Abstract
The explosion that took place at Fukushima Daiichi Nuclear Power Plant Unit 3 (1F3) is supposed to be caused by a leakage of gas from the primary containment vessel (PCV). To deepen the understanding of the 1F accident scenario and improve the safety measures to prevent a similar accident from happening again, the relationship between the PCV->Reactor Building (R/B) gas-leak mechanism and the hydrogen explosion in 1F3 is estimated. The leak spots and the leakage behaviors (beginning time, flowrate, composition, etc.) were assumed according to the evidence shown in R/B inspections and PCV pressure behavior, respectively. Due to the uncertainties of these assumptions, a series of numerical analyses were performed using GOTHIC code by varying the leak spot and flow rate until the simulated leaked-gas distribution right before the explosion be interpretable to the observations from the explosion video and R/B inspections. As a result, the study suggested that; (1) most of the gas leaked from the drywell top head flange to the 5th floor (operating floor) through the shield plug, and then distributed gradually to the whole building through the hatch and the stairs. (2) As the continuous leakage proceed, the hydrogen and steam ratio in R/B increased rapidly, while the oxygen ratio decreased correspondingly. (3) Until the last moment before the explosion, an oxygen deprivation condition has been formed on the 5th floor. (4) Meanwhile, a proper portion of the hydrogen from the 5th floor and the residual oxygen was accumulated on the 4th floor, which makes it explosive. It may be one of the reasons that lead to the explosion observed on the 4th floor before the big eruption. (5) On the other hand, incomplete combustion might take place on the 5th floor due to oxygen deprivation. The remained hydrogen might keep burning after escaping into the atmosphere, and eventually led to the big eruption.
