TOKYO — The chairman of the U.S. Nuclear Regulatory Commission gave a far bleaker appraisal on Wednesday of the threat posed by Japan's nuclear crisis than the Japanese government had offered. He said American officials believed that the damage to at least one crippled reactor was much more serious than Tokyo had acknowledged, and he advised Americans to stay much farther from the plant than the perimeter established by Japanese authorities.
The announcement opened a new and ominous chapter in the five-day effort by Japanese engineers to bring the six side-by-side reactors under control after their cooling systems were knocked out by an earthquake and a tsunami last Friday.
The congressional testimony by Gregory Jaczko, the chairman of the commission, was the first time the Obama administration had given its own assessment of the condition of the plant, apparently mixing information it had received from Japan with data it had collected independently.
Jaczko's most startling assertion was that there was now little or no water in the pool storing spent nuclear fuel at the No. 4 reactor of the Fukushima Dai-ichi Nuclear Power Station, leaving fuel rods stored there exposed and bleeding radiation into the atmosphere.
As a result, he said, "We believe that radiation levels are extremely high, which could possibly impact the ability to take corrective measures."
His statement was quickly but not definitively rebutted by officials of Tokyo Electric Power, the plant's operator, and Japan's nuclear regulatory agency.
In a later bit of better news, the company said today that a nearly completed new power line could restore cooling systems at the nuclear plant, raising some hope of easing the crisis.
But on Wednesday, the conditions at the plant appeared to worsen, with white smoke pouring from the complex and a surge in radiation levels forcing workers to retreat for hours from their struggle to cool the overheating reactors.
The new line would revive electric-powered pumps, allowing the company to maintain a steady water supply to troubled reactors and spent fuel storage ponds, keeping them cool. The company is also trying to repair its existing disabled power line.
Wednesday's pullback by workers who have been pumping seawater into the reactors cost valuable time in the fight to prevent a nuclear meltdown, a nightmare scenario after Friday's horrific earthquake and tsunami. The disasters pulverized Japan's northeastern coast and are feared to have killed more than 10,000 people.
Japan's emperor, in an unprecedented made-for-TV speech, called on the country to work together. "It is important that each of us shares the difficult days that lie ahead," said Akihito, 77. "I pray that we will all take care of each other and overcome this tragedy."
He also expressed his worries over the nuclear crisis, saying: "With the help of those involved I hope things will not get worse."
But officials are also taking increasing criticism for poor communication about efforts at the complex. There has been growing unease at the U.N.'s International Atomic Energy Agency's 35 board member nations, who have complained that information coming from Japan on the rapidly evolving nuclear disaster is too slow and vague.
IAEA head Yukiya Amano spoke of a "very serious" situation and said he would leave for Tokyo within a day.
He said it was "difficult to say" if events were out of control, but added, "I will certainly have contact with those people who are working there who tackled the accident, and I will be able to have firsthand information."
Jaczko's testimony came as the U.S. Embassy in Tokyo, on advice from the NRC, told Americans to evacuate a radius of "approximately 50 miles" from the Fukushima plant.
The advice to Americans in Japan represents a graver assessment of the risk in the immediate vicinity of Dai-ichi than the warnings made by the Japanese themselves, who have told everyone within about 12 miles to evacuate. That assessment seems bound to embarrass, if not anger, Japanese officials, suggesting they have miscalculated the danger or deliberately played down the risks.
It was not immediately clear how many people live within the zone around the plant that U.S. officials believed should be evacuated. But the zone gets far closer to the city of Sendai, with its population of 1 million, which took the brunt of the earthquake last week.
A U.N. forecast of the possible movement of the radioactive plume coming from crippled Japanese reactors shows it churning across the Pacific and touching the Aleutian Islands today before hitting Southern California late Friday.
Health and nuclear experts emphasize that radiation in the plume will be diluted as it travels and, at worst, would have extremely minor health consequences in the United States, even if hints of it are ultimately detectable. In a similar way, radiation from the Chernobyl disaster in 1986 spread around the globe and reached the West Coast of the United States in 10 days, its levels measurable but minuscule.
The projection, by the Comprehensive Test Ban Treaty Organization, an arm of the United Nations in Vienna, gives no information about actual radiation levels but only shows how a radioactive plume would probably move and disperse.
The nuclear crisis has partly overshadowed the human tragedy caused by Friday's 9.0 magnitude earthquake, one of the strongest recorded in history.
Millions of Japanese have been with little food and water in heavy snow and rain since Friday. In some towns, long lines of cars waited outside the few open gas stations, with others lined up at rice-vending machines.
More than 4,300 people are officially listed as dead, but officials believe the toll will climb to well over 10,000. Police say more than 452,000 people are staying in temporary shelters such as school gymnasiums.
Wednesday's radiation spike was believed to have come from the complex's Unit 3. But officials also admitted that they were far from sure what was going on at the four most troubled reactors, including Unit 3, in part because high radiation levels made it difficult to get very close.
While white smoke was seen rising Wednesday above Unit 3, officials could not ascertain the source. They said it could be spewing from the reactor's spent fuel pool — cooling tanks for used nuclear rods — or may have been from damage to the reactor's containment vessel, the protective shell of thick concrete.
Masahisa Otsuki, an official with Tokyo Electric Power Co., which owns the complex, said officials are most concerned about the spent fuel pools, which are not encased in protective shells.
"We haven't been able to get any of the latest data at any spent fuel pools. We don't have the latest water levels, temperatures, none of the latest information for any of the four reactors," he said.
Authorities have doubled the number of workers at the nuclear plant to 100 and by late Wednesday, helicopters were dumping water on the stricken reactors. Government officials also said they'd asked special police units to bring in water cannons — normally used to quell rioters — to spray water onto the spent fuel storage pool for the complex's Unit 4.
The cannons are thought to be strong enough to allow emergency workers to remain a safe distance from the complex while still able to get water into the pool, said Minoru Ogoda of the Japanese nuclear safety agency.
Given the reported radiation levels, John Price, an Australian-based nuclear safety expert, said he saw few risks for the public so far. But he 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."
Chief Cabinet Secretary Yukio Edano said the government expects to ask the U.S. military for help, though he did not elaborate. He said the government is still considering offers of help from other countries.
There are six reactors at the plant. Three have been rocked by explosions. Compounding the problems, on Tuesday a fire broke out in a storage pond, an area where used nuclear fuel is kept cool, causing radioactivity to be released into the atmosphere.
Information from the New York Times and Associated Press was used in this report.