Coolant activity in the primary coolant system and reactor pool in an open pool-type research reactor is very important in view of operational and radiological safety considerations. As pool water acts as the shielding medium to minimize the radiation dose at the top of the reactor pool, an estimation of the activities of radionuclides in pool water is essential to establish the safety of the operating personnel and researchers working at the reactor pool top. A system is provided to create a hot water layer (HWL) at the top of the pool by supplying water at a temperature more than that of the pool water so that the dose rate at the reactor pool top can be minimized. This HWL system helps in breaking the natural convection current of reactor pool water by maintaining a higher temperature at this layer so that high-density pool water below this layer cannot replace this low-density HWL. Therefore, pool water that is comparatively more radioactive will not be able to cross this HWL by convection. Hence, diffusion will be the only mechanism by which radioactivity can reach the pool top. So eliminating the convection current keeps the activity at this topmost layer of the pool at a minimum value. The estimation of the activity of the radioactive nuclides is required to assess the radiation field at different locations in the primary coolant loop for designing proper shielding requirements of the system. The radionuclides of interest are the activation products of aluminum (24Na, 27Mg, 28Al) and 41Ar. In this technical note, a transient code is presented for estimating the activity of radioactive nuclides in the coolant loop and reactor pool of a nuclear research reactor. The reduced activity level at the pool top is estimated considering the presence of the HWL at the top of the pool. It is observed that purification flow plays a major role on the activity level of radioactive nuclides in reactor pool water. The variation of the activity dose rate at the reactor pool top with purification flow is also discussed.