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Report No.
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Experimental study on inter-wrapper flow phenomena during natural circulation decay heat removal in fast reactors; Effects on natural circulation flow rate and Core cooling

Momoi, K.; Hayashi, Kenji; Kamide, Hideki ; Nishimura, Motohiko; Kokaki, Nobuhisa

The evaluation of core thermohydraulics under natural circulation conditions is of significance in order to utilize 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. IWF can decrease the temperature in the subassemblies. On the other hand, the natural circulation head will be reduced by temperature reduction in the upper non-heated section of subassemblies due to the IWF cooling. These positive and negative effects of IWF are our main concerns in this report. Sodium experiments were carried out to investigate these phenomena. In a test section, seven subassemblies are bundled and connected to an upper plenum with a heat exchanger. The experiments were carried out under steady state conditions. Experimental parameters were power in the core and flow resistance in the primary loop. Decrease of natural circulation flows in the subassemblies were recognized. Inter-subassembly flow redistribution was also seen due to larger cooling in outer 6 subassemblies and smaller cooling in the center subassembly. In the extremely low flow conditions (large flow resistance in the primary loop), reverse flow was registered in 2 or 3 outer subassemblies. Cooling effect of IWF was also observed. It consisted of direct cooling through the wrapper tube, flow redistribution among subassemblies (higher flow rate for hotter subchannel), and cold reverse flow from the upper plenum. When the flow resistance was small in the primary loop, i.e., flow rate was larger than 1% of reactor rated conditions (based on subassembly averaged flow velocity), the cooling effects and ...

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