Engineers have resumed work to restore the cooling system of reactor 3 at Japan's Fukushima Daiichi nuclear power plant.
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.
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.
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.
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 stabilized. 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".
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.