Estimation of flow residence time in a decay tank for a pool type research reactor using CFD

A decay tank shall be designed to provide enough flow residence time to ensure that the N-16 activity decreases before the coolant leaves the decay tankʹs shielding room. Therefore, estimation of the residence time in a decay tank is necessary before the conclusion of such a design. To estimate the flow residence time in a decay tank, the transport equation of a user-defined additional variable, which has a time unit and zero kinematic diffusivity, is solved using steady and unsteady assumptions. The minimum flow residence time is compared with that of an analytic method with an assumption of a uniform flow in a decay tank, and a particle tracking method. For the simulations, a commercially available CFD code, ANSYS-CFX, is used. When comparing the results, the unsteady simulation is seen to be much more efficient approach than steady simulation to obtain the minimum flow residence time of the decay tank.