A building housing a reactor was destroyed on March 12 blast at the Fukushima No.1 plant.

The authorities said the reactor itself was intact inside its steel container.

The devastating earthquake and tsunami is believed to have left more than 1,000 people dead.

The Japanese government has sought to play down fears of a meltdown at Fukushima No.1, saying that radiation levels around the stricken plant have now fallen.

But on March 13, concerns were raised about the safety of a second reactor at the plant after operator Tokyo Electric Power (Tepco) said the cooling system of another reactor had failed.

Since the earthquake, radioactive air and steam has been released from several reactors at both Fukushima No.1 and No.2 plants in an effort to relieve the huge amount of pressure building up inside. Sea water and boron is being pumped into the site to lower temperatures.

Tepco said four of its workers were injured in the explosion, but that their injuries were not life-threatening. The Japanese government doubled the size of the evacuation zone around No.1 plant to 20km (12.4 miles) after the blast.

Reuters news agency quotes a Japanese nuclear safety agency official as saying that tests indicate that at least nine people have been exposed to radiation from the plant, and local authority estimates suggest this figure could rise as high as 160.

The government has urged local people to remain calm and is preparing to distribute iodine to anyone affected.

The UN's nuclear watchdog, the International Atomic Energy Agency (IAEA), said in a statement: "In the 20-kilometre radius around Fukushima Daiichi (No.1), an estimated 170,000 people have been evacuated.

"In the 10-kilometre radius around Fukushima Daini (No.2) an estimated 30,000 people have been evacuated. Full evacuation measures have not been completed."