Damage of reactor buildings at Fukushima Daiichi nuclear power plants. Why hydrogen explosion occurred

Fukushima Daiichi Nuclear Power Plants unit 1 - unit 4 suffered much damage in the earthquake and resulting tsunamis. A large amount of radioactive materials was discharged to the environment from four units, which was directly caused by damage of reactor buildings due to hydrogen explosions. This article focused why hydrogen explosion occurred, based on plant information available until now. At first specifications and plant conditions of each unit until around the time hydrogen explosion occurred were described chronologically. Isolation condenser of unit 1 seemed work for eight hours and lost core cooling after. Dry well pressure increased up to 840 kPa of maximum and decreased to almost constant lower pressure for ten hours, which suggested gases inside dry well leaked into reactor building. Reactor core isolation cooling system (RCIC) of unit 2 worked keeping reactor pressure almost 6 MPa, while dry well pressure was increasing due to discharge of RCIC turbine driven steam. Damage of wet well occurred due to hydrogen explosions. As for unit 3, RCIC and High Pressure Coolant Injection System worked, and then venting of containment and fresh water and seawater injection were performed intermittently but hydrogen explosion occurred in operating floor of reactor building. Plant behavior of unit 1 was analyzed by simulation code and the amount of hydrogen accumulated after all fuel meltdown was estimated 500 kg. Amount and concentration of hydrogen in the operating floor of unit 1 was about 250 kg and 15% and after about 20 sec of burning maximum pressure of operating floor increased to about 5 bar, which damaged walls of operating floor. As for unit 3, concentration of hydrogen in the operating floor was about 30% and after about 0.02 sec of burning (transition from deflagration to detonation) maximum pressure of operating floor increased up to about 60 bar, which damaged walls of operating floor more seriously. (T. Tanaka)