A Study on gap heat transfer between boron carbide pellet and cladding (IV)

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The purpose of this study is to investigate gap heat transfer between a boron carbide pellet (BC) and a cladding in a control rod of the Fast Breeder Reactor. The present report describes discussions of the experimental data obtained previously and results of the following two experiments and one analytical calculation. The first experiment was performed to measure conductances of an annular gap between the BC pellet and the cladding. The test piece used a sheath heater to obtain an accurate heat transfer rate and ceramic spacers to minimize an axial heat dissipation and an eccentric effect. The gap width was 0.5 mm and the test range was up to 600C on a surface temperature of the BC pellet. The second experiment was to measure an emissivity of radiative heat of the BC and the stainless steel materials. The analytical calculation was made by the finite element method to evaluate an eccentric effect on a circumferential profile of the heat transfer rate. The results were summarized as follows. The emissivities of the BC and the stainless were 0.85 and 0.25, respectively. When the minimum gap width was 0.125 mm due to the eccentricity of the BC and the cladding, heat transfer rates in the circumferential direction distributed within 25%. If the gap width is around 0.5 mm and the gap is charged with helium gas, the gap conductance can be predicted by thermal conductance and radiation.