Tokyo, Mar 12 (AP) 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.