All workers from a crippled reactor at a nuclear power plant in Fukushima, Japan, have been evacuated, Chief Cabinet Secretary Yukio Edano said Wednesday.
Edano said that white smoke was rising from the No. 3 reactor at the Fukushima Daichi power plant, adding officials were investigating the cause. He said radiation levels had been fluctuating by the hour; they had spiked rapidly Wednesday morning, and that even the minimal workforce was evacuated.
Edano said that something similar to what previously happened at reactor No. 2 may be happening at reactor no. 3 — a possible problem with the reactor's containment vessel.
Meanwhile, efforts continue to try to cool the storage pool at reactor No. 4. Edano said officials were still trying to decide the best course of action, and that simply adding a lot of water quickly could pose risks.
The development came after a fire broke out Wednesday at reactor No. 4.
Tokyo Electric Power Co. spokesman Hajimi Motujuku said the blaze erupted early Wednesday in the outer housing of the reactor's containment vessel. It wasn't clear if the second fire at reactor No. 4 in as many days was new, or if the first fire wasn't fully extinguished. Firefighters are trying to put out the flames. Japan's nuclear safety agency also confirmed the fire, whose cause was not immediately known.
Tuesday's fire and an apparent explosion damaged the reactor's roof, and there are concerns that the spent fuel rods are overheating.
About three hours after the blaze erupted Wednesday, Japan's nuclear safety agency said fire and smoke could no longer be seen at Unit 4, but that it was unable to confirm that the blaze had been put out.
Also Wednesday, the agency said 70 percent of the nuclear fuel rods may have been damaged at another Fukushima Daiichi reactor that was first stricken last week, triggering the crisis.
"But we don't know the nature of the damage, and it could be either melting, or there might be some holes in them," said an agency spokesman, Minoru Ohgoda.
Desperate plant operators considered dramatic plans to stave off a meltdown in the reactor, including dumping water on it by helicopter. But plant operators worried that the water wouldn't reach the fuel rods.
According to NHK television, officials at Tokyo Electric Power Co. decided a hole in the roof of the reactor was "dozens of meters" from the swimming-pool-like chamber where the spent fuel rods are overheating. So a helicopter dump, similar to putting out a forest fire, probably wouldn't reach the pool.
Moreover, officials say helicopters can't carry enough water to do the job. And Japanese Defense Ministry officials are worried about the safety of military personnel on the helicopters, according to Kyodo News.
TEPCO, which operates the Fukushima power plant, is still considering the use of high-pressure fire hoses to spray cooling water into the spent fuel pool.
Radiation levels are far too high to permit workers to bring hoses anywhere near the pool's edge to re-flood it manually.
U.S. nuclear safety experts agreed. David Lochbaum, director of the nuclear safety program at the Union of Concerned Scientists, says a study done for Connecticut nuclear power plants concluded that in a situation such as this one, radiation would be so intense that a worker at the pool's edge "would receive a lethal dose in something like 16 seconds."
The spent-fuel problem is a new wild card in the potentially catastrophic failure of the Fukushima power plant. Since last Friday's 9.0 earthquake, the plant has been wracked by repeated explosions in three different reactors.
Some experts are now concerned that spent fuel rods may overheat in two other reactors, even though they were not in service at the time of the earthquake. Those two units, Nos. 5 and 6, have not yet reported problems.
"There are accounts that they're having difficulties cooling those three spent fuel pools, and they need to regain control of that," says Lochbaum, a nuclear engineer by training. "Or as a minimum, need to be able to replace the water that may be evaporating or boiling away to prevent the water from dropping below the level of irradiated fuel in the bottom of those spent fuel pools to prevent their damage from overheating as well."
Radiation Concerns From Spent Fuel Rods
The problem at reactor No. 4 was apparently brewing for some time before Tuesday's fire. The company says the temperature of the spent fuel pool reached 183 degrees Fahrenheit on Monday — twice the normal level. It apparently went higher, but a "technical failure" prevented later readings.
One concern is that the spent fuel pool may not have the radiation containment barriers that protect the reactor's fission vessel. Thus, melting fuel rods could become a significant source of radiation leakage into the environment.
Radiation levels spiked as high as 400 milliSieverts per hour at the plant's main gate during Tuesday's fire and explosion — a potentially dangerous and even fatal level if it's sustained. But the International Atomic Energy Agency (IAEA) reports the levels later dropped to much lower readings.
Still, company officials evacuated all but 50 of the facility's 800 workers for their own safety. The IAEA says it's seeking "details about the status of all workers, reactors and spent fuel at the Fukushima Daiichi plant."
It is not clear how much radiation exposures increased in the 12-mile evacuation zone around the plant or beyond. Radiation levels went up transiently in Tokyo, but not to levels that pose any public health concern, officials say.
The problems with spent fuel rods mean there are now two potential sources of radioactive leakage at the damaged plant.
A second explosion on Tuesday in the plant's No. 2 reactor probably has caused a breach in the part of the heavy containment vessel known as the suppression chamber, or torus. That potentially allows radioactive steam or water to escape from a new route.
Previously, radioactivity emanated from controlled releases of steam in an attempt to depressurize the reactors and allow cooling seawater to be pumped in.
The Associated Press contributed to this report.
Copyright 2011 National Public Radio.