The Japanese are looking to the U.S. for help after frantic efforts to cool the overheating Fukushima Daiichi nuclear reactors and fuel ponds have failed to bring the plant under control.
There is hope that water pumps the U.S. is sending could help to avert disaster.
The entire crisis began when the plant lost power after last week's 9.0 earthquake and tsunami. Plant operators have now connected a new power line that could restore electricity. However, if the Japanese flip the switch and the critical water pumps that cool the reactors do not work, the American pumps may come to the rescue.
The Pentagon has shipped in the pumps, but no U.S. personnel. Japanese workers will risk their lives to operate them. The pumps were not shipped in earlier because the Japanese had not requested them.
The power line, if it works, might help in the effort to cool the reactors, but not the fuel ponds.
"From what I see they are working to get electricity back to the site so that they can restart the backup cooling pumps. If this happens then that would be good news for the reactors themselves," Kirby Kemper, a nuclear physicist and professor at Florida State University, said. "As far as the holding ponds are concerned, you probably also need to get some boron-loaded fluid in there [if] you think that any of the rods have melted through and released material, so that there is no danger of having fissions from the clump of material falling to the bottom of the tank."
Today, in new video, close-ups of reactors three and four were visible for the first time. Reactor three was charred and billowing steam and the walls of reactor four were blown out.
One U.S. official told ABC News the most serious problem was the spent fuel rods at reactor four, which are extremely hot and "probably close to a crisis situation."
The water in the pool is desperately low, and without water, the rods could ignite and fill the sky with radioactive smoke.
The situation at reactor three is very similar. The fuel stored in its pool is likely on the verge of burning up, engineers said, and because it includes plutonium, it would produce a highly dangerous toxic plume. Also, the reactor's five-foot-thick concrete containment vessel is likely cracked, which means that if the core melts down, radioactive lava would pool at the bottom of the vessel and seep out.
Some engineers call that a "core on the floor" situation, meaning that the containment vessel gives way at the bottom. There is no longer a way to cool the nuclear cores so they melt down, bleed out and send toxic nuclear clouds into the air.
On Thursday the Japanese used water cannons, trucks and helicopters to dump seawater on the plant in northern Japan.
First, Japanese pilots, wearing radiation protection suits, flew two CH-47 Chinook helicopters over the damaged unit 3, dumping four huge buckets of water. The helicopters had lead plates on them to help shield the pilots from radiation. Following the drop, no significant changes in radiation were detected at the nuclear plant. Much of the water appeared to dissipate in the wind as it fell.
Unit 3 has both a damaged reactor and a pond for spent fuel where the temperature is rising.