Controlled cooling of the fuel channels in a calandria based reactor is important to avoid any hazardous aftermath. On the other hand, efficient cooling of the fuel channels can also ensure better utilization of the fuel in electricity generation. In this regard, simple modifications which can cause significant increase in the efficiency are appreciable; in particular, in the case of existing nuclear reactors. In this paper, we have discussed (a) the thermal and flow characteristics of the heavy water inside the traditional calandria based reactor and, (b) the effect of the introduction of a splitter near the heavy water inlet on the cooling efficiency. It is observed that the mass flow rate and inlet orientation have a significant effect on the convective heat transfer from the fuel channels to the heavy water. The underutilization of the heavy water in cooling the fuel channels in the traditional calandria design was speculated from the CFD analysis. The effect of introduction of a simple splitter (aerofoil shaped body) near the inlet was hence studied by carrying out CFD simulations. The latter seems to cause considerable decrease in the temperatures inside the calandria. The comparative analysis has shown that the splitting of the flow near the inlet can keep the calandria operating temperature under the safe limits for the same mass flow rate but for higher thermal power output. © 2019, Chinese Mechanical Engineering Society. All right reserved.