The International Reactor Innovative and Secure (IRIS) is an integrated small modular reactor (SMR) designed by Westinghouse Electric Company and a consortium of organizations. It has a thermal electric power rating of 1000 MWth and an electric power rating of 335 MWe. Being an SMR that features a derivative pressurized water reactor design, it contains the core, upper plenum, lower plenum, pressurizer, reactor coolant pumps, and steam generators in a single reactor vessel. An important feature of the IRIS is passive safety systems. Passive safety systems don’t require mechanical/electrical inputs for operation and rely on natural laws, material properties, and stored internal energy for hazard mitigation. In this work, a RELAP5 model of IRIS was constructed. A small-break loss-of coolant accident (SBLOCA) is simulated where the six major stages of SBLOCA response: (i) break initiation; (ii) blowdown; (iii) vessel depressurization by condensation and automatic depressurization system (ADS); (iv)
pressure equalization; (v) reactor vessel/containment vessel depressurization; and (vi) long-term cooling are demonstrated. This paper demonstrates how efficiently the passive cooling systems work in mitigating the hazard