N-plant trouble after Japan quake, 3,000 evacuated

Tokyo: Japan's massive earthquake caused a power outage that disabled a nuclear reactor's cooling system, triggering evacuation orders for about 3,000 residents as the government declared its first-ever state of emergency at a nuclear plant.

Japan's nuclear safety agency said pressure inside one of six boiling water reactors at the Fukushima Daiichi plant had risen to 1. 5 times the level considered normal.

Hours after the evacuation order, the government announced that the plant in northeastern Japan will release slightly radioactive vapour from the unit to lower the pressure in an effort to protect it from a possible meltdown.

Chief Cabinet Secretary Yukio Edano said the amount of radioactive element in the vapour would be "very small" and would not affect the environment or human health.

"With evacuation in place and the ocean-bound wind, we can ensure the safety," he said at a televised news conference early today.

After the quake triggered a power outage, a backup generator also failed and the cooling system was unable to supply water to cool the 460-megawatt No 1 reactor, though at least one backup cooling system was being used. The reactor core remains hot even after a shutdown.

The agency said plant workers are scrambling to restore cooling water supply at the plant but there is no prospect for immediate success.

Edano said the 40-year-old plant was not leaking radiation.

The plant is in Onahama city, about 270 kilometres northeast of Tokyo.

If the outage in the cooling system persists, eventually radiation could leak out into the environment, and, in the worst case, could cause a reactor meltdown, a nuclear safety agency official said on condition of anonymity, citing sensitivity of the issue.

Another official at the nuclear safety agency, Yuji Kakizaki, said that plant workers were cooling the reactor with a secondary cooling system, which is not as effective as the regular cooling method.

Kakizaki said officials have confirmed that the emergency cooling system, the last-ditch cooling measure to prevent the reactor from the meltdown, is intact and could kick in if needed.

"That's as a last resort, and we have not reached that stage yet," Kakizaki added.

Japan's nuclear safety agency said the evacuation, ordered by the local government of Fukushima, affects at least 2,800 people.

Edano said residents were told to stay at least three kilometres from the plant and to stay inside buildings.

He said both the state of emergency and evacuation order are precautionary measures.