An experimental study of inter-subassembly heat transfer during natural circulation decay heat removal in fast breeder reactors Original Research Article

The evaluation of core thermohydraulics under natural circulation conditions is significant to utilize passive safety features of fast breeder reactors (FBRs). Under low flow conditions, it is predicted that buoyancy effects and heat transfer through wrapper tubes, i.e. inter-subassembly heat transfer, will significantly influence the flow and temperature distributions in subassemblies. Thus, steady-state sodium experiments were carried out using a three-subassembly model. The transverse temperature distributions in the subassemblies were measured under conditions wherein inter-subassembly heat transfer occurred. A wall subchannel factor was introduced to estimate the sodium temperature near the wrapper wall, which characterizes the inter-subassembly heat transfer. This factor enables a one-dimensional system code to predict the inter-subassembly heat transfer accurately. The characteristics of the factor were studied experimentally. It was shown that a buoyancy parameter, Gr*/Re, and a heat flux ratio of wrapper wall to pin surface were essential to predict the wall subchannel factor and also the peaking factor. Experimental analyses were also carried out using a three-dimensional analysis code that modeled the multi-bundle system. Good agreement between experiments and calculations was obtained for temperature distributions influenced by the inter-subassembly heat transfer.