Abstract: A CFD analytical model of flow domain outside the tube of heat exchanger with C-type tubes was established to simulate typical two-phase natural circulation characteristics in IRWST of reactor. The analytical model was firstly validated by experimental data in open literature, and proved that the turbulence model and wall boiling model were feasible to describe the heating up process and two-phase flow characteristics. The calculation results show that the natural circulation in large pool is enhanced with C-type tube which increases the flow turbulence by mixing hot and cold water intensively and reduces their temperature difference. Whereas, bubble accumulation and maximum gas volume fraction occur in some areas of elbow and horizontal pipe, because of bubbles blocked by walls. The calculation results can contribute to a better understanding of the processes and give hints for heat exchanger design of the passive residual heat removal system.