Nuclear Crisis: NRC Says Spent Fuel Pool at Unit Four Lost Massive Amounts of Water; Japan Disputes Claims

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WATCH Teetering on the Brink of Meltdown

America's top nuclear official told Congress today that the pool cooling spent fuel rods at the crippled Japanese nuclear complex had lost most of its water or all of its water, a potentially catastrophic situation.

The Japanese quickly challenged that statement, but gave few details saying only that the situation at the holding pool was "stable."

Nuclear Regulatory Commission Chairman Gregory Jaczko said that the fuel pool at unit 4 at the the Fukushima Daiichi nuclear power plant had lost massive amounts of water.

"We believe at this point that unit 4 may have lost a significant inventory, if not lost all of its water," Jaczko told a hearing before the House Energy and Commerce Committee. "What we know at unit three, and again our information is limited, what we believe is that there is a crack in the spent fuel pool for unit three as well, which could lead to a loss of water in that pool."

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.

Japan's nuclear safety agency and Tokyo Electric Power Co., which operates the complex, deny water is gone from the pool. Utility spokesman Hajime Motojuku told the Associated Press the "condition is stable" at unit 4.

Radiation levels have risen rapidly at the plant and there is a fear that the situation is heading for the worst. If levels continue to rise the doses emergency workers experience near the reactors could be lethal. One U.S. Official told ABC News that "it would be hard to describe how alarming this is right now" and that a suicide mission might not even be enough to avert disaster.

Jaczko recommends that American citizens living within 50 miles of the Fukushima nuclear power plant evacuate the area.

"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's current evacuation zone is 12 to 19 miles.

The recommendation comes as the Tokyo Electric Power Co. announced that the power line to the plant is almost complete and that the company plans to try it "as soon as possible." The line would revive electric-powered pumps, enabling a steady water supply to be maintained at the troubled reactors and spent fuel storage ponds, keeping them cool.

Surging radiation levels temporarily halted work to cool the troubled reactors at the plant earlier today, raising worries that officials are running out of options to stabilize the escalating catastrophe.

"We're very close now to the point of no return," Dr. Michio Kaku, a theoretical physicist, said. "It's gotten worse. We're talking about workers coming into the reactor perhaps as a suicide mission and we may have to abandon ship."

Crisis At the Japanese Nuclear Complex Escalates

A group of 180 workers rotate shifts working at the plant in teams of 50 men. The men have been nicknamed the "Fukushima Fifty."

When radiation levels surged following a fire at Unit 4 and a rising cloud of radioactive vapor from unit 3, officials deemed it too risky for the plant workers to continue their critical work of pumping sea water on the damaged reactors and fuel ponds.

"The workers cannot carry out even minimal work at the plant now," Chief Cabinet Secretary Yukio Edano told the Associated Press. "Because of the radiation risk we are on standby."

Radiation levels were as high as 10 millisieverts per hour today, 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.

In the aftermath of the 9.0 magnitude earthquake and tsunami, the growing fear of a nuclear meltdown has spread throughout Japan.

Emperor Akihito, a figure deeply respected in Japan, spoke for the first time since the Mar. 11 earthquake that has left at least 4,340 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 434,00 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."

In another sign of escalating nuclear danger, Cabinet Secretary Edano acknowledged that the containment vessels of some of the reactors are likely damaged. The Japan Atomic Industrial Forum confirmed damage to Units 2 and 3.

The last step in a nuclear meltdown is the breaching of the containment vessels. The fact that at least two containment vessels are damaged makes nuclear experts nervous.

"We have cracks now, cracks in the containment vessels...and if those cracks grow or if there's an explosion, we're talking a full blown Chernobyl, something beyond Chernobyl," Kaku said.

Japan Aborts Helicopter Mission Over Damaged Fukushima Plant

Some scientists believe that the accident level at the troubled plant should be escalated to a level 6, just one level lower than Chernobyl and two levels higher than the accident at Three Mile Island.

"I think the last ace in the hole is the Japanese Air Force, the military at some point may have to take over, may have to bury these reactors in concrete just like we did at Chernobyl, sandbagging the reactor with 5,000 tons of concrete, boric acid and sand," Kaku said.

Earlier today, government officials called off a plan for helicopters to dump seawater on the troubled reactors because of the heightened radiation levels.

The Japanese government has asked for the United States' help in the crisis.

Already, seven additional experts from the U.S. Nuclear Regulatory Commission arrived in Japan today.

The United States government may be sending in a special nuclear team, made up of hundreds of U.S. military personnel trained specifically for nuclear emergencies. They would be help respond to the disaster and offer aid to the local population if they suffered decontamination.

At least 140,000 people in the 12 mile radius around the plant have been evacuated. Those in a 12 to 19 mile radius of the plant have been ordered to stay indoors.

The mayor of Minami Soma, a town within that radius, said that residents are being stigmatized, Japanese broadcaster NHK reported.

"We are being labeled as contaminated lepers," he said.

The mayor said that drivers are refusing to transport supplies to them, NHK reported.

As Japan continues its rescue efforts, strong aftershocks continue to jolt the nation. Two aftershocks of magnitude 6.0 have hit Japan in the last 24 hours.

The Japanese are also bracing for a cold snap. Rain and snow is expected in the north. The worry for some is whether the snow will be radioactive.

To learn more about nuclear radiation, click here.

ABC News' Jim Hill, Juju Chang, Martha Raddatz, Luis Martinez, Lauren Pearle, Sunlen Miller and The Associated Press contributed to this report.