A large-break loss of coolant accident (LBLOCA) occurring in the CALLISTO test loop facility (capability for light water irradiation in steady state and transient operation) is studied CALLISTO, installed at the BR-2 research reactor, is mainly dedicated for PWR (pressurized water reactor) fuel bundle irradiation and qualification; it operates so at 155 bar and 300 °C. It is composed of an out-of-pile part and three in-pile sections connected to the primary system. Each one of these IPSs is provided with a nine fuel rod clusters. The scenario under consideration is a large brittle fracture in the inner pressure. The simulation is conducted using the RELAP5/SCDAP3.2 thermal hydraulic system code. The assessment study was carried out by comparison of the calculated values against the facility design data and the available experimental data. The steady-state results obtained using RELAP5 are consistent with experimental data. Transient results are compared with results reported in the safety analysis report data for the same accident sequence simulated by the constructor. The RELAP5 model for the CALLISTO test facility might be considered in the framework of future safety analysis including small and medium break scenarios taking into account the out-of-pile regulating system.