New Approach for Reactor and Containment Pressure Changes after Loss of Core Cooling In the Fukushima Meltdown Accident
Matsuoka T
Published on: 2021-10-14
Abstract
The purpose of this Commentary is to clarify as much as possible the whole history of reactor and containment pressure changes in the Fukushima meltdown accident. It is based on a new approach for film boiling, which is kept after the Zr-H2O reactions. As the reaction rate is proportional to the reactor or containment pressure under film boiling, it increases rapidly and stops suddenly, keeping the film boiling. The containment pressure change consists of three phases, namely pressurizing, keeping the high pressure and de-pressurizing. The containment is pressurized by H2 gas and steam produced by the Zr-H2O reactions and de-pressurized by a heatsink such as the containment wall and inner concrete shield after reaction stops. The high pressure between these pressure changes is kept by balancing the H2 gas produced by reaction with the leaked gas from the gap between the top lid and the containment. Core decay heat is large but its change is negligibly small. So, the pressurization is calculated from H2 gas and steam produced by the Zr- H2O reactions. The heatsink balances with the reaction during the high pressure condition. The de-pressurization occurs after the reaction is over, so the reaction heat rate can be calculated by the heat rate of the heatsink, which is equal to the condensation rate during de- pressurization. The leak rate of the leak gas can be calculated using the reaction rate. It is very important that the rection rate is slowed by the insufficient steam supply, as the melted reactor cores in the Fukushima accident were covered with H2 gas and steam (film boiling) at 0.8MPa or lower pressure. This is different from the rate (at approx. 7MPa) in the Three Mile Island accident, as the steam specific volume at 0.8MPa 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.