TOKYO (AFP) - Japan scrambled Saturday to prevent nuclear accidents at two atomic plants where reactor cooling systems failed after a massive earthquake, as it evacuated tens of thousands of residents.

   Radiation 1,000 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 into the atmosphere at one plant to relieve building reactor pressure, but said 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 Friday, without specifying which plant.

   The two nuclear plants affected are the Fukushima No. 1 and No. 2 plants, both located about 250 kilometres (160 miles) northeast of greater Tokyo, an urban area of 30 million people.

   Tokyo Electric Power vented radioactive vapour at the No.1 plant to release building reactor pressure.

   The evacuation area was expanded. A total of 45,000 people living within a 10-kilometre (six-mile) radius of the No. 1 plant were told to evacuate -- raising the number from the fewer than 6,000 people within three kilometres told to leave Friday.

   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 amid worries over cooling problems.

   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 great 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.

   Around 160 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 percent of the world's major earthquakes.

   As an industrial powerhouse nation poor in energy resources, Japan also draws about 30 percent of its total power from its 53 nuclear plants.