TOKYO, Japan - An explosion sent plumes of smoke spewing from an ageing Japanese nuclear power plant Saturday, raising fears of radioactive meltdown a day after a massive quake struck the facility's cooling system.
After declaring an atomic emergency, the government said tens of thousands of people living within 20 kilometres (12 miles) of the Fukushima No. 1 plant should leave, widening the evacuation zone from 10 kilometres.
Authorities called for calm and said radiation was within "assumed levels", suggesting they remained safe.
"We are doing our best to grasp what happened and to analyse it. We are monitoring the radiation," Prime Minister Naoto Kan's right-hand man, Chief Cabinet Secretary Yukio Edano, told a press conference.
Public broadcaster NHK reported that a blast had been heard at about 3:30 pm (0630 GMT) and NTV showed recorded footage of smoke billowing from the site, then reported that the reactor building had been destroyed.
Japan's Nuclear and Industrial Safety Agency later said serious damage to the stricken reactor's container, which shields the external environment from the effects of nuclear fission, was unlikely.
Operator Tokyo Electric Power confirmed the explosion at the plant 250 kilometres northeast of Tokyo, and that the roof of the reactor building had collapsed, saying this had happened during an aftershock from the quake.
Tepco also confirmed that four of its workers had been injured in the blast, saying that the injuries were not life-threatening.
Edano told the 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, which can be used against radiation sickness.
TV channels warned nearby residents to stay indoors, turn off air-conditioners and not drink tap water. People going outside were told to avoid exposing their skin and to cover their faces with masks or wet towels.
The operator Tepco said that the radiation detected after the explosion was about twice the level at which companies are required to notify the government.
Kyodo News said the hourly radiation matched the allowable annual dose.
Reactor cooling systems were damaged in the 8.9-magnitude quake that hit Friday, sending pressure levels soaring and leaving the government scrambling to fix the problem.
The evacuation zone affected 45,000 residents, while thousands more were also evacuated from near a second plant, Fukushima No. 2, which also suffered damage to its cooling systems.
When Friday's quake hit, the Fukushima plants immediately shut down as they are designed to do, but then the cooling system failed.
When reactors shut down, cooling systems must kick in to bring down the very high temperatures. These systems are powered by either the external electricity grid, backup generators or batteries.
The fear is that, when this fails, fuel rods that cannot be cooled are exposed to the outside air and release radioactivity.
In a meltdown, or severe nuclear power plant failure, the reactor core is unable to properly cool, leading nuclear fuel assemblies to overheat and melt, potentially releasing radioactive materials into the environment.
Parts of the No. 1 reactor's nuclear fuel rods were briefly exposed to the air Saturday after cooling water levels dropped through evaporation, and a fire engine was pumping water into the reactor, Jiji reported.
On Friday the US Air Force, which has many bases in Japan, had delivered coolant to a Japanese nuclear plant, US Secretary of State Hillary Clinton said Friday, without specifying which plant.
Japan -- located on the "Pacific Ring of Fire," where several continental plates meet and create a string of volcanoes and seismic hot spots -- records 20% of the world's major earthquakes.
Japan draws about 30% of its power from about 50 nuclear plants.