CNN Wire Staff
(CNN) - One day after black smoke prompted an evacuation, workers returned Thursday to the Fukushima Daiichi nuclear plant - employing myriad methods to try to prevent more radiation from seeping into the atmosphere.
After several days of setbacks and billowing smoke, authorities Thursday addressed issues at each of the facility's six reactors.
"We are working to resume (operations)," Japanese Chief Cabinet Secretary Yukio Edano said. "We cannot be too optimistic, and we are still taking cautious measures."
Japan Atomic Industrial Forum, an industry trade group, reported Thursday that - despite previous fears to the contrary - the No. 3 reactor's containment vessel was "not damaged."
This news came the same morning that smoke stopped rising above the reactor, according to Hidehiko Nishiyama, an official with Japan's Nuclear and Industrial Safety Agency.
He said the cause of the smoke remains unknown, speculating it may have come from burning oil or machinery nearby.
On Wednesday, the same day the black smoke appeared, Edano said three workers were exposed to water contaminated by radioactive material while laying cable in the No. 3 reactor's turbine building. They stepped into the water, which seeped into the shoes of two of the men, according to Tokyo Electric Power Company.
All three men were exposed to between 173 and 181 millisieverts of radiation, and two went to a hospital for treatment, a Tokyo Electric Power Co. official said.
A person in an industrialized country is naturally exposed to 3 millisieverts a year. But Japan's health ministry recently raised the maximum level of exposure for a person working to address the crisis at the nuclear plant from 100 millisieverts to 250 millisieverts per year.
The three workers reached the highest level of millisieverts recorded so far, Tokyo Electric said. The two admitted to the hospital were a man in his 30s who was exposed to 180.7 millisieverts, and a man in his 20s who tested at 179.37 millisieverts. The third man, who was exposed to 173 millisieverts, did not go to the hospital, as his boots were high enough to cover his skin, Tokyo Electric said.
Seventeen workers so far have been exposed to radiation at levels over 100 millisieverts, Tokyo Electric said Thursday, including the three involved in the water incident.
By Thursday, work had resumed at that reactor. Beginning shortly after 5:30 a.m., crews began injecting about 500 tons of seawater into it, Nishiyama said.
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