A hydrogen explosion could occur at Unit 3 of the Fukushima Dai-ichi nuclear complex, said Chief Cabinet Secretary Yukio Edano. That would follow a blast that took place Saturday at the same power plant as operators attempted to prevent a nuclear meltdown of another unit by injecting sea water into it.
"At the risk of raising further public concern, we cannot rule out the possibility of an explosion," Mr Edano said. "If there is an explosion, however, there would be no significant impact on human health."
Mr Edano said that mechanical problems with the pump delayed the injection of cooling water into the reactor, leaving the fuel rods briefly exposed, raising the risk of both a nuclear meltdown and an explosion.
The death toll from Japan's devastating earthquake and tsunami is likely to exceed 10,000 in Miyagi prefecture alone, a regional police chief said.
Millions of Japanese were without drinking water or electricity Sunday, surviving on instant noodles and rice balls, two days after a powerful earthquake and tsunami hammered the northeastern coast, killing at least 1,000 people.