Application of the REPAS methodology to analyze the reliability of the EHRS in the decay heat removal strategy for an SMR

Within the European framework, the Small Modular Reactors(SMRs), in particular the Light Water-SMRs, are one of the technology options for a nuclear reactor short-term deployment. The reasons come from the well-proven and established large Light Water Reactor technology, incorporating their operational plant knowledge, and to which they include evolutionary design modifications, as the use of passive safety systems, to increase the inherent safety of the plant. Passive safety systems determine several advantages as no reliance on external power sources and minimization of components, but also some drawbacks as the functional failure. In fact, the system can potentially fail due to the failure of the Natural Circulation, phenomenology at the base of its safety function. Therefore, passive system failures must be addressed by SMR designers and must be considered also in an independent safety review process. Considering this, in relation with deterministic safety demonstration, guidance on requirements specific to passive safety systems and their features have still to be consolidated together with guidance on the methodologies appropriate to model the system failure modes. Therefore, a short-term research needs are the reliability analysis for assessing the failure related to the ThermalHydraulic phenomena driving the operation of the systems and asses the related Uncertainty Analysis. Starting from the MELCOR input-deck developed by ENEA along the Horizon Euratom Safety Analysis of SMR with Passive Mitigation Strategies-Severe Accident (SASPAM-SA) project, this thesis aims to apply the REPAS (Reliability Evaluation of Passive Safety Systems) methodology to the Emergency Heat Removal System (EHRS), a passive safety system which, in advanced designs, can represent one of the key mitigation features for decay heat removal. REPAS will help analyze various system states, even low-probability ones, to understand EHRS behavior and its impact on the plant.