An open-pool type research reactor has a submerged reactor core in the lower part of the pool to provide core cooling and reactor safety. Since the reactor core transfers core heat to the water in the lower part of the pool, the natural circulation due to unstable stratification should be suppressed to reduce transfer of radioactive materials to the pool surface. For that purpose, hot water is supplied in the upper part of the pool to build stable stratification which could restrict the natural circulation. In this study, the formation of hot water layer and the resulting stable stratification in the pool is simulated using conventional CFD method. To confirm the accuracy of the numerical method, the calculated results are compared to experimental results which are obtained during the performance test of the research reactor. Although there exists some deviations, the predictions are shown to be reliable within an aceptable accuracy.