[en] This paper presents the numerical studies of several parts of decay tank from a viewpoint of residence time. The main objective of decay tank installation is to decrease the N-16 activity by increasing the residence time. To assess the design adequacy of the tank, the residence time should be predicted. The dead zone inside the device is represented to long tail of time distribution. To obtain the accurate time distribution, several numerical methods are compared by simulating the simple geometric cases. This paper describes a comparison of residence time distributions using various numerical methods. To assess the design adequacy of decay tank, an accurate residence time calculation is required. In the present study, two particle methods, DPM and streamline, and two tracer methods, mixture and UDS, are utilized and compared by simulating the validation case. DPM shows higher exit particle ratio and accurate time estimation than streamline method. UDS methods shows better performance than mixture model for the scalar conservation. Therefore, it is concluded that the DPM methods is more proper at the stage of initial design. And at detailed design phase, UDS method could be utilized to assess the accurate tank performance. This study will be applied to design the decay tank and internal flow devices.