The author independently conducted a detailed analysis of plant conditions, based on a new meltdown process, during the 14 hours after all power loss at the Fukushima Daiichi Nuclear Power Plant Unit 1. After 3：30 PM on March 11, 2011, all power loss caused reactor core cooling function loss and the vaporization of water in the reactor vessel(RPV), and the RPV was filled with high-temperature hydrogen gas generated by decay heat and Zr-H2O reactions. The reactor core melted and the peripheral stainless-steel structures and nuclear vessel walls were heated by the radiation energy of decay heat. Then, the shrouds and other components near the reactor core started melting and the wall temperature of the RPV was raised. Along with the wall temperature rising to nearly 650℃, the aluminum insulation near the wall started melting(aluminum melting point, about 650℃). The heat from the collapsed insulation caused the superheated state of the containment pressure and temperature of 0.84 MPa and 400℃, respectively, around 3 AM from the saturated state of 0.6 MPa around 1 AM on March 12. After that, the containment vessel was depressurized gradually and kept under stable cooling 14 hours after all power loss, for the time from 4 to 6 AM. By the first water injection to the reactor core at 4 AM, radioactivity release increased slightly and the containment pressure was stable. However, the situation changed significantly after continuous water injection to the reactor core started at around 6 AM. The water injection into the high-temperature reactor core generated film boiling (poor heat transfer)and good reactor core cooling was no longer available; thus, increased heating by the Zr-H2O reactions started. The results of the author’s analysis showed good consistency with the measured reactor pressure, containment pressure, containment temperature and radioactivity near the main gate. Although decay heat cannot be decreased intentionally, the occurrence of the Zr-H2O reactions can be inhibited. It is proposed that the best way to mitigate the effects of a meltdown is to stop water injection to the core after detecting the initiation of film boiling or inferring that the fuel rods are not covered with water. © 2022 Atomic Energy Society of Japan, All Rights Reserved.
