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Fundamental water experiment on subassembly with porous blockage in 4 sub-channel geometry; Influence of flow on temperature distribution in the porous blockage

Tanaka, Masaaki  ; Kobayashi, Jun ; Isozaki, Tadashi; Nishimura, Motohiko; Kamide, Hideki 

In the liquid metal cooled Fast Breeder Reactor, Local Fault incident is recognized as a key issue of the local subassembly accident. In terms of the reactor safety assessment, it is important to predict the velocity and temperature distributions not only in the fuel subassembly but also in the blockage accurately to evaluate the location of the hottest point and the maximum temperature. In this study, the experiment was performed with the 4 sub-channel geometry water test facility. Dimension is five times larger than that of a real FBR. The porous blockage is located at the center sub-channel in the test section and surrounded with three unplugged sub-channels. The blockages used in this study were (1)the solid metal, (2)the porous medium consisted of metal spheres, (3)the porous blockage with end plates covering the side or top faces of the blockage to prevent the horizontal and axial flows into the blockage. The experimental parameters were the heater output provided by the electrical heater in the simulated fuel pins and the flow rate. Temperature of the fluid was measured inside/outside the blockage and velocity profiles outside the blockage were measured. From the comparison of velocity profiles, the flow field inside the blockage depended remarkably on the blockage conditions. Such variation of flow fields affected the temperature distributions. Efficient heat transportation by horizontal flow existed in the upper part of the porous blockage. While, in the lower part of the blockage, the axial flow from the bottom face of the blockage was pre-dominated for the heat removal. Nusselt number defined by the temperature difference between the heater pin surface and the bulk temperature of the unplugged sub-channel was proportional to the power of 0.5$$sim$$0.6 of Reynolds number. This result shows that the dependency of the Nusselt number to the Reynolds number was decided by the heat transfer from the blockage matrix to the coolant at the side of porous blockage.

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