Numerical study on ignition and failure mechanisms of hydrogen explosion accident in Fukushima Daiichi Unit 1

This paper studied ignition and failure mechanisms of hydrogen explosion accident in the container building of Fukushima Daiichi Nuclear Power Plant Unit 1, which occurred a day after the Great East Japan Earthquake in 2011. To understand the mechanism of hydrogen explosion, an unsteady three-dimensional numerical analysis of the flow field in the container building was conducted in real time by assuming that the high-temperature hydrogen gas (500 ?C) issued from the head flange of the reactor vessel. The numerical results show that the hydrogen gas spread not only on the 5th floor but also on the 4th floor of the container building covering the stairs. It was found that the hydrogen concentration on the 5th and 4th floors was highly uniform owing to the long real-time accumulation (15 h) of hydrogen, and it spread gradually from the 5th to the 4th floor through the stair. As a result, the hydrogen concentration reached the ignition level on the 4th floor (5%), which could result in the ignition of hydrogen by a tank heater on the 4th floor and the flame propagated to the 5th floor, where fast deflagration and detonation might occur because of the high concentration of hydrogen gas (19%). This scenario is consistent with the observations of failure mechanism after the explosion accident.

Automated summary

The study on the hydrogen explosion accident at Fukushima Daiichi Nuclear Power Plant Unit 1 revealed that the high-temperature hydrogen gas emitted from the reactor vessel head flange spread uniformly in the container building, reaching an ignition level on the 4th floor. This could have been ignited by a tank heater on the 4th floor, leading to fast deflagration and detonation on the 5th floor due to the high concentration of hydrogen gas.