(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.
Authorities intend to eventually switch to fresh water, he said, without providing an explanation as to why or a timetable.
In addition, firefighters from Tokyo and Yokohama are preparing to spray another 500 tons of water toward that reactor's nuclear spent fuel pool, which Nishiyama said could happen Thursday or Friday.
These pools contain fuel rods that, if not cooled down, can overheat and, in the process, release radioactive vapors into the air.
Edano said Thursday "there has been no spike in temperatures" at the No. 3 reactor.
The secretary also downplayed concerns about white smoke reportedly rising from other reactors.
The "vapor" rising near the No. 1 reactor at the plant is "only natural" and not a cause for alarm, he said, especially since water is now in that unit's nuclear spent fuel pool.
He added that the temperature at the No. 1 reactor "right now is going down."
Nishiyama added that by decreasing the rate of water being pumped into the reactor early Thursday, authorities had also decreased pressure that had earlier been rising.
Previous buildups of hydrogen gas have contributed to at least three explosions -- several of which caused injuries and coincided with spikes in radiation -- at the power plant's Nos. 1, 2 and 3 units.
Light was restored Thursday in the No. 1 reactor's central control room, a Tokyo Electric Power Co. official told CNN.
But it was just a partial restoration, as workers continued to try to get electricity going for control panels and cooling system pumps at the reactor. Nishiyama said that the hope is to begin cooling the unit's spent fuel pool Friday using outside power.
While describing the No. 2 unit as "quite stable," Nishiyama did note -- but did not explain -- "high radiation readings" nearby.
Seawater continues to be pumped in an effort to cool down nuclear fuel rods and prevent the further emission of radioactive material into the atmosphere.
Members of Japan's self-defense forces Thursday doused the nuclear spent fuel pool at the No. 4 reactor. Nishiyama estimated that this effort should wrap up Friday.
And Nos. 5 and 6 reactors appear safe for now, the nuclear safety official said.
The temperatures at both units are relatively low, though that could change after their cooling system gave out Wednesday.
Nishiyama said workers hope to get that machinery back into working order soon.
"Until we get power restored to the power plant, continuing doing what they have been doing for the last 12 days is, quite honestly, the only game in town," Michael Friedlander, a former senior operator at three U.S. nuclear power plants who has been following the situation at the Fukushima Daiichi plant, said Wednesday.
"It's absolutely essential that they keep the reactor covered with water as well as continuing to refill it," he said. "But getting the power restored and getting the equipment moving so they can get back on track is essential. At that point, we can determine the emergency is in the final stages."
He predicted power would be restored gradually to the plant over the next few days. "The power source is going to the main control room because that is the main nerve center of the entire plant," he said. "Once they get that restored, then we have some instrumentation and we can figure out what's going on in the power plant that up to this point has been almost impossible to figure out."
Once that is done, he said, power will be sent to individual pieces of equipment and the situation will be analyzed. He estimated it could take two weeks or more to get the plant "in a stable, cold shut-down configuration."
Lake Barrett, a nuclear engineer who led the initial cleanup and response of the Three Mile Island plant in Pennsylvania following a partial core nuclear meltdown in 1979, said there's likely no saving the plant -- though much can, and still needs, to be done to keep the crisis under control.
"It is an industrial catastrophe," Barrett said. "It's a huge plant, and it's been basically destroyed internally and has high contamination levels inside. There are areas in the building where no human is going to go for a long time."
But Barrett said the situation should be controlled and the radioactive fallout limited enough such that the long-term repercussions for the public health will be relatively minimal.
"It's also not a health catastrophe -- as long as the people follow the instructions from the government, they're going to be safe in Japan," he said.
Meanwhile, Japanese authorities added another vegetable to the list of restricted foods originating in prefectures near the Fukushima plant. On Wednesday, health officials said they found high levels of radioactive substances in the mizuna, or potherb mustard, shipped to Kyoto from Ibaraki Prefecture. While there was no immediate health risk, authorities instructed distributors and buyers to remove the vegetable from stores. Consuming 20 grams of the vegetable would be the equivalent of 1/400th the radiation of a stomach X-ray, officials said.
CNN's Whitney Hurst contributed to this report.