Abstract: A systematic study on entry criterion of severe accident management (SAM) for different hot leg break sizes in a 3-loop PWR was carried out using the detailed system thermal-hydraulic and severe accident analysis code package, RELAP/SCDAPSIM. Best estimate calculations of the large break loss of coolant accident (LBLOCA) of 15, 20 and 25 cm without accident management and in the case of high-pressure safety injection as the accident management were performed in this paper. The analysis results show that the core exit temperatures are not reliable indicators of the peak core temperatures and core damage states when peak core temperature reaches 1 500 K, and the proposed entry criteria for SAM is not effective to prevent core melt when the core outlet temperature reaches 900 K. Then other analyses were performed with a parameter of fluid temperature just above the hot channel. The latter analysis results show that earlier water injection is effective to prevent further core melt when the fluid temperature just above the hot channel reaches 900 K. The maximum exit temperature of 900 K above the hot channel can be used as entry criteria to enter into the SAM.