Desperate operators of the deteriorating nuclear power plant in Fukushima, Japan, on Wednesday are having second thoughts about a dramatic plan to stave off a meltdown in a fire-damaged reactor by dumping water on it by helicopter.
Plant operators are worried that the water wouldn't reach its intended target — overheated spent-fuel rods in reactor No. 4.
According to NHK television, officials at Tokyo Electric Power Co. decided a hole in the roof of the reactor, which suffered an apparent explosion and fire on Tuesday morning, 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.
