Black smoke has been seen emerging from Unit 3 of the crippled nuclear plant in north-eastern Japan, prompting a new evacuation of the complex, according to officials.
The Tokyo Electric Power company said workers from the entire Fukushima Dai-ichi plant have been temporarily evacuated.
Operators of the power station have been desperately trying to cool the reactors and spent fuel pools at the plant after it was damaged by this month’s tsunami, which knocked out power to the cooling systems.
Yukio Edano, Japan’s chief cabinet secretary, has told reporters that is has proved extremely difficult to see inside the stricken reactors to find out what is causing the vapour.
Meanwhile, Japan’s government has said the economic costs of the catastrophic March 11 earthquake and tsunami could reach £210bn.
The damage to housing, infrastructure and businesses in north east Japan could cost between 16 trillion yen and 25 trillion yen (£189bn), according to the cabinet office.
The 9.0-magnitude quake and tsunami devastated Japan’s north eastern coast, killing thousands of people and triggering a crisis at a nuclear power plant.
Tens of thousands of people living near the plant were evacuated.
Utilities have imposed power rationing, many factories remain closed and key rail lines are impassable.