Japan’s nuclear regulator said Wednesday that the tsunami following the March 11, 2011, earthquake–not the quake itself–was the main cause of the accident at the Fukushima Daiichi nuclear power plant.
The conclusion matters because of the implications for other nuclear-power plants. Virtually all of Japan is prone to earthquakes, but some places are relatively protected from tsunamis. Currently all of the nation’s 48 reactors are offline, and the government is weighing whether to restart some next year.
In the March 2011 nuclear accident, three reactors melted down after the plant lost main and backup power, paralyzing cooling systems.
The Nuclear Regulation Authority studied why the No.1 reactor lost backup power and concluded on Wednesday in a report that the tsunami was the main cause, based on data about temperature, pressure and other parameters. Those data were stable immediately after the earthquake hit at 2:46 p.m., suggesting the plant didn’t suffer critical damage until the arrival of the tsunami some 45 minutes later.
A previous investigation by Japan’s parliament had left more room for the possibility that the earthquake itself did significant damage.