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.
Next, the National Police Agency used a pumper truck to launch 11 water cannons from the ground. The results were deemed a failure by officials, Japanese broadcaster NHK reported. High radiation forced them to a location too far away from the plant for them to be effective.
"It's like a squirt gun, using a squirt gun against a raging forest fire. They're overwhelmed, they're floundering, they don't know what to do," said Dr. Michio Kaku, a theoretical physicist who has been consulting for ABC News.
Next, Japan's Self Defense Forces poured 30 tons of water from special pumper trucks. Workers were able to pump the water without leaving the trucks. Officials are still determining how effective the water was in cooling the fuel pond.
The water trucks and helicopters were used, in part, because radiation levels are too high for workers to be directly in the plant for extended periods.
"At a certain point, they're going to have to abandon ship, they're committing a suicide mission to go in there. The radiation levels are near lethal right now…you're committing suicide to spend large amount of time there," Kaku said.
The International Atomic Energy Agency said at a press conference today that the situation at the plant remains "serious" but has not worsened since yesterday.
Radiation levels have soared in some places around the plant in the last 24 hours. The highest readings outside the area have come 30 kilometers northwest of the plant, where levels of 170 millisieverts were measured.
The level of radiation in Tokyo remains low, officials said, and is not a danger to humans.
The IAEA said it was concerned about unit 4's fuel pond. Officials said that they have received no new information on how much water remains in the pond. The last time they received a temperature reading was three days ago.
Experts say that the spent fuel ponds might be more dangerous than the core reactors. Fuel ponds hold old fuel rods that must be kept in water at all times. Of particular concern are the fuel ponds for units 3 and 4.
"Hollywood likes to focus in on the meltdown, the melted core exposed uranium. But old fuel is actually more dangerous than the meltdown because there's more radiation in an unguarded spent fuel pond than the reactors," Kaku said. "You could have a fire. It would go up like fireworks, like Roman candles."
The spent fuel rods are kept in pools of water to prevent them from overheating and ultimately melting down. The outer shell of the rods could also ignite with enough force to propel the radioactive fuel inside over a wide area. Along with the escalating temperatures in unit 3 and 4, unit 5 and 6 are also experiencing rising temperatures in their fuel ponds.
Japan's nuclear safety agency and Tokyo Electric Power Co., which operates the complex, denies water is gone from the pool. Utility spokesman Hajime Motojuku told the Associated Press the "condition is stable" at unit 4.
After days of silence on the issue of the nuclear crisis, President Obama delivered a statement today promising Japan the full support of the U.S. and reassuring Americans that they are not at risk.
"I want to be very clear," said Obama. "We do not expect harmful levels of radiation to reach the West Coast, Hawaii, Alaska or U.S. territories in the Pacific."
Overnight, Obama spoke with the Japanese Prime Minister Naoto Kan to express his sympathy and offer assistance.
"The president expressed his extraordinary admiration for the character and resolve of the Japanese people, and his confidence that Japan will make a full recovery from this disaster," said the White House in a statement.
Shortly after, the U.S. State Department authorized the first evacuations of Americans out of Japan. Taking no chances, U.S. officials have ordered all Americans within 50 miles of the plant to evacuate.
It also issued a warning to Americans to avoid traveling to the quake and tsunami-ravaged nation.
Chartered planes will begin assisting American citizens wishing to leave the country.
"For a comparable situation in the United States we would recommend an evacuation to a much larger radius than has been provided in Japan," he said. "As a result of this recommendation, the ambassador in Japan has issued a statement to American citizens that we believe it is appropriate to evacuate to a larger distance, up to approximately 50 miles."
Japan has not expanded its radius of evacuation, which requires those living within 12 miles to leave their homes and those between 12 and 19 miles from the plant to stay indoors.
U.S. Official: Next 28 to 48 Hours 'Critical'
One hundred and eighty workers rotate shifts, working at the plant in teams of 50 men. The men have been nicknamed the "Fukushima Fifty."
One U.S. official told ABC News that they are urging the Japanese to get more people to help the workers inside the plant.
"This is very, very radioactive material...if there is core on the floor and containment penetration, there will be significant public health consequences," Ken Bergeron, a physicist and nuclear reactor safety expert, said.
Radiation levels were as high as 10 millisieverts per hour yesterday, the equivalent of getting a CT scan for every hour of exposure. Radiation levels have since dropped and the plant workers are planning to return to work, officials said.
The Japanese government has actually amended its national safety standard on how much radiation workers can be exposed to so that workers can return to the plant. The limit is now 250 millisieverts, 2.5 times the previous limit.
Emperor Akihito, a figure deeply respected in Japan, spoke for the first time yesterday since the March 11 earthquake that has left at least 5,300 people dead. He tried to ease worries about the country's nuclear crisis.
"With the help of those involved I hope things will not get worse," Akihito, 77, said.
He offered his condolences to a grieving nation where at least 9,083 people are still missing and 434,000 are homeless.
"It is important that each of us shares the difficult days that lie ahead," Akihito said. "I pray that we will all take care of each other and overcome this tragedy."
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ABC News' Martha Raddatz, David Muir, Jim Hill, Juju Chang, Dan Arnall and The Associated Press contributed to this report.