The Pohang University of Science and Technology facility (POSTECH) experiments to mimic the siphon breaker system (SBS) of an open-type nuclear research reactor were simulated by computational fluid dynamics (CFD). Two siphon breaker lines (SBL) and one siphon breaker hole (SBH) designs were solved with the open field operation and manipulation code (openfoam). The air–water system was solved using the volume of fluid (VOF) method, and the turbulence was modeled by means of the standard k– model. The pool drainage by siphonage was captured and the siphon break phenomena due to the entrance of air through the SBS were captured. The velocity and pressure fields in the main line were analyzed in depth. The pool undershooting height (hun) as well as the flow rate through the main line were quite well estimated without the need of introduce any fitting parameter in the computational model.