The POSTECH facility experiments to mimic the siphon breaker system of an open-type nuclear research reactor were simulated by computational fluid dynamics. Two siphon breaker lines and one siphon breaker hole designs were solved with OpenFOAM. The air-water system was solved using the Volume of Fluid method, and the turbulence was modelled by means of the standard k-epsilon model. The pool drainage by siphonage was captured and the siphon break phenomena due to the entrance of air through the siphon breaker systems was captured. The velocity and pressure fields in the main line were analysed in depth. The pool undershooting height 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.