The heat transfer characteristics of passive containment cooling system (PCS) are very important to protect the nuclear reactor from early release in some severe accident scenarios. In this paper, based on the natural convection and the VOF (Volume of Fluid) multiphase model, the two-phase flow and heat transfer characteristics of the water film of PCS at postulated accident was numerically investigated. To economize the calculation resource, a two dimensional axis symmetry cooling system was included in the simulation model owing to the symmetry of the system. The transportation of mass and energy during the phase change at film interface was established by adding source terms to the mass and energy equations with User Defined Function (UDF). Because of the lower heat capacity of the cooling gas, the air flow rate and flow pattern of natural convection were the governing factor of the heat transfer through containment. Generally, the heat transfer rate is very lower because the heat and flow attached layer at air baffle and water film are very thick due to the flat plate air baffle, but stronger vortex produced by air baffle will reduce the heat transfer because of damage of the water film. The dimensions and the shape of the air baffle have important effect on air flow rate and flow pattern of the natural convection, and the optimal option was chosen based on the simulation results.

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