In a typical small modular reactor, an emergency cooling tank acts as additional safety feature that is intended to cope with an emergency situation. Water inside the cooling tank absorbs residual heat from the reactor core when an accident occurs through a system designed for that purpose. In the emergency cooling tank, a heat exchanger is used to transfer the heat from the core to the water coolant under natural convection. For studying this emergency cooling system, an experimental facility was designed, namely FASSIP-02. The objective of this study was to investigate the temperature distribution inside the water cooling tank under natural convection. The numerical code Fluent 6.3 and RELAP5 were used to investigate the heat transfer phenomena from the heat exchanger to the water cooling tank. The water cooling tank, which was one of the important components in simulating the emergency cooling tank, was made from carbon steel with length of 3100 mm, width of 1100 mm, and height of 2750 mm. The water with initial temperature of 300 K was filled into the cooling tank as high as 2000 mm. A U-shaped heat exchanger that was made from copper tube with inner diameter of 25.4 mm, outside diameter of 27.4 mm, length of 4200 mm, and thickness of 1 mm was immersed in the cooling tank. The hot water with various temperatures of 343, 353, and 363 K was assumed to flow inside the heat exchanger, while the velocities of the hot water were varied from 0.05, 0.5, to 1 m/s. The simulation results obtained show that an increasing of temperature and velocity of the hot water inside the U-shaped heat exchanger resulted in increasing of heat transferred to the water and water temperature in the cooling tank. The natural single-phase convection affected the heat transfer.