By the CNN Wire Staff
Tokyo (CNN) -- Japanese authorities launched an urgent attempt Thursday to avert a nuclear disaster by air, but initial attempts to drop tons of water at the quake-ravaged Fukushima Daiichi plant appeared to do little to lower potentially perilous radiation levels.
Helicopters made four passes in about a 20-minute span Thursday morning, dropping 7.5 tons of seawater each time on the facility's No. 3 reactor in order to cool its overheated fuel pool. Experts believe that boiling steam rising from that pool, which contains at least partially exposed fuel rods, may be releasing radiation into the atmosphere.
But hours later, the Tokyo Electric Power Company -- which runs the plant -- told Japan's Kyodo News that the operation didn't appear to lower radiation levels. The report suggested levels actually rose to about 3,000 microsievert per hour. It takes a year for a person to be naturally exposed to that level of radiation.
Officials from the government and Tokyo Electric had said Thursday that cooling down the No. 3 reactor was top priority.
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Japanese Defense Minister Toshimi Kitazawa, in conjunction with Prime Minister Naoto Kan, said he decided early Thursday to address the crisis from the air and ground despite concerns about exposing workers to radiation.
"We could not delay the mission any further, therefore we decided to execute it," Kitazawa told reporters.
Authorities said they suspended the water-drop operation, so that they could get new readings on radiation levels. Kitazawa said that plans are in place to bring in 11 special water cannon trucks, along with one from Tokyo's police department, later Thursday to spray water at the No. 3 unit from the ground.
Since a 9.0-magnitude earthquake and tsunami devastated northeast Japan Friday, officials have been working to resolve cooling problems at four of Fukushima's six reactors.
On Wednesday, Gregory Jaczko, the head of the U.S. Nuclear Regulatory Commission told Congress that spent fuel rods in the No. 4 reactor had been exposed because there "is no water in the spent fuel pool," resulting in the emission of "extremely high" levels of radiation.
But Japanese authorities disputed Jaczko's assertion, citing information gathered from a helicopter flight over the plant.
"We have been able to confirm that there is water in the spent nuclear fuel pool," a Tokyo Electric official said Thursday. "But we do not know how much water."
The government itself further came under fire, with the release Thursday of a WikiLeaks cable attributed to the U.S. State Department that notes claims by Japanese legislator Taro Kono accusing Japan's Ministry of Economy, Trade and Industry of "covering up nuclear accidents, and obscuring the true costs and problems associated with the nuclear industry."
Also Thursday, engineers were planning to begin the process of restoring power to the stricken nuclear complex using power lines from outside. The plant lost power when the quake struck.
"This is one of the high-priority issues that we have to address," said an official with Japan's Nuclear and Industrial Safety Agency.
Once the power supply has been re-established, the cooling system will be operated using seawater, he said. But he warned that the process will not be immediate.
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"It will take time to restore the function of the main part of the facilities, because the pumps were contaminated by seawater and must be repaired before reuse," he said, adding that temporary pumps would be used initially.
A Japanese government spokesman, Noriyuki Shikata, warded off fears of an imminent meltdown, telling CNN Thursday, "We have not seen a major breach of containment" at any of the plant's troubled nuclear reactors.
A meltdown occurs when nuclear fuel rods cannot be cooled and the nuclear core melts. In the worst-case scenario, the fuel can spill out of the damaged containment unit and spread radioactivity through the air and water.
That, public health officials say, can cause both immediate and long-term health problems, including radiation poisoning and cancer.
Tests in Fukushima city, 80 kilometers (50 miles) away, found radiation measuring 12.5 microsieverts per hour -- well above the average reading of 0.04, but still well below that considered harmful to humans.
Small, and for now, harmless amounts of iodine -- a potential bi-product of a nuclear meltdown -- were found in the city's water.
Citing Fukushima Prefecture officials, Kyodo News reported Thursday that about 10,000 people had gotten screening tests for radiation exposure the previous day at evacuation centers and medical offices.
About 200,000 people living within a 20-kilometer (12-mile) radius of the plant have been evacuated; those living 20 to 30 kilometers from the site have been told to remain inside. Authorities also have banned flights over the area.
Koichi Shiga described the town of Minamisoma, 25 kilometers from the plant and site of the Hotel Iseya that he owns, as ghostly quiet with most all shops closed and empty streets. Electricity and water were still flowing, though evacuation efforts have been hampered by a paucity of gasoline.
"People have not evacuated, they are staying at home," Shiga told CNN. "There was a ration of 10 liters of gas, and I saw a long line of people."
Several countries, including the United States, have called for a broader range, urging their citizens in Japan to evacuate or at least stay indoors if they live within 80 kilometers of the Fukushima Daiichi plant.
One positive development is that winds have been blowing from the northwest, helping push emitted radiation off to sea.
Asked about the report of a high level of radioactivity near the plants -- which may be related to at least a partial meltdown in some reactors -- Shikata said, "We have not seen the level that is, for example, dangerous to human bodies beyond the very close vicinity of the reactors."
Still, at least 20 people have fallen ill due to possible radiation contamination (in addition 19 injured and two missing at the plant), the International Atomic Energy Agency reported. And experts and Japanese authorities fear that overheating and evaporation of water in spent fuel pools around the plant could lead to the release of further radiation.
The IAEA said the water temperature in spent fuel pools is typically kept below 25 degrees Celsius (77 degrees Fahrenheit). That requires a constant cooling source, which requires a constant power source -- both unavailable at the damaged plant.
"The concern about the spent fuel pools at Fukushima Daiichi is that sources of power to cool the pools may have been compromised," the agency, whose chief Yukiya Amano is heading to Japan, said.
On Tuesday, temperatures at the the fuel pools in Unit 4, 5 and 6 all registered far above the recommended levels: 84 degrees Celsius; 60.4 degrees Celsius and 58.5 degrees Celsius respectively, the U.N. nuclear watchdog agency said.
By the following day, it was reporting "no data" for Unit 4 and worrying trends for the other two:
Unit 5 had risen to 62.7 degrees Celsius and Unit 6 to 60 degrees Celsius.
The water in the fuel pool served to both cool the uranium fuel and shield it. But once the uranium fuel was no longer covered by water, the zirconium cladding that encases the fuel rods heated, generating hydrogen, said Robert Alvarez, senior scholar at the Institute for Policy Studies and a former official with the Department of Energy.
That caught fire, resulting in a situation that is "very, very serious," he told CNN.
He said the next step may involve the remaining 180 nuclear plant workers taking heroic acts.
"This is a situation where people may be called in to sacrifice their lives," Alvarez said. " It's very difficult for me to contemplate that but it's, it may have reached that point."