A decay tank is installed to decrease the N-16 radioactivity in PCS (Primary Cooling System) of research reactor. As the flow velocity is sharply decreased by huge volume of the decay tank, the flow of primary coolant is delayed inside the tank. The N-16 activity is rapidly decreased by passing through the tank, since the half-time of N-16 activity is only 7.13 seconds. For the cooling system design of research reactor, the accident scenario of pipe break should be considered to secure the safe shutdown of the reactor. The loss of coolant or air inflow conditions could be occurred at the break position in accordance with the system design. For the air inflow accident, the coolant flow of air volume fraction over certain level could give rise to serious problems for pumps. And, the air inflow of reactor core could cause the structural problems of nuclear fuels. Therefore, in the present system design, a study on the role of trapping the air inside the decay tank is performed by using the numerical methods. As the air inflows inside the tank, the air-water stratification phenomenon is observed by the density difference of two fluids. The efficiency of trapping the air is evaluated by the estimation of air volume fraction inside the tank when the air is leaved the tank. And, the trapping efficiency comparison is also performed according to the decay tank types.