Courtesy: CTV News Channel
The Japanese government says an explosion Saturday blew off the roof and walls of the building containing a nuclear reactor, but did not damage the reactor itself.
The explosion at the Fukushima Dai-ichi plant had earlier triggered fears of a nuclear meltdown.
On Saturday, Japan's government spokesperson Yukio Edano said radiation levels around the plant did not rise, but actually decreased after the explosion. He added that pressure in the reactor was also decreasing.
The walls of the building have crumbled, leaving only a skeletal metal frame standing, said Reuters reporter Kei Okamura.
"The government says there are no large leaks of radiation but just in case they have initiated an evacuation that has been expanded from an initial radius of 10 kilometres to now 20 kilometres," Okamura told CTV News Channel, reporting from Tokyo.
Four workers suffered fractures and bruises in the explosion, and puffs of smoke can be seen spewing out of the facility.
Earlier there were reports that large amounts of radiation were leaking from the plant.
The facility was badly damaged in Friday's devastating 8.9 magnitude earthquake.
Pressure and heat have been building at the nuclear reactor since the disaster, when the earthquake and ensuing tsunami caused its cooling system to fail.
Chris Johnson, a freelance reporter in Tokyo, said the message from the Japanese government has been confusing and journalists have had trouble getting accurate information. No one is being allowed to approach the site of the blast.
"It doesn't seem to be under control," Johnson said.
"I think if it was under control you'd have the government saying very strongly 'we have things under control.' But they're not doing that. They're still talking about analyzing the situation and asking people to stay calm."
A meltdown is not a technical term -- it describes the inability of a nuclear plant to manage temperatures in the system -- and nuclear scientists in Japan and around the world have said a meltdown may not pose a widespread danger.
Fukushima Dai-ichi plant is 30 kilometres from Iwaki, and about 260 kilometres north of Tokyo.
Five nuclear reactors at two power plants lost cooling ability on Friday, and a state of emergency was declared at both plants.
At one plant, radiation inside one reactor jumped to 1,000 times the normal level, but officials stressed there was no "immediate health hazard."
Both plants lost electrical power, and then suffered critical failures in their backup generators used to power cooling systems. Batteries were being used temporarily to keep the cooling systems going.
A lack of power to the cooling systems results in a pressure buildup beyond what the nuclear reactors are designed to handle.
Philip White, of the Citizens Nuclear Information Center, said his organization for years has warned about the risk of a combined earthquake and nuclear disaster.
The two events, if they were to occur simultaneously, could cause terrible damage, he said.
"If you have a nuclear disaster you need people to be inside away from the radiation," White told CTV News Channel.
"If you have an earthquake disaster, you need people outside so the buildings don't fall on them. You also need people to be able to move away from the situation when there's a nuclear disaster but they can't because their roads have been destroyed and transport is no longer available."