The Calandria in the pressurized heavy water reactors acts as a moderator and has a cylindrical structure. 380 fuel channels are located inside the cylindrical structure of Calandria and the heat from the fuel channels is removed through the heat exchanger of the moderator cooling system. In the event of a loss of coolant accident involving loss of the emergency core cooling systems, the Calandria shall be the only heat sink of the fuel channel. Therefore, the relevant cooling performance is required to ensure that sufficient heat removal performance is present and that no boiling in the Calandria is generated during the accident. In this study, the thermal hydraulic behavior inside the Calandria was analyzed through CFD. Testing results demonstrate the reliability of turbulence model, buoyancy model and grid generation method. The analysis of the thermal hydraulic characteristics and sub-cooling margin were calculated and heat removal performance of the moderator system was reviewed.