Fuel debris retrieval in atmospheric conditions is proposed as one candidate in the Fukushima Daiichi NPS since it can suppress generation of the contaminated water. However, there are unknowns and uncertainties to evaluate the air cooling performance of the fuel debris. Solving the problems, we have been developing the air-cooling analysis method for accumulated debris in the pedestal. A numerical simulation code JUPITER, which is based on a CFD technics and treats a multi-phase, multi-component thermal-hydraulics, is used as the basis of the method. We preliminary evaluated the effect of debris shapes and heat source conditions on free convection and heat transfer in the simplified structure of the pedestal region using the JUPITER. As a result, it was confirmed that the flow pattern of the free convection in the pedestal clearly differs with debris shapes and a correlation exists between the temperature distribution and the velocity fluctuation around the debris surface.

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