[en] This paper describes the numerical simulations of a decay tank, which will be utilized in KIJANG Research Reactor (KJRR) project. In the research reactor, the heat generated in the core is removed by primary cooling system (PCS), thus primary coolants contain many kinds of radioactive nuclides. N-16 comprises a majority of radiation level for PCS due to the generation of high strength γ-rays. Since the half-life of N-16 is 7.13 seconds, the radiation level could be decreased by installing the decay tank at the core outlet pipe of PCS. The N-16 activity is decreased by the sufficient flow residence time in the decay tank. The residence time distribution at outlet boundary is shown at Fig. 2. The increased time is about 6.0 seconds from the time distribution, so N-16 activity is reduced about 45%. This result is consistent with the preliminary calculation. The numerical simulations were conducted with a commercial program, ANSYS-CFX. More accurate and elaborate flow simulation will be performed with hexahedral meshes to enhance the resolution of unsteady flow phenomena. Furthermore, Fluid Structure Interaction (FSI) simulation will be also conducted to assess the structural safety analysis on decay tank