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Japanese television showed what appeared to be steam rising from reactor No. 3 at the Fukushima Daiichi plant about 10 a.m. Wednesday.
The announcement marked a new and ominous chapter in the five-day long effort by Japanese engineers to bring four side-by-side reactors under control after their cooling systems were knocked out by an earthquake and tsunami last Friday. It also suggested a serious split between Washington and Tokyo, after American officials concluded that the Japanese warnings were insufficient, and that, deliberately or not, they had understated the potential threat of what is taking place inside the nuclear facility.
Gregory Jaczko, the chairman of the commission, said in Congressional testimony that the commission believed that all the water in the spent fuel pool at the No. 4 reactor of the Fukushima Daiichi Nuclear Power Station had boiled dry, leaving fuel rods stored there exposed and bleeding radiation. As a result, he said, "We believe that radiation levels are extremely high, which could possibly impact the ability to take corrective measures."
If his analysis is accurate and Japanese workers have been unable to keep the spent fuel at that inoperative reactor properly cooled - it needs to remain covered with water at all times - radiation levels could make it difficult not only to fix the problem at reactor No. 4, but to keep workers at the Daiichi complex from servicing any of the other problem reactors at the plant.
Mr. Jaczko (the name is pronounced YAZZ-koe) said radiation levels may make it impossible to continue what he called the "backup backup" cooling functions that have so far helped check the fuel melting at the other reactors. Those efforts consist of using fire hoses to dump water on overheated fuel and then letting the radioactive steam vent into the atmosphere.
Those emergency measures, implemented by a small squad of workers and firemen, are the main steps Japan is taking at Daiichi to forestall a full blown fuel meltdown that would lead to much higher releases of radioactive material.
Mr. Jaczko's testimony came as the American Embassy in Tokyo, on advice from the Nuclear Regulatory Commission, told Americans to evacuate a radius of "approximately 50 miles" from the Fukushima plant.
The advice represents a graver assessment of the risk in the immediate vicinity of Daiichi than the warnings made by the Japanese themselves, who have told everyone within 20 kilometers, about 12 miles, to evacuate, and those between 20 and 30 kilometers to take shelter.
Mr. Jaczko's testimony, the most extended comments by a senior American official on Japan's nuclear disaster, described what amounts to an agonizing choice for Japanese authorities: Send a small number of workers into an increasingly radioactive area in a last-ditch effort to cover the spent fuel, and the fuel in other reactors, - with water, or do more to protect the workers but risk letting the pools of water protecting the fuel boil away - and thus risk a broader meltdown.
The Japanese authorities have never been as specific as Mr. Jascko was in his testimony about the situation at reactor No. 4, where they have been battling fires for more than 24 hours. It is possible the authorities there disagree with Mr. Jascko's conclusion about the exposure of the spent fuel, or that they have chosen not to discuss the matter for fear of panicking people.
Experts say workers at the plant probably could not approach a fuel pool that was dry, because radiation levels would be so high. In a normally operating pool, the water provides not only cooling but also shields workers from gamma radiation. A plan to dump water into the pool, and others like it, from helicopters was suspended because the crews would be flying right into a radioactive plume.
Mr. Jaczko's analysis suggests that a potentially dangerous chain of events could unfold, as workers trying to cool the adjacent reactors at the facility could also be exposed to intolerable levels of radiation. If they, too, had to withdraw, the problem could worsen, as reactor cores were go uncooled and spent fuel pools run dry.
Earlier in the day, Japanese authorities announced a different escalation of the crisis at Daiichi when they said that a second reactor unit at the plant may have suffered damage to its primary containment structure and appeared to be releasing radioactive steam.
The break, at the No. 3 reactor unit, worsened the already perilous conditions at the plant, a day after officials said the containment vessel in the No. 2 reactor had also cracked.
