Workers at Fukushima nuclear plant enter ravaged reactor building

They are installing ventilation systems in reactor Number 1 to filter out radioactive material from the air.

The quake on March 11 disabled reactor cooling systems, causing fuel rods to overheat.

Radiation levels inside reactor buildings must be lowered before new cooling systems can be installed, following fears in the past weeks of extreme radiation levels.

The Number 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.

The BBC reports spokesman Junichi Matsumoto, who told reporters "Groups of four will go in one by one to install the ducts. They'll be working in a narrow space.”

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.

Tepco has said it expects to bring the crisis under control and achieve a cold shutdown of the plant by the end of the year, but some doubt whether this target can be achieved.

A 20km (12 mile) evacuation zone has been put in place around the plant because of concern about radiation levels.

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

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