A numerical investigation of the siphon breaker of an open-pool type nuclear research reactorwas performed by Computational Fluid Dynamics. The computational model was assessed by solving a siphon break line design, for which experimental and numerical data are available. The multiphaseproblem was solved with Volume of Fluid Method and k-epsilon for turbulence modeling. Numerical results were in very good agreement with experimental data. The siphon breaker occurrence was verified and the undershooting height, measured from the pool level to the siphon break line end as well as the liquid mass flow rate inside the main pipe were well captured. The implemented model showed to bereliable for assessing this kind of passive safety systems.