For the research reactor, the primary coolant system (PCS) connected to the reactor is designed to penetrate the pool. During a loss of coolant accident (LOCA), the pool water should be prevented from draining below the top of the reactor for removing the residual heat using the natural circulation in the pool. The siphon breakers are installed to the branch line of the PCS to maintain the pool level during LOCA. In this study, the siphon break phenomena for a LOCA of the research reactor are analyzed by ANSYS-CFD. The calculations are performed according to the characteristic of the broken pipe. When the siphon and main pipe size are 4 and 16 inch respectively, and the height between the pool and the break location is 12.2m, the undershooting height is estimated lower than 0.45m.