A dry method is one of fuel debris retrieval methods for decommissioning of TEPCO’s Fukushima Daiichi nuclear power station. If stopping the water leakage from a damaged primary containment vessel (PCV) is impossible, the dry method may be adopted. Besides, the dry method may be also adopted to retrieve the fuel debris in a pedestal under a reactor pressure vessel (RPV) because accessing and retrieval of the fuel debris from the lateral sides of PCV may be adopted for retrieval of the fuel debris in the pedestal. However, the decay heat must be removed from the fuel debris without water because fuel debris is exposed to the air in the dry method. Thus, we must evaluate the air cooling performance for the fuel debris in the dry method.
Japan Atomic Energy Agency (JAEA) has started the research project to develop an evaluation method by the JUPITER to understand the air cooling performance for fuel debris in the dry method. In the project, free convection heat transfer and the radiation heat transfer experiments for air are also conducted for validation of the JUPITER. In this paper, the free convection heat transfer, the radiation heat transfer, the conduction heat transfer and the Rayleigh number for the free convection in the simplified model were estimated to decide experimental conditions. We confirmed the free convection and the radiation heat transfer were important to estimate heat transfer of the debris exposed to the air in the RPV pedestal. Considering the estimations, the experiment for heat transfer and flow visualization of free convection adjacent to upward-facing horizontal surface was conducted to validate the calculation of the turbulent free convection by the JUPITER. From comparing between the calculation result and the experimental result, we confirmed that the calculation result of the JUPITER was qualitatively reasonable.