The explosion at an earthquake-damaged nuclear plant at Fukushima, 240 km north of Tokyo, was not caused by damage to the nuclear reactor but by a pumping system that failed even as crews were at work trying to bring the reactor's temperature down, chief cabinet secretary, Yukio Edano said Saturday.
After the explosion the workers at the Fukushima Daiichi plant will now attempt to flood the reactor containment structure with sea water in a bid to bring the reactor's temperature down to safe levels, he said.
This may take up to two days.
Edano also said radiation levels had fallen since the explosion and there was no immediate danger, but by way of ample precaution authorities had expanded the evacuation zone, which will now encompass a radius of 20 kilometres (about 12.5 miles) around the plant. Previously, the evacuation zone extended to 10 kilometres.
The explosion, about 3:30 pm Saturday, had sent white smoke shooting above the plant.
A day earlier a massive earthquake and a follow-on tsunami had crippled cooling systems at the plant located in northeastern Japan.