This project deals with the CFD modelling of a free surface flow. The aim is to develop and validate a fast and accurate numerical model for stratified two-phase flows. Volume of Fluid (VOF) multiphase model is employed. The purpose is to use the developed numerical model for the design of an element within a compact nuclear reactor.Unsteady Reynolds Averaged Navier-Stokes (RANS) simulations are conducted. Two free surface test cases are simulated to verify and ensure robustness of the model: a dam break and a vertical cylindrical obstacle set in a channel. From there, an optimization is performed in order to find the best compromise between accuracy and rapidity with the solver. The proper set of parameter models is found by carrying out extensive sensitivity studies and compare the solutions with available measurements.The obtained numerical results show a reasonable good agreement with the experimental data for the dam-break. Significant time savings are achieved thanks to the implemented optimization process while maintaining accuracy. The optimized model is then applied to the second test case and comparisons with experimental measurements are carried out. The same physical behavior of the flow as in experiments is captured with the simulations. The differences found between the simulation data and experiments are partly due to the difficulty to monitor experimentally with a high accuracy the highly non uniform regions within the flow.