COMPUTATIONAL FLUID DYNAMIC INVESTIGATIONS ON EFFECTS OF CIRCULAR ROD INSERTS IN FAST REACTOR FUEL BUNDLE
In sodium cooled fast reactors (FBR) the core is very compact with large heat flux on clad surface. FBR core consists of fuel pins (clad tubes) vertically held in the form of bundle, arranged in a triangular pitch within a hexagonal wrapper tube. The clad tubes are separated by spacer wires helically wound around the tubes and the gap between the clad tubes forms the sub-channels for the coolant flow and the heat transfer coefficient of the coolant increases because of the cross flow movement of sodium in these sub-channels. The heat generation is relatively more near the central subchannels and low near the peripheral sub-channels. However peripheral sub-channels have more coolant flow area than the central sub-channels. The higher coolant flow near the peripheral sub-channels and lower coolant flow at the central sub-channels result in temperature variations around the clad tubes leading to local hot spots. Also a cooled sodium flow at the periphery and hot sodium flow in the centre create nonuniform outlet temperature in the subassembly. A proper balanced sub-channel flow can be achieved by insertion of solid devices in the gap between clad and hexcan walls. This insert reduces the flow area and hence the flow of coolant in the peripheral sub-channels so as to get an improved sodium outlet temperature and lower hot spot temperature. In the present study, a computational thermal-hydraulic study has been carried out to understand the effect of circular inserts in a 7 pin bundle. The focus of the study has been on friction factor, outlet temperature and the efficiency of circular inserts in reducing the flow bypass as well as increasing the heat transfer. From the CFD analysis, it was observed that with the circular inserts, the Nusselt number increases from 8.4 to 12, i.e. a rise of 45%. The friction factor for 7 pin bundle has been calculated from the CFD results as 0.0193 for a flow with Reynolds number 0.845×105 and with the insert the value
increases to 0.0226. The rise in friction factor is 17%.