Hearld Sun -The explosion was heard at 3:36 p.m. following large tremors and white smoke was seen at the facility in Fukushima Prefecture, the company said.
The four workers were working to deal with problems caused by a powerful earthquake that hit northeastern Japan on Friday. However there is no word on injured worker's condition, Jiji says.
Radioactivity at the plant was 20 times over the normal level, and Japan's Nuclear Safety Commission has said it may be experiencing meltdown.
Tokyo Electric Power Co (TEPCO) says explosion may have been hydrogen used to cool Fukushima plant, Kyodo reports.
TV reports have confirmed the Tokyo fire department is sending special nuclear rescue team to Fukushima.
Pressure has reportedly been growing at the plant, with Japanese officials racing against time to cool the reactors that were disabled by yesterday's massive earthquake and tsunami or face a nuclear meltdown.
TEPCO is racing to cool down the reactor core after a highly unusual "station blackout" - the total loss of power necessary to keep water circulating through the plant to prevent overheating.
Daiichi Units 1, 2 and 3 reactors shut down automatically at 246pm local time yesterday due to the earthquake. But about an hour later, the on-site diesel back-up generators also shut, leaving the reactors without alternating current (AC) power.
That caused TEPCO to declare an emergency and the Government to evacuate thousands of people from near the plant. Such a blackout is "one of the most serious conditions that can affect a nuclear plant," according to experts at the Union of Concerned Scientists, a US based nuclear watchdog group.
"If all AC power is lost, the options to cool the core are limited," the group warned.
TEPCO also said it has lost ability to control pressure at some of the reactors at its Daini plant nearby.
The reactors at Fukushima can operate without AC power because they are steam-driven and therefore do not require electric pumps, but the reactors do require direct current (DC) power from batteries for its valves and controls to function.
If battery power is depleted before AC power is restored, the plant would stop supplying water to the core and the cooling water level in the reactor core could drop.
Officials are now considering releasing some radiation to relieve pressure in the containment at the Daiichi plant and are also considering releasing pressure at Daini, signs that difficulties are mounting. Such a release has only occurred once in US history, at Three Mile Island.
"(It's) a sign that the Japanese are pulling out all the stops they can to prevent this accident from developing into a core melt and also prevent it from causing a breach of the containment (system) from the pressure that is building up inside the core because of excess heat," said Mark Hibbs, a nuclear expert at the Carnegie Endowment for International Peace.
While the restoration of power through additional generators should allow TEPCO to bring the situation back under control, left unchecked the coolant could boil off within hours. That would cause the core to overheat and damage the fuel, according to nuclear experts familiar with the Three Mile Island accident in Pennsylvania in 1979.
It could take hours more for the metal surrounding the ceramic uranium fuel pellets in the fuel rods to melt, which is what happened at Three Mile Island. That accident essentially froze the nuclear industry for three decades.
Seven years later the industry suffered another blow after the Chernobyl plant in Ukraine exploded due to an uncontrolled power surge that damaged the reactor core, releasing a radioactive cloud that blanketed Europe.
The metal on the fuel rods would not melt until temperatures far exceed 1000 degrees F. The ceramic uranium pellets would not melt until temperatures reached about 2000 degrees F, nuclear experts said.
The Government declared an atomic emergency amid growing international concern over its reactors after an 8.9 magnitude earthquake, the biggest in Japan's history, unleashed tsunamis that swept all before them.
The US Air Force, which has many bases in Japan, delivered coolant to a Japanese nuclear plant, US Secretary of State Hillary Clinton said yesterday, without specifying which plant.
The two nuclear plants affected are the Fukushima No.1 and No.2 plants, both located about 250km northeast of greater Tokyo, an urban area of 30 million people.
Tokyo Electric Power vented radioactive vapour at five reactors between both plants to release building pressure.
"We are not in a situation in which residents face health damage," Chief Cabinet Secretary Yukio Edano told reporters today, according to Jiji.
However, the evacuation area was expanded. A total of 45,000 people living within a 10km radius of the No.1 plant were told to evacuate.
Officials today ordered the evacuation of people living within a 3km-radius of the second plant, with those up to 10km away told to stay indoors.
When Friday's massive quake hit, the plants immediately shut down, along with others in quake-hit parts of Japan, as they are designed to do, but the cooling systems failed, the Government said.
The major fear is that fuel rods, which create heat through a nuclear reaction, could become exposed and release radioactivity.
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.
This is key to prevent a "nuclear meltdown" and major radioactive release.
Japan's Prime Minister Naoto Kan early today left on a helicopter to Fukushima to assess the situation at the plants operated by Tokyo Electric Power, and other areas in the disaster zone.
Military personnel have been dispatched to Fukushima, including a chemical corps and an aircraft on a "fact-finding mission".
The UN's International Atomic Energy Agency (IAEA) said Japanese officials had kept it informed of their efforts to restore power to the cooling systems while monitoring a pressure build-up.
Early in the crisis Prime Minister Kan said no radiation leaks were detected among the country's reactors after the quake.
According to the industry ministry, 11 nuclear reactors automatically shut down at the Onagawa plant, the Fukushima No.1 and No.2 plants and the Tokai No.2 plant after the strongest earthquake ever to hit the country.
As an industrial powerhouse nation poor in energy resources, Japan also draws about 30 per cent of its total power from its 53 nuclear plants.
Earlier today the Tokyo Electric Power, which runs the plants, had released some radioactive vapour at the plants in a bid to relieve building reactor pressure.