Japanese military helicopters dumped water Thursday from huge buckets onto the Fukushima Dai-ichi nuclear plant's damaged reactors in a bid to raise hopes of easing the crisis that has threatened a meltdown.
Three twin-rotor CH-47 Chinooks from the Japanese Self-Defense Forces were used in the operation. Two helicopters dumped seawater on the plant's damaged No. 3 reactor, while the other dropped seawater on the No. 4 reactor.
A nearly completed new power line could also restore electric cooling systems in the facility, its operator said Thursday.
Tokyo Electric Power Co. spokesman Naoki Tsunoda said the new power line is almost complete, but did not specify when the project would be finished.
The line would revive electric-powered pumps, allowing the company to maintain a steady water supply to troubled reactors and spent fuel storage ponds.
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The situation at Fukushima Dai-ichi grows more dire as the U.S. nuclear agency chief says there is no more water in the spent fuel pool at the facility, where unexplained white smoke is pouring from the complex.
National Regulatory Commission Chairman Gregory Jaczko did not say Wednesday how the information was obtained, but the NRC and U.S. Department of Energy both have experts on site at the Fukushima Dai-ichi complex of six reactors.
"There is no water in the spent fuel pool and we believe that radiation levels are extremely high, which could possibly impact the ability to take corrective measures," Jaczko said at a House Energy and Commerce subcommittee hearing.
The White House said it would cooperate closely with Japan during the recovery period, and President Obama spoke with Japanese Prime Minister Naoto Kan by phone on Wednesday evening.
Obama briefed the prime minister on U.S. support being provided, such as military teams that specialize in nuclear response and consequence management.
Kan updated Obama on the latest efforts to contain the nuclear disaster, and thanked him for U.S. support.
The International Atomic Energy Agency confirmed Wednesday that three reactors had partially melted down. Yukiya Amano, the head of the nuclear watchdog agency, says he plans on going to Japan as soon as possible.
When asked if events were out of control, he answered: "It is difficult to say."
Japan's nuclear safety agency and Tokyo Electric Power Co., which operates the complex, deny water is gone from the spent fuel pool. Utility spokesman Hajime Motojuku said the "condition is stable" at Unit 4.
Without water, there's nothing to stop the fuel rods from getting hotter and ultimately melting down.
Meanwhile, 50 employees at the Fukushima Dai-ichi nuclear plant -- dubbed the Fukushima 50 -- have put their health and well-being on the line as they try to prevent a total nuclear meltdown with conditions worsening at the plant, while the National Police Agency has been asked to send a water cannon to cool a pool storing spent fuel rods at the Unit 4 reactor, according to NHK.
Two of the workers are missing after a fire at the Unit 4 reactor, according to the Sydney Morning Herald. Japanese officials have told the International Atomic Energy Agency that they are worried about the state of a pool holding spent nuclear fuel, according to Reuters.
"Japanese authorities have reported concerns about the condition of the spent nuclear fuel pool at Fukushima Daiichi Unit 3 and Unit 4," the IAEA told Reuters.
Japan's defense minister says helicopters plan to drop water onto Unit 3, while officials prepare to use water cannons to spray water into Unit 4.
The workers, who have been regularly rotated through the danger zone to minimize their radiation exposure, were briefly pulled out of the plant after radiation levels spiked while they were dousing the nuclear reactors with seawater to cool them, but were ordered to return to the site several hours later after radiation levels decreased. Officials believe the radiation levels spiked when white clouds drifted up from one of the reactors.
The government has now raised the maximum radiation dose allowed for nuclear workers to 250 millisieverts from 100 millisieverts.
It is not known how much radiation has leaked from the crippled nuclear plant because the computer system that forecasts the spread of radioactivity has not been working due to malfunctioning monitoring posts, according to NHK.
The Nuclear and Industrial Safety Agency does not know when the system will be back in operation. The system, called SPEEDI, predicts how radiation will spread in case of leakage from nuclear power plants.
A U.S. nuclear expert said he feared the worst.
"It's more of a surrender," said David Lochbaum, a nuclear engineer who now heads the nuclear safety program for the Union of Concerned Scientists, an activist group. "It's not like you wait 10 days and the radiation goes away. In that 10 days things are going to get worse."
"It's basically a sign that there's nothing left to do but throw in the towel," Lochbaum said.
The U.S. authorized evacuations of Americans out of Japan on Wednesday, taking a tougher stand on the deepening nuclear crisis and warning U.S. citizens to defer all non-essential travel to any part of the country as unpredictable weather and wind conditions risked spreading radioactive contamination, reported the Associated Press.
There are six reactors at the plant. Units 1, 2 and 3, which were operating last week, shut down automatically when the quake hit. Since then, all three have been rocked by explosions.
Compounding the problems, on Tuesday a fire broke out in Unit 4's fuel storage pond, an area where used nuclear fuel is kept cool, causing radioactivity to be released into the atmosphere.
Units 4, 5 and 6 were shut at the time of the quake, but even offline reactors have nuclear fuel -- either inside the reactors or in storage ponds -- that need to be kept cool.
The Nuclear and Industrial Safety Agency estimated that 70 percent of the rods have been damaged at the No. 1 reactor.
Japan's national news agency, Kyodo, said that 33 percent of the fuel rods at the No. 2 reactor were damaged and that the cores of both reactors were believed to have partially melted.
"We don't know the nature of the damage," said Minoru Ohgoda, spokesman for the country's nuclear safety agency. "It could be either melting, or there might be some holes in them."
Meanwhile, the outer housing of the containment vessel at the No. 4 unit erupted in flames early Wednesday, said Hajimi Motujuku, a spokesman for the plant's operator, Tokyo Electric Power Co.
Japan's nuclear safety agency said fire and smoke could no longer be seen at Unit 4, but that it was unable to confirm that the blaze had been put out.
Japan ordered 140,000 people to seal themselves indoors after a series of explosions and fires at the plant. Hospitals had to be evacuated and thousands were screened for radiation exposure.
The grim search for survivors continues in areas devastated by Friday's magnitude 9 earthquake and subsequent tsunami as the mayor of a northeastern city says his town has been wiped out.
Mayor Kameyama Hiroshi told Kyodo News that 10,000 people remain missing in the city of Ishinomaki. Before the tsunami wiped out the coastal town, 164,000 people were living there.
Japan's National Police Agency says more than 12,000 people are reported missing or dead. The official death toll is above 4,000, but officials believe it will exceed 10,000.
The country's Defense Ministry tells Japan news agency NHK that more than 25,000 have been rescued and another 23,000 are still believed to be stranded on islets near the coast.
Days after Friday's twin disasters, millions of people were struggling along the coast with little food, water or heat, and already chilly temperatures dropped further as a cold front moved in. Up to 450,000 people are staying in temporary shelters, many sleeping on the floor of school gymnasiums.
In an extremely rare address to the nation Thursday, Emperor Akihito expressed condolences and urged Japan not to give up.
"It is important that each of us shares the difficult days that lie ahead," said Akihito, 77, a figure deeply respected across the country. "I pray that we will all take care of each other and overcome this tragedy."
"We have received so [much] support from across the globe," Noriyuki Shikata, the Japanese prime minister’s spokesman, told Fox News' Bret Baier on "Special Report." "U.S. forces in Japan have been actively helping the Japanese people."
According to the Pentagon, there are 50,000 active-duty U.S. forces in Japan. The 7th fleet has brought in 17,000 sailors and Marines onboard ships
The Associated Press and NewsCore contributed to this report.