The possibility of high radiation levels above the plant prompted the Japanese military to put off a highly unusual plan to dump water from helicopters - a tactic normally used to combat forest fires - to lower temperatures in a pool containing spent fuel rods that was dangerously overheating at the No. 4 reactor. The operation would have meant flying a helicopter into the steam rising from the plant.
But in one of a series of rapid and at times confusing pronouncements on the crisis, the authorities insisted that damage to the containment vessel at the No. 3 reactor - the main focus of concern earlier on Wednesday - was unlikely to be severe.
Yukio Edano, the chief cabinet secretary, said the possibility that the No. 3 reactor had "suffered severe damage to its containment vessel is low." Earlier he said only that the vessel might have been damaged; columns of steam were seen rising from it in live television coverage.
The reactor's operator, Tokyo Electric Power Company, said it had been able to double the number of people battling the crisis at the plant to 100 from 50, but that was before the clouds of radioactive steam began billowing from the plant. On Tuesday, 750 workers were evacuated, leaving a skeleton crew of 50 struggling to reduce temperatures in the damaged facility. An increasing proportion of the people at the plant are soldiers, but the exact number is not known.
The Pentagon said Wednesday that American military forces in Japan were not allowed within 50 miles of the plant and that some flight crews who might take part in relief missions were being given potassium iodide to protect against the effects of radiation. Tokyo Electric said Wednesday that some of those at the plant had taken cover for 45 minutes on site, and left water pumps running at reactors Nos. 1, 2 and 3. There was no suspension of cooling operations, said Kazuo Yamanaka, an official at Tokyo Electric. The vessel that possibly ruptured on Wednesday had been seen as the last fully intact line of defense against large-scale releases of radioactive material from the stricken reactor, but it was not clear how serious the possible breach might be.
The possible rupture, five days after a devastating earthquake and tsunami crippled the plant, followed a series of explosions and other problems there that have resulted in the world's worst nuclear crisis since the Chernobyl accident in 1986.
The head of the Vienna-based International Atomic Energy Agency, Yukiya Amano, who is Japanese, said he would leave for Japan as soon as possible to assess the situation.
The revised official assessment of the severity of the damage at the No. 3 reactor may have been intended to reduce some concerns about the containment vessel, which encloses the core, but the implications of overheating in the fuel rod pool at No. 4 seemed potentially dire.
There are six reactors at the plant, all of which have pools holding spent fuel rods at the top level of the reactor building. Reactors 4, 5 and 6 were out of service when the earthquake and tsunami struck, and there were concerns about the pools at 5 and 6 as well, and possibly those at the other reactors.
At a hearing in Washington on Wednesday held by two subcommittee of the House Energy and Commerce Committee, Energy Secretary Steven Chu said, "We think there is a partial meltdown" at the plant.
"We are trying to monitor it very closely," he said. "We hear conflicting reports about exactly what is happening in the several reactors now at risk. I would not want to speculate about what is happening."
He said that his agency had sent 39 people to the American Embassy and to United States consulates in Japan "with the skills, expertise and equipment to help assess, survey and monitor areas." The department has also shipped survey equipment that can measure radiation levels from the air, he said.
The developments were the latest in Japan's swirling tragedy since the quake and tsunami struck the country with unbridled ferocity last Friday. Emperor Akihito made his first ever televised appearance on Wednesday to tell the nation he was "deeply worried" about the nuclear crisis.
International alarm about the nuclear crisis appeared to be growing, as several nations urged their citizens in Japan to head to safer areas in the south or leave the country. Prior advisories had largely been limited to simply avoiding nonessential travel. Germany urged its citizens to move to areas farther away from the stricken nuclear plant.
Earlier Wednesday morning, Tokyo Electric reported that a fire was burning at the No. 4 reactor building, just hours after officials said flames that erupted Tuesday had been doused.
A government official at Japan's nuclear regulatory agency soon after said that flames and smoke were no longer visible, but he cautioned that it was unclear if the fire had died out. He also was not clear if it was a new fire or if the fire Tuesday had never gone out.
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