They are installing ventilation systems in the No 1 reactor to filter out radioactive material from the air.

The quake disabled reactor cooling systems, causing fuel rods to overheat.

Radiation levels inside reactor buildings must be lowered before new cooling systems can be installed.

The No 1 reactor was one of four damaged by explosions in the days immediately after the earthquake and tsunami. Water is being pumped in to cool the reactors.

The Tokyo Electric Power Company (Tepco) said that 12 engineers would work inside the reactor building in shifts of 10 minutes.

Footage filmed by cameras mounted on robots sent into the reactor had already established that there were no leaks of potentially radioactive water inside the building.

Tepco said it hoped to begin operating the ventilator later in the day. It then plans to connect up a new cooling system outside the reactor to bring temperatures down.

The company faces similar problems at three other reactors at the six-reactor plant.

Tepco is also dealing with highly radioactive waste-water leaking from the No 2 reactor which it is moving to secure storage on site.

A total of 14,785 people are confirmed to have been killed by the earthquake and resulting tsunami. Another 10,271 remain missing, according to the latest police figures.

Japan's recovery bill has been estimated at US$300 billion - making this already the most expensive disaster in history.