Officials worked desperately to stop fuel rods in the damaged reactors from overheating after some controlled radiation leaks into the air to relieve pressure.
The government said a building housing a second reactor was at risk of exploding after a blast blew the roof off the first the day before at the complex, 240 km (150 miles) north of Tokyo.
The fear is that if the fuel rods do not cool, they could melt the container that houses the core, or even explode, releasing radioactive material into the wind.