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All six reactors at Fukushima Daiichi had their external power supplies restored on TuesdayEngineers have resumed work to restore the cooling system of reactor 3 at Japan's stricken Fukushima Daiichi nuclear power plant, reports say.
Work had been suspended after a plume of black smoke was seen coming from the overheating reactor on Wednesday.
Later on Thursday, two workers at reactor 3 were taken to hospital after being exposed to radiation.
The plant was badly damaged by the 9.0-magnitude earthquake and tsunami which struck north-eastern Japan on 11 March.
Radiation has contaminated Tokyo's water supply and food produced nearby.
The science ministry has begun to monitor levels of radioactive iodine and caesium 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 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.
But radiation readings on Thursday showed levels in water in Tokyo had fallen back below the danger level.
Concern is growing among Japan's neighbours. Australia has become the latest country to ban food imports from the affected region.
Workers exposed
Tepco, the operator of Fukushima Daiichi, said it had allowed work to resume on reactor 3 on Thursday morning because it was safe for workers to return, Kyodo news agency reported.
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If this is long term, I think we have a lot to worry about”
Riku KatoFather of one-year-old baby living in Tokyo
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 early on Thursday, although steam continues to rise from reactors 1 to 4, according to NHK television.
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.
Later on Thursday, Japan's nuclear safety agency said three workers stationed at reactor 3 had been exposed to high levels of radiation as they were laying power cables.
Spokesman Hideyuki Nishiyama said the workers had been "exposed to radiation ranging from 170 to 180 millisieverts", AFP reports. An exposure of 100 millisieverts per year is considered the lowest level at which any increase in cancer risk is evident.
Two of the workers were taken to hospital.
Power restored
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 remains "of serious concern", though there have been "some positive developments".
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The plant is 250km (155 miles) north-east of Tokyo. The government has declared a 20km exclusion zone and evacuated 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 were warned on Wednesday not to give tap water to babies less than a year old, because levels of radioactive iodine - which can cause thyroid cancer -in some areas are twice the recommended safe level.
"If this is long term, I think we have a lot to worry about," Riku Kato, the father of a one-year-old baby, told the Reuters news agency.
But officials have stressed that children would have to drink a lot of it before it harmed them. There is no immediate health risk to others.
Nevertheless, reports said the shelves of supermarkets in the capital had been cleared of bottled water by Thursday morning.
The municipal authorities plan to distribute 240,000 bottles of water to households with infants on Thursday. They estimate that there are 80,000 babies in the affected area. Each will get three 550ml bottles.
The authorities in the nearby city of Kawaguchi, Saitama prefecture, also reported radiation levels above safety norms in its water supply on Thursday.
Levels in Tokyo were later reported to have fallen below the danger level.
Rescue operation
Meanwhile, exhausted and traumatised rescuers are still sifting through the mud and wreckage of towns devastated by the earthquake and tsunami.
Police believe the final death toll from the earthquake and tsunami may be more than 18,000The National Police Agency said on Wednesday night that the official death toll has risen to 9,523. Another 16,094 people are listed as missing.
Most of the deaths - 5,700 - have been reported in the prefecture of Miyagi. Iwate prefecture has found 3,000 bodies, and Fukushima 776.
Police believe the final death toll may be more than 18,000.
At least 18,000 houses were destroyed and 130,000 damaged.
More than 200,000 people are living in emergency shelters. NHK said some survivors had attempted to return to their homes in areas where essential services had been restored, but that there were shortages of basic supplies.
The Japanese government has said it will cost as much as 25 trillion yen ($309bn; £189bn) to rebuild the country after the disaster.
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