Abstract: With reference to the parameters of integral natural circulation test facility OSU-MASLWR, the system analysis code RELAP5/MOD3.3 was applied to investigate the thermal-hydraulic characteristics. Transient safety studies were done for loss-of-coolant accidents within the containment. The results show that the reactor core can be provided with a stable cooling source adequate to remove decay heat without significant cladding heatup under all scenarios. Further, the heat rejected through the containment wall to the surrounding pool of water will be greater than the amount of the decay heat produced by the reactor core. The thermal stratification phenomenon occurs in the containment. Comparison of calculated results and experiment data shows that the phenomena of interest are predicted by the code.