© 2021 Elsevier LtdThe pool-type research reactor operations sometimes require personnel to stand on top of the reactor tank. The radioactive N-16 isotopes with very high energy gamma rays that are produced through (n,p) reaction of O-16 in water are dragged with the coolant to the top of the tank, therefore, increase the radiation dose to workers. Since the half-life of N-16 is only 7.13 s, the solution for reducing the radiation risk is to increase the rise time of the N-16 radioisotopes in the tank and allow them to decay away naturally along the way. The diffuser in Istanbul Technical University TRIGA Mark II Research Reactor is designed for this purpose. In this article, the effect of diffuser on the pathways of N-16 radioisotopes were investigated with computational fluid dynamics simulations. The distribution of the N-16 radioisotopes in the tank and the rise time were obtained. The results showed that the diffuser approximately doubles the rise time and significantly reduces the exposure due to N-16 concentration on top of the reactor tank.