Two Japanese self-defence helicopters dumped loads of seawater over a crippled nuclear reactor on Thursday morning, in an effort to avoid a full meltdown as operators raced to restore the plant's cooling system.

Each of the CH-47 Chinook helicopters was able to dump 7.5-ton loads of water on the Fukushima Daiichi complex's Unit 3 reactor in northern Japan. More than a dozen similar air drops were expected after the operation began. However, much of the water appeared to be dispersed in the wind in the first four rounds of drops.

A similar plan was abandoned Wednesday after radiation levels were deemed too high to proceed safely.

Water cannon trucks borrowed from riot police were also on hand to spray down the reactors.

At a news conference Thursday morning, Japanese officials said that contrary to some reports, the Fukushima plant's Unit 4 reactor still had water left in its spent fuel cooling pool. However, it was not known how much water was in the pool, a spokesperson with the Tokyo Electrc Power Co. said, adding that the information had been obtained through aerial surveys from Wednesday.

Dire appraisal from U.S. nuclear commission

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International Atomic Energy Agency Director General Yukiya Amano attends a news conference at the United Nations headquarters in Vienna on Tuesday. (Herwig Pramme/Reuters)

Japanese officials had earlier rejected a dire appraisal of the Fukushima Daiichi nuclear facility by the head of the U.S. Nuclear Regulatory Commission, who said water was absent from a reactor's spent fuel pool, raising the potential of a meltdown.

"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," NRC chairman Gregory Jaczko said.

Without any water in the spent fuel pool, the fuel rods would eventually overheat and melt down, causing the outer rods to burn and spread radioactive fuel widely.

Japan's nuclear safety agency and Tokyo Electric Power Co., the operator of the Fukushima complex, denied that water was gone from the spent fuel pool. Company spokesman Hajime Motojuku said "the condition is stable," at the Unit 4 reactor, one of six at the plant.

Jaczko made his statement Wednesday before a U.S. House energy and commerce subcommittee in Washington, D.C., and did not provide the source for his claim, but the NRC and the U.S. Department of Energy have experts in Japan.

Canada increases evacuation radius

The spent fuel rods are located on the top floor of each reactor's building, without the greater protection of thick steel walls that surround the reactor cores. As well, the containment building for Unit 4 has been punctured.

The International Atomic Energy Agency in Vienna said late Wednesday it is seeking further information about the water levels, temperature and condition of all spent fuel pool facilities at the plant. 

"The concern about the spent fuel pools at Fukushima Daiichi is that sources of power to cool the pools may have been compromised," it said.

Spent fuel is normally kept in a pool at a temperature of 25 C, the IAEA said, which requires a power source for constant cooling. It said it had no data for the Unit 4 spent fuel pool on Wednesday, but that the temperature had reached 84 C on Tuesday, and that temperatures had risen to 62.7 C and 60.0 C for units 5 and 6. respectively.

Canada's Department of Foreign Affairs and International Trade advised its citizens to evacuate from within 80 kilometres of the site.

The U.S. Embassy in Tokyo also advised Americans on Wednesday to evacuate an area 80 kilometres surrounding the troubled nuclear facility, while the Japanese government has ordered a 20-kilometre evacuation zone and told those within 30 kilometres to stay indoors.

The U.S. State Department has said it is bringing in chartered aicraft to Tokyo to help Americans exit Japan, according to Reuters.

Power line could reduce threat

A new power line could restore electricity to cooling systems at the Daiichi plant, its operator said early Thursday, a development that would reduce the threat of a meltdown.

Construction of the line is nearly complete, said TEPCO spokesman Naoki Tsunoda, and officials hoped to try it "as soon as possible."

Workers were forced out of the plant Wednesday after another fire broke out and radiation levels surged, but they returned in the evening after levels subsided.

Japanese officials have tried a number of methods to cool the reactors, but the plan to douse one of the stricken reactors from the air with water from helicopters was aborted because radiation levels were too high.

Wednesday's radiation spike was believed to have come from Unit 3, where workers are struggling with a fuel storage pond believed to be leaking radiation, as well as possible damage to the steel containment vessel, which would allow radiation to escape.

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.

As the crisis continued, Yukiya Amano, head of the IAEA, described the situation in Japan as "very serious."

Amano, who has urged the Japanese government to provide the agency with better information, said he hoped to fly to Japan on Thursday.

Earlier Wednesday, Japan's Emperor Akihito urged people to pull together in a rare televised appearance.

The 77-year-old emperor expressed his condolences to those affected by the quake and said it is important that people take care of each other in the difficult days ahead.

He also said he was deeply concerned about the unpredictable situation at the Fukushima Daiichi complex.

"It is my deepest hope that the concerned people can prevent the situation from worsening further," he said.

The workers at the forefront of the fight — a core team of 50 to 70 people who work in protective gear — are being regularly rotated in and out of the danger zone to minimize their radiation exposure.

CBC's  Curt Petrovich said the company won't release the names of the workers who have been at the site.

"They're the ones pouring the seawater onto the reactors to prevent the meltdown," Petrovich said, noting that the anonymous workers have become "folk heroes" to many people in Japan.

Japanese health officials said Wednesday they were raising the maximum allowable radiation dose for nuclear workers  to 250 millisieverts from 100 millisieverts. It described the move as "unavoidable" because of the circumstances, The Associated Press reported.

Meanwhile, the governor of Fukushima prefecture, Yuhei Sato, said the anxiety and anger felt by people living around the plant has reached a "boiling point."

Fukushima Daiichi nuclear complex

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Of the six Daiichi reactors, Units 1, 2 and 3, which were operating last week, shut down automatically when the quake hit.

The quake and the ensuing tsunami knocked out the backup diesel generators needed to keep nuclear fuel cool, setting off the crisis that frantic workers have been struggling to contain.

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.

Source: The Associated Press

Sato criticized preparations for an evacuation if conditions worsen and said centres already housing people moved from nearby the plant do not have enough hot meals and basic necessities.

Since the quake and wave hit, authorities have been struggling to avert an environmental catastrophe at the nuclear complex.

Steam and pressure build up in the reactors as workers try to cool the fuel rods, leading to controlled pressure releases through vents — as well as uncontrolled explosions.

Radiation worries

Meanwhile, the Nuclear and Industrial Safety Agency estimated that 70 per cent of the rods have been damaged at the Unit 1 reactor.

Japan's national news agency, Kyodo, said 33 per cent of the fuel rods at Unit 2 were damaged, and 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."

John Price, an Australia-based nuclear safety expert, said he was surprised by how little information the Japanese were sharing.

"We don't know even the fundamentals of what's happening, what's wrong, what isn't working. We're all guessing," he said. "I would have thought they would put on a panel of experts every two hours."

Given the radiation levels, he saw few health risks for the general public so far, though he was concerned for the workers, who he said were almost certainly working in full bodysuits and breathing through respirators.

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A medical worker screens a photographer for possible radiation exposure at a centre in northern Japan on Wednesday. (Yuriko Nakao/Reuters)

The government has ordered some 140,000 people in the vicinity of the plant to seal themselves inside their homes.

A little radiation was also detected in Tokyo, triggering panic buying of food and water.

In Tokyo's Shibuya shopping district, the normally bustling streets were unusually quiet Wednesday, CBC's Peter Armstrong said.

"Most people are staying home, giant TV screens over the main square are turned off and most of the huge electric and neon signs have been shut down to conserve energy," Armstrong said, noting that rolling blackouts are being used to try to deal with power shortages.

With files from The Associated Press