A satellite image from DigitalGlobe shows the Fukushima Daiichi nuclear… (Associated Press )
Workers struggling to contain radioactive releases from the Fukushima power plant face two critical tasks to avoid turning a nuclear disaster into a catastrophe: preventing a runaway chain reaction into the nuclear fuel and maintaining a massive flow of seawater through the damaged pools and reactor vessels.
There are few options, none of them good.
"The most imaginative engineers in the world couldn't have dreamed up a situation like this," said Najmedin Meshkati, a USC professor and nuclear power expert.
But to deal with the grim reality at hand, utility workers and some of the top U.S. experts on nuclear reactors are working around the clock, attempting to calculate how to avoid further damage to the reactors and how to get sufficient cooling water through the plant with improvised water cannons, fire hoses and helicopters.
Photos: Japan grapples with crisis
If they can buy enough time, possibly several weeks, the nuclear material will cool off and become less radioactive, significantly reducing the risk of a further meltdown or chain reaction event. In coming years, experts agree, the damaged fuel will have to be removed from the plant to a safer location. Most experts say that the plant cannot be entombed in concrete, as was the Chernobyl plant after the 1986 disaster.
"If those water cannons are getting water to the cooling pools, they should keep that up," said Elmer Lewis, an expert on nuclear power plant safety at Northwestern University in Evanston, Ill. "Until that fuel cools down, they have a real mess on their hands."
Edward Morse, a professor of nuclear engineering at UC Berkeley, added that it would take huge amounts of water to compensate for the cracks in a containment pool that were uncovered by U.S. surveillance aircraft on Friday.
"The best thing to do is use as much of the Pacific Ocean as possible," he said.
The other crucial task is to prevent the fuel in any of the reactors or pools from going critical, an event in which nuclear fission starts on its own and generates tremendous amounts of heat. Tokyo Electric Power Co., which operates the Fukushima plant, said it was importing tons of additional boron to ensure it could flood the reactor with the material, which absorbs neutrons that trigger the breakdown of uranium nuclei.
Not only will water absorb heat, it also forms a protective barrier against radiation, making it safer for workers at the plant, said David Lochbaum, head of nuclear safety policy at the Union of Concerned Scientists and a former nuclear plant operator.
"If they can cover the fuel, it will reduce the radiation levels and they can use the plant equipment," Lochbaum said.
If the fuel overheats, as it has already done in some cases, it will release additional radioactive contamination into the environment.
But Per Peterson, chairman of the UC Berkeley nuclear engineering department, said he believes that most of the rods will not burn even if they are not covered by water. Only a few extremely radioactive rods may be vulnerable, said Peterson, who is advising U.S. officials on their response.
But if the fuel does overheat, that would destabilize the situation further and vastly complicate future efforts to clean up the plant.
While the spraying continues, workers continue to be exposed to relatively high levels of radiation, with doses of 20 millisieverts per hour measured in the control room at the site. Although workers are wearing protective gear that prevents them from coming in contact with radioactive particulates and are working for only one or two hours at a time, they are still being exposed to potentially lethal levels of gamma radiation.
The French nuclear agency IRSN said Friday that the Fukushima Daiichi plant had already released 10% as much radioactivity as Chernobyl, though the agency has been accused by some as being alarmist.
The cooling job may get easier when power is fully restored to the plant.
Company workers were able to lay a new power line to the plant early Saturday morning and began connecting it to reactor No. 2, whose containment vessel is believed to be cracked. They will then hook up the buildings housing reactors No. 1, No. 3 and No. 4, which they hope to have connected by the end of Sunday, they said.
But experts think it is unlikely that cooling pumps in the three reactors that were operating when the magnitude 9 Tohoku quake struck — Nos. 1, 2 and 3 — will work even with an outside source of electricity. Those pumps were probably damaged in a series of hydrogen explosions that occurred in the first four days of the crisis. The power lines could provide electricity to operate valves and controls, however.
There is no indication that the cooling pump servicing the cracked spent fuel pond on the upper floor of reactor No. 4 has been damaged, so officials hope to restore it to action.