Company officials said the fire started at 9:38 am local time Tuesday on the fourth floor of the building that houses the reactor, which was not operating due to a periodic inspection outage.
This is the third explosion at Fukushima Daiichi, which was hit by Friday's massive 8.9 magnitude earthquake and inundated by the resulting tsunami.
Chief Cabinet Secretary Yukio Edano told reporters the reactor was not operating after the earthquake, but hydrogen is being produced because spent fuel creates its own heat.
Handout satellite photo of the Fukushima Daiichi with smoke rising from the Unit 3 explosion
Another explosion at the unstable Fukushima Daiichi nuclear power plant on Japan's Pacific coast occurred Monday at 11 am local time, followed by white smoke billowing into the air. Eleven workers were injured and radiation readings near the plant jumped after the blast at the Unit 3 reactor.
The company said the Unit 3 reactor explosion was due to hydrogen gas generated in the containment building after nuclear fuel rods were left exposed by falling water levels after power to the cooling water pumps failed.
On Saturday, a similar explosion blew out the wall and roof of the building housing the containment vessel at the plant's Unit 1 reactor.
The company said it believes the reactor containment vessels in Units 1 and 3 are intact but officials are currently investigating the status of all onsite reactors.
Also Monday, a nuclear emergency was declared at the Unit 2 reactor at the same power plant.
TEPCO said Monday afternoon that fuel rods were exposed at Unit 2 reactor after the level of coolant water dropped. At around 6:20 pm, the power company began pumping in seawater.
Tokyo Electric says it had to halt the cooling process due to loss of power to the pumping system, possibly leaving the fuel rods in the reactor exposed. The firm says a core meltdown might have occurred.
Crews work to bring order to the chaos in northeastern Japan. (Photo courtesy UNICEF)
The Japan Nuclear and Industrial Safety Agency says that pumping seawater into the reactor is working now. But the company said it might have to release more radioactive hydrogen gas to relieve pressure inside the Unit 2 containment vessel.
"In light of the incidents that have occurred at Units 1 and 3, we are considering applying prevention measures to the wall of the reactor building to ventilate the hydrogen gas contained in Unit 2," TEPCO said.
Later, a top Japanese official said the fuel rods in all three of the nuclear reactors may be melting, with Unit 2 the most worrisome.
“Units 1 and 3 are at least somewhat stabilized for the time being,” said nuclear safety agency official Ryohei Shiomi. “Unit 2 now requires all our effort and attention.”
Prime Minister Naoto Kan has urged all residents to evacuate from within 20 kilometers of the Fukushima Daiichi power station. He told people living within 20 to 30 kilometers to stay indoors.
Due to power shortages the Prime Minister warned that rolling blackouts would be necessary over much of the country.
TEPCO cut power in some areas of Tokyo and neighboring prefectures early Tuesday morning, affecting about 700,000 households while a second outage at about 10 am affected about 250,000 households.
Wreckage from the 8.9 magnitude earthquake washed out to sea by the resulting tsunami, March 13, 2011 (Photo courtesy U.S. Navy)
TEPCO says the rolling blackouts are expected to continue for some time.
While the country and world watches warily, Japan is counting the casualties and searching for the missing.
Nearly 2,500 people have been confirmed dead so far from the earthquake and tsunami. More than 17,000 others are missing.
Nearly 450,000 people are taking shelter at over 2,500 evacuation centers across northeastern Japan.
Rescuers are struggling to reach 24,000 people who remain stranded in the tsunami-struck region. These isolated outposts have been confirmed in 80 places in Miyagi, Iwate and Fukushima prefectures.
In Souma City, Fukushima prefecture, rescuers still cannot reach residents trapped in their homes because of flooded roads.
More than 310,000 evacuees are taking shelter at schools and other public institutions. They must endure extreme cold at night as heating systems are short of fuel.
In total, the Japanese government estimates that more than 46,000 homes and buildings were damaged by the disaster.
The National Police Agency says the quake damaged roads and bridges in at least 600 locations. Railroads were damaged at seven locations in two prefectures.
The quake also caused landslides at 66 locations in seven prefectures.
Government officials told reporters that aftershocks with an intensity of five and over on the Japanese scale of zero to seven have been registered nearly 200 times since Friday, when northeastern Japan was rocked by a record earthquake.
The roll call of numbers cannot possibly indicate the massive devastation across northeastern Japan.
And there are more shocks to come. Japan's Meteorological Agency has warned of a major aftershock within the next three days.
Agency expert Takashi Yokota said the likelihood of a tremor with a magnitude above five within the next three days is 40 percent. He said such a tremor may trigger another tsunami.
Many countries are now checking the safety and performance of their nuclear reactors. The United States has 104 civilian nuclear reactors operating at 65 power plants across the country.
Twenty-three of those power plants use the same General Electric Mark 1 boiling water reactors as the ones at the Fukushima Daiichi power plant.
This design has been criticized by nuclear experts as being susceptible to explosion and containment failure.
But a spokesman for the Nuclear Regulatory Commission said the NRC's rigorous safety regulations ensure that U.S. nuclear facilities are designed to withstand tsunamis, earthquakes and other hazards.
Copyright Environment News Service (ENS) 2011. All rights reserved.