonstration.