Blast reported at nuclear plant amid worries that quake-hit reactor can no longer cool radioactive substances.
Radiation has leaked from a Japanese nuclear reactor and authorities say there is a possibility of a meltdown at a plant about 250km north of the capital, Tokyo.
But the government insisted on Saturday that radiation levels have reduced since pressure built rapidly last night near the Fukushima Daiichi nuclear plant, where five reactors were earlier under a state of emergency.
The cooling system of the plant was damaged in the massive earthquake that struck northeastern Japan and triggered a tsunami, killing hundreds of people.
An explosion at the nuclear facility tore down the walls of one building on Saturday, leaving smoke billowing out. But authorities said the explosion did not affect the reactor core container.
However, local media reported that three workers have suffered radiation exposure.
“We are now trying to analyse what is behind the explosion,” Yukio Edano, a government spokesman, said. “We ask everyone to take action to secure safety.
“We’ve confirmed that the reactor container was not damaged. The explosion didn’t occur inside the reactor container. As such there was no large amount of radiation leakage outside, so we’d like everyone to respond calmly.”