A proper assessment of core thermohydraulics under natural circulation conditions is important so that the full potential of the inherent, passive feature of a fast reactor can be used. When the heat exchangers of the decay heat removal system are operated in the upper plenum of a reactor vessel, cold sodium exiting the heat exchangers may penetrate into the gap regions between fuel subassemblies; this gap flow between the wrapper tubes of subassemblies is called interwrapper 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, using a test section consisting of seven subassemblies housed and connected to an upper plenum. The cooling effect of IWF on the fuel subassemblies was evaluated and a new nondimensional parameter was deduced to characterize this effect. On the other hand, IWF reduced the natural circulation flow in the primary loop due to a temperature decrease in the upper part of the core. A balance between the cooling effect and the flow reduction effect is discussed. Three-dimensional analyses were performed to establish an estimation method for IWF. For the temperature decreases due to IWF at the hottest point in the subassemblies there was good agreement between experiments and predictions.
