New approach for describing reactor and containment pressure change after loss of core cooling at Fukushima meltdown accident

Abstract

The purpose of this comment is to clarify the whole history of reactor and containment pressure change during the Fukushima meltdown accident. It is based on a new approach for film boiling, which is sustained after the Zr–H2O reaction. As the reaction rate is proportional to the reactor or containment vessel pressure under film boiling, it increases rapidly and stops abruptly while sustaining film boiling. The containment vessel pressure change consists of three phases, namely, pressurization, holding a high pressure and depressurization. The containment vessel is pressurized with H2 gas and steam produced by the Zr–H2O reaction and depressurized by heat removal by heatsinks such as the containment vessel wall and inner concrete after the reaction stops. The high pressure between these pressure changes is sustained by balancing the amount of H2 gas produced by the reaction and that of gas leaking from the gap of the top hat of the containment vessel. The amount of core decay heat is large, but the change of this is negligible. Thus, pressurization is calculated from the amounts of H2 gas and steam produced by the Zr–H2O reaction. The amount removed by the heatsink balances with that produced by the reaction during the high-pressure phase. Depressurization occurs after the reaction is over, so the reaction heat rate can be calculated from the heat removal rate of the heatsink, which is equal to the condensation rate during depressurization. The rate of gas leakage can be calculated from the reaction rate. It is very important that the reaction rate was slow owing to the insufficient steam supply, as the melted core in the Fukushima accident was covered with H2 gas and steam at a pressure of 0.8 MPa or lower. This is different from the rate (at approximately 7 MPa) in the Three Mile Island accident, as the specific volume of steam at 0.8 MPa is ten times larger than that at 7 MPa. The calculation results based on this assumption show that almost all the Zr in each core of Units 1, 2 and 3 reacted with water. The location of a small penetration hole produced by the contact of the high-temperature H2 gas with the suppression chamber wall, is estimated in Unit 2.