Explosion at quake-hit nuclear plant
A Japanese nuclear power plant has been hit by an explosion, raising fears of a radioactive meltdown, a day after a massive quake damaged the facility's cooling system.
A government spokesman said tens of thousands of people within a 10km radius of the plant should evacuate, calling for a "calm response" and saying radiation was "at assumed levels".
Kyodo News said the hourly radiation matched the allowable annual dose.
TV channels warned nearby residents to stay indoors, turn off air-conditioners and not to drink tap water.
People going outside were also told to avoid exposing their skin and to cover their faces with masks and wet towels.
Nuclear authorities had earlier warned that the Fukushima No. 1 plant, located about 250km northeast of Tokyo, an urban area of 30 million people, "may be experiencing a nuclear meltdown".
The plant's cooling system was damaged in the 8.9-magnitude quake that hit on Friday, leaving the government scrambling to fix the problem and evacuate more than 45,000 residents within a 10km radius.
Public broadcaster NHK reported that a blast had been heard at about 3.30pm local time (5.30pm AEDT) and showed delayed footage of smoke billowing from the site, then reported that the reactor building had been destroyed.
Operator Tokyo Electric Power confirmed the explosion and that the roof of the reactor building had collapsed, saying this had happened during an aftershock to Friday's quake, Kyodo News reported.
Tepco also confirmed that four of its workers had been injured, saying that the injuries were not life-threatening.
Prime Minister Naoto Kan's right-hand man, Chief Cabinet Secretary Yukio Edano, told a press conference that the Tokyo Fire Department would send a "hyper rescue team" to the site.
He also said the government was taking contingency measures and collecting iodine, with can be used against radiation sickness.
Opposition to the nuclear industry is strong in Japan, where every year thousands still gather to mourn the victims of the two nuclear bombs dropped on Hiroshima and Nagasaki in the closing days of World War II.
But scarce natural energy resources mean atomic power is seen as a necessary evil, with 54 nuclear plants providing about 30 percent of the country's electricity needs.
Its nuclear facilities are built to withstand seismic shocks and automatically shut down in a significant earthquake, but Fukushima's emergency backup cooling systems failed, leaving its core glowing hot.
Greenpeace said the damage left Japan "in the middle of a nuclear crisis with potentially devastating consequences", with experts fearing the fuel rods could melt.
"Then you have the possibility of a greater release of radioactivity into the environment," said James Acton, a physicist who examined Japan's Kashiwazaki-Kariwa plant after a 2007 earthquake.
If that happens Acton told CNN "there's a possibility of cancer in the long term — that's the main hazard here."
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An explosion has been reported at Japan's stricken  nuclear power plant after its cooling system was damaged by a massive 8.9 magnitude earthquake that hit off the North-East coast of Japan on Friday. The explosion occurred at around 3:00pm local time (6:00am UTC). According to the plant's operator, , four workers were injured in the explosion.

The explosion occurred as workers were attempting to cool a reactor by injecting water into its core. Local media are reporting that the plant's exterior walls have been destroyed and only a skeleton structure remains.

More than 45,000 residents within a 10-kilometre radius of the plant have been evacuated.

Television reports show white smoke coming from the plant. Local authorities are warning residents in the vicinity of the plant to stay indoors, turn off air-conditioners and not to drink tap water. According to Japan's nuclear agency, radioactive  and  has been identified at the plant. Authorities speculate that this indicates that containers holding uranium fuel at the site may have ruptured and are leaking.