Officials fear a meltdown at the reactor, which lost its cooling system. Yesterday's double disaster, which pulverized Japan's northeastern coast, has left hundreds dead or missing.
Smoke poured out of the building in the region crippled by a massive earthquake and tsunami.
Another official said the utility that runs the Fukushima Daiichi plant is reporting that several workers may have been injured.
Meanwhile, Japan launched a massive, military-led rescue operation today to the stricken area.
Prime Minister Naoto Kan said he is sending 50,000 troops following the 8.9-magnitude quake that unleashed one of the greatest disasters Japan has witnessed. A 23-foot tsunami washed far inland over fields, smashing towns, airports and highways in its way.
