Abstract: In order to conduct a deterministic safety assessment of a WWR-S research reactor, a thermal-hydraulic transient model by RELAP5 code is developed and used to simulate the Reactivity Insertion Accident (RIA). The reactor power is 2 MW with downward flow direction and different types of fuel bundles with different power densities and different coolant flow-rates. The model is verified by comparing its results with TPRTHA program results for steady-state normal operation. RIA is simulated for four different scenarios including inadvertent withdrawal of a control rod at full power operation regime, inadvertent withdrawal of a control rod at start-up operation regime, control rod ejection accident at full power operation regime and control rod ejection accident at start-up operation regime. Except the last scenario, the reactor is shutting down safely in the first three scenarios without any damage to the fuel bundles integrity. In the fourth scenario where a control rod is ejected at start-up operation regime, both the fuel and clad are melted for the hot channel calculations. The model results for the four scenarios are analyzed and discussed