Investigation of inter-wrapper flow in fast reactors during natural circulation decay heat removal - seven-subassembly sodium experiments

The evaluation of core thermohydraulics under natural circulation conditions is of significance in order to utilize and enhance passive safety features of fast reactors. When the heat exchangers of the decay heat removal system are operated in the upper plenum of a reactor vessel, cold sodium provided by the heat exchangers can penetrate into the gap regions between fuel subassemblies; this natural convection phenomenon is called inter-wrapper flow (IWF). During natural circulation decay heat removal, IWF will significantly modify the flow and temperature distributions in the subassemblies. Sodium experiments were carried out to investigate these phenomena. In a test section, seven subassemblies are bundled and connected to an upper plenum. The cooling effect of IWF on the fuel subassemblies were recognized and a new non-dimensional parameter was deduced to characterize this effect. Three-dimensional analyses were performed to establish an estimation method for IWF. The temperature decreases due to IWF at the hottest point in the subassemblies were in good agreement between experiments and analyses. 7 refs., 14 figs., 2 tabs