Abstract
The internal investigations of TEPCO’s Fukushima Daiichi Nuclear Power Station (1F) Unit 2 indicate multiple breaches in the lower head of the reactor pressure vessel (RPV), which led to discharges of molten core materials. In addition, a large enough breach(es) is expected near the vessel periphery, which allowed relocation of a fuel assembly upper tie plate to the pedestal floor. However, the muon radiography indicates that massive fuel debris are still retained within the RPV lower head. This study aims to provide a comprehensive explanation of such observations by considering interactions of the fuel debris with the thermal insulation plates below the RPV lower head at the time of the accident. The Moving Particle Semi-implicit (MPS) method has been developed and pieces of the debris were modeled by rigid bodies to analyze thermal behavior of the fuel debris and their interactions with the insulation plate. The results showed that whether the insulation plate failed or not depended on the initial enthalpy and temperature distribution of the relocated fuel debris on the plate. The results implied that thermal load on the plate was greater below the outer region of the vessel than the central region, because there was larger space between the plate and the vessel for the debris to pileup.
