Smoke billows from Japan's Fukushima I nuclear reactor as a man watches the scene unfold on TV Saturday—the result of damage to the reactor's cooling system during yesterday's earthquake and tsunami. (See earthquake and tsunami pictures from Friday.)

At the plant, an explosion blew the roof off a building and destroyed exterior walls, Japan's Kyodo News reported. The steel container housing the reactor, however, was reportedly not damaged.

The explosion has heightened fears of radiation exposure in Japan, which has declared a precautionary nuclear emergency for the country.

(Find out why nuclear reactors may be at risk due to climate change.)

—John Roach