Japan is scrambling to prevent nuclear accidents at two atomic plants where reactor cooling systems failed after a massive earthquake, as it evacuates tens of thousands of residents.

Radiation 1000 times above normal was detected in the control room of one plant, although authorities said levels outside the facility’s gates were only eight times above normal, spelling “no immediate health hazard”.

Tokyo Electric Power, which runs the plants, said it had released some radioactive vapour at the plants to relieve building reactor pressure, but stressed the move posed no health risks.

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 on Friday, without specifying which plant.

The two nuclear plants affected are the Fukushima No.1 and No.2 plants, both located about 250 kilometres 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 on Saturday, according to Jiji.

However, the evacuation area was expanded. A total of 45,000 people living within a 10-kilometre radius of the No.1 plant were told to evacuate.

Officials on Saturday ordered the evacuation of people living within a three kilometre radius of the second plant, with those up to 10 kilometres 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 on Saturday morning 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.

Kan had at first, on Friday afternoon, 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.

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 per cent of the world’s major earthquakes.

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.

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