In order to cope with the absence of the external electricity source condition, such as the accident occurred in Fukushima Dai-ichi nuclear power plant, several designs of nuclear reactor incorporate a natural circulation based emergency cooling system (ECS). To understand the phenomena and assess the reliability of the natural circulation in the ECS, one experimental facility named FASSIP-02 is being constructed. The facility consists of a water heater tank (WHT), a water cooling tank (WCT), which contains a C-shaped heat exchanger (CHX) and a straight wickless heat pipe (SHP), and a piping system. The current work focuses on numerical simulation of the thermal-hydraulic characteristics in the WCT. The study uses RELAP5 code as a numerical simulating tool. The objective is to assess the applicability of the numerical model, to study the characteristics of the heat transferred to the cooling water from the CHX submerged in the water cooling tank, and the SHP performance to remove heat from WCT by numerical simulation methodology. Two conditions in the WTC are simulated, i.e. with and without SHP. With the given hot source condition in CHX, the simulation results show that without any SHP, the water temperature in the WCT increases continuously reaching saturation temperature in about 130,000 s due to the heat transferred from the CHX. However, using five SHPs with a defined geometry of 1 inch diameter, 2 m total length and water filling ratio of 80% and inside temperature of 25°C, the WCT's water coolant temperature could be maintained about 40°C. It is concluded that the RELAP's model provides a good results showing the general thermal-hydraulic characteristics of the WCT and that the given SHP could transfer the heat from the WCT to the a heat sink preventing continuous increase of WCT's water temperature.