Under the design conditions of a research reactor, the siphon phenomenon induced by pipe rupture can cause continuous efflux of water. In order to prevent water efflux, an additional facility is necessary. A siphon breaker is a type of safety facility that can resist the loss of coolant effectively. However, analysis of siphon breaking is complex since it comprises two-phase flow and there are many inputs to be considered. For this reason, in order to facilitate the analysis and design of the siphon breaker, a simulation program based on fluid mechanics was developed using MFC (Microsoft Foundation Class) programming. From Bernoulli's equation, the velocity and quantity as well as undershooting height, water level, pressure, friction coefficient, and factors related to the two-phase flow could be calculated. The Chisholm model, which was included in the program to analyze the two-phase flow, can predict the results in a manner similar to those obtained from a real-scale experiment. By modifying the values of the input parameters and analyzing the results with respect to loss of coolant accident (LOCA) locations, the size of pipe and coefficients could be compared easily. Since simulation results are shown in the form of a graph, the user is able to confirm the total breaking situation. Furthermore, it is possible to save the entire simulation results. The simulation results were shown to be similar to those obtained from the real-scale experiment and the program functioned correctly. By using the program, the user is easily able to confirm the status of the siphon breaking, and the program is also helpful in the design of the siphon breaker. (C) 2016 Elsevier Ltd. All rights reserved.