Engineers have resumed work to restore the cooling system of reactor¹ 3 at Japan's stricken Fukushima Daiichi nuclear power plant, reports say.

据报道，日本福岛核电站的工程师们正全力修复3号反应堆的冷却系统。

The Tokyo Electric Power Company had suspended work after a plume² of black smoke was seen from the overheating reactor on Wednesday afternoon.

The plant was badly damaged by the 9.0-magnitide earthquake and tsunami³ which struck north-eastern Japan on 11 March.

Radiation has contaminated Tokyo's water supply and food produced nearby.

The science ministry⁴ has begun to monitor levels of radioactive iodine⁵（碘） and caesium（铯） in soil, water and air around the plant to determine the extent of the contamination, and how it will affect the farming and fishing industries.

There were also reports that shops in the capital had run out of bottled water, after Tokyo Governor Shintaro Ishihara said that levels of radioactive iodine in tap water were more than twice what is considered safe for babies.

Concern is growing among Japan's neighbours. Australia has become the latest country to ban food imports from the affected⁶ region.

Power supply

Tepco, the operator of Fukushima Daiichi, said it had allowed work to resume on reactor 3 on Thursday morning because it was certain it was safe for workers to return, the official Kyodo news agency reported.

While the cause of the dark smoke remained unknown, there was no fire and radiation levels in the vicinity did not rise, the company added.

The Japanese nuclear safety agency confirmed that the smoke had stopped as of 0450 on Thursday (1950 GMT on Wednesday), although steam continues to rise from reactors⁷ 1 to 4, according to NHK television.

The AFP news agency reported that fire crews were again using high-pressure water jets to spray water into the storage pond on top of reactor 3 to stop the spent nuclear fuel rods inside being exposed the air.

The unit is of special concern because it used a type of fuel known as Mox, a mixture of plutonium oxide⁸ and uranium oxide. The radioactive fallout from any meltdown would be far worse than from the other reactors.

All six reactors had their external power supplies restored on Tuesday night, but each piece of equipment has to be tested before it can be turned back on.

Once the cooling systems are restated, the reactors can be stabilised. However, the process could take weeks or even months, Tepco said.

The International Atomic Energy Agency (IAEA) has said the situation remains⁹ "of serious concern", though there have been "some positive developments".

The plant is 250km (155 miles) north-east of Tokyo. The government has declared a 20km exclusion¹⁰ zone and evacuated¹¹ tens of thousands of people. Those living up to 30km away have been told to stay indoors to minimize exposure.

People in Fukushima prefecture have been told not to eat 11 types of green leafy vegetables grown locally because of contamination worries. Local producers have been ordered not to send the goods to market.

Residents of Tokyo have meanwhile been warned not to give tap water to babies less than a year old, because levels of radioactive iodine - which can cause thyroid（甲状腺） cancer -in some areas are twice the recommended safe level.