Surge tank is provided in the secondary sodium circuit of Sodium cooled Fast Reactors (SFR) to protect the secondary sodium circuit components from the pressure surges due to sodium water reaction in Steam Generator (SG). The pressurized argon gas inside the surge tank above sodium will act as a cushion and absorb the pressure surges. The entrainment of argon gas into the sodium due to free level fluctuations, turbulence etc. can cause operational difficulties in reactor. It is necessary to develop effective gas entrainment mitigating devices which keeps the sodium free surface calm but the development only through experiments is difficult and time consuming. Therefore a CFD model of surge tank is developed to predict the surge tank hydraulics and it is validated through experiments. Velocity measurement in the model at different directions and different elevations has been carried out using Ultrasonic Velocity Profiler (UVP).