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

Small Modular Reactors (SMRs), particularly Light Water-SMRs (LW-SMR), represent a viable option for near-term deployment of nuclear technology in Europe, building upon established Light Water Reactor technology while incorporating evolutionary design modifications like passive safety systems. While these systems offer advantages, such as independence from external power for the operation and component minimization, they face potential functional failures due to natural circulation mechanism. Therefore, such failure modes must be systematically addressed in SMR design and safety reviews. Current guidance on passive safety system requirements and failure mode modelling needs consolidation. Therefore, short-term research priorities for SMR deployment include reliability analysis of thermal-hydraulic phenomena that drive passive system operation, as well as related uncertainty quantification. Building on ENEA's MELCOR input-deck of a generic SMR of about 300 MWe, developed within the Horizon Euratom SASPAM-SA (Safety Analysis of SMR with Passive Mitigation Strategies-Severe Accident) project, this work applies the REPAS (Reliability Evaluation of Passive Safety Systems) methodology to the Emergency Heat Removal System (EHRS), a key passive safety system for decay heat removal in advanced designs. REPAS will help analyses various system states, including low-probability scenarios, to understand EHRS behaviour and its impact on plant safety.