Safety implications of multiple passive safety valve failures in an i-SMR design

Abstract

Small Modular Reactors (SMRs) are being developed as next-generation nuclear options in response to increasing global power demand driven by data centers, artificial intelligence, and industrial electrification. Korea's innovative SMR (i-SMR) is designed as an integral pressurized water reactor with a thermal output of 520 MWth, aiming to achieve a core damage frequency below 1.0E-09 per year. To meet these goals, the i-SMR adopts multiple Passive Safety Systems (PSSs), including the Passive Emergency Core Cooling System (PECCS), which ensures core cooling through depressurization and natural recirculation. While previous studies have validated the effectiveness of PSSs under nominal conditions, their performance under multiple passive valve failures remains underexplored. In particular, the emergency depressurization valves, and emergency recirculation valves are critical components that enable passive cooling. If one or more of these valves fail to function as designed, coolant may be lost rapidly or recirculation hindered, accelerating fuel exposure. In this study, a detailed MELCOR input model of the i-SMR was developed to analyze accident progression behavior under multiple passive valve failures, particularly focusing on PECCS-related valves. MELCOR simulation results revealed that partial valve failures can lead to faster core degradation than complete system failure scenarios. These findings highlight the critical need to incorporate design redundancy and diversity in passive valve components to ensure reliable accident mitigation capabilities in the i-SMR.