Abstract: In the Fukushima Daiichi Nuclear Power Station core melt accident, core degradation processes in the Unit 1 and Unit 3 would have proceeded along the wet core degradation scenario in which core damage and melt would have occurred in a condition that there was water within the active core to feed steam for zirconium-steam reaction. Then, large amount of hydrogen would have been generated by the reaction, which was enough for hydrogen explosion in the reactor building. Concerning core material relocation, metallic melt would have moved down and froze at or near the water/steam interface to form coherent crust above the core plate. On the crust, fuel material mixture of melt and solid would have accumulated. The fuel material mixture would have moved down into the lower plenum through two step relocation with the sequential abrupt failures of the crust and the core plate. The two distinguished steep pressure spikes observed in the Unit 3 reactor around 10:00 and 12:10 on 2011/3/13 can be interpreted with the two step fuel material relocation into the lower plenum. On the other hand, the core degradation process in the Unit 2 would have proceeded along the dry core degradation scenario in which core damage and melt would have occurred in a condition that there was no water within the core region above the core plate. The situation would have resulted in no steam generation in the core for zirconium-steam reaction. Then, there would not have been enough hydrogen for hydrogen explosion. It can interpret the fact that hydrogen explosion did not occur in the Unit 2. Concerning core material relocation, zirconium rich metallic melt would have gradually relocated into the lower plenum through existing coolant flow paths in the lower core structure. It can interpret the distinguished but gradual pressure increase observed in the Unit 2 reactor from 20:30 on 2011/3/14. Fuel material mixture of melt and solid would have accumulated on the core plate and then moved down into the lower plenum with abrupt failure of the core plate. It can interpret the distinguished rapid pressure increase in the Unit 2 reactor around 22:40 on 2011/3/14.     

Keyword: Fukushima accident, core melt, relocation into lower plenum, zirconium-steam reaction, hydrogen