The Nuclear Regulatory Commission will conduct a "comprehensive review" of the safety of all U.S. nuclear plants following what U.S. officials are calling the dangerous and complicated situation at Japan's damaged Fukushima Dai-ichi reactors.
President Barack Obama took the rare step and called upon the independent commission to conduct the review.
"When we see a crisis like the one in Japan, we have a responsibility to learn from this event and to draw from those lessons to ensure the safety and security of our people," Obama said Thursday.
Obama's statement came as he tried to reassure a worried nation that "harmful levels" of radiation from the Japanese nuclear disaster are not expected to reach the U.S., even as other officials conceded it could take weeks to bring the crippled nuclear complex under control.
Meanwhile, the first evacuation flight of U.S. citizens left Japan, the State Department said.
"We've seen an earthquake and tsunami render an unimaginable toll of death and destruction on one of our closest friends and allies in the world," Obama said in brief remarks at the White House after a visit to the Japanese Embassy to offer his condolences.
There are 104 nuclear reactors in the United States, providing roughly 20 percent of the nation's electricity. "Nuclear energy is an important part of our own energy future," Obama said.
A leading industry group agreed with the review.
"A review of our nuclear plants is an appropriate step after an event of this scale, and we expect that the Nuclear Regulatory Commission will conduct its own assessment," said Marvin Fertel, president of the Nuclear Energy Institute. "The industry's highest priority is the safe operation of 104 reactors in 31 states and we will incorporate lessons learned from this accident..."
In the U.S., Customs and Border Protection said there had been reports of radiation being detected from some cargo arriving from Japan at several airports, including ones in Chicago, Dallas and Seattle.
Radiation had not been detected in passengers or luggage. And none of the reported incidents involved harmful amounts.
Homeland Security Secretary Janet Napolitano said the agency was screening passengers and cargo for "even a blip of radiation."
On Friday, Deputy Energy Secretary Daniel Poneman reiterated that no U.S. land _ the West Coast, Hawaii, Alaska and the American territories _ is in peril.
Poneman noted that Japan had moved Friday to upgrade the nuclear crisis from a Category 4 to Category 5, and said "the most pressing concern is the fate of the Japanese people as they struggle with this tragedy. Our most important concern is, we're doing everything we can."
"The first thing we've got to do is cool down the reactors and the spent fuel ponds," Poneman said on NBC's "Today" show. "Those are two significant areas of vulnerability."
Asked on ABC's "Good Morning America" how long that process should last, he said, "We're all trying to bring the assets to bear that will help bring the water, cool down the reactors, cool down the spent fuel and in the days and weeks ahead we hope that's going to take us in the right direction."
Obama said he knows that Americans are worried about potential risks from airborne radiation that could drift across the Pacific. "So I want to be very clear," he said. "We do not expect harmful levels of radiation to reach the United States, whether it's the West Coast, Hawaii, Alaska or U.S. territories."
Gregory Jaczko, chairman of the Nuclear Regulatory Commission, told reporters at a White House briefing it could be some time before the crisis is brought under control as crews work to cool spent-fuel rods and get the damaged Japanese reactors under control. The activity could continue for days and "possibly weeks," Jaczko said.
He said the U.S. recommendation that American troops and citizens stay 50 miles away from the nuclear complex was "a prudent and precautionary measure to take." But he also said "basic physics" suggested there was little risk to anyone in the United States or its Pacific territories.
Poneman told the briefing that a "very dangerous situation" remains in Japan. Information at the nuclear plant is "genuinely complex and genuinely confusing," he said.
As the officials spoke, Japanese emergency workers sought to regain control of the dangerously overheated nuclear complex, dousing it with water from police cannons, fire trucks and helicopters to cool nuclear fuel rods that were threatening to spray out more radiation.
The U.S. Energy Department said it had conducted two separate aerial tests to measure how much radioactive material had been deposited in Japan. Those data, Poneman said, were consistent with the recommendation for Americans to evacuate a 50-mile radius around the plant.
The U.S. officials declined to criticize the Japanese call for a smaller evacuation zone.
"We're analyzing the information, and we're sharing it with the Japanese," said Poneman. "The preliminary look has indicated that the measures that have been taken (by the Japanese) have been prudent ones. And we have no reason to question the assessment that has been made or the recommendation that has been made by the Japanese authorities."
At his visit to the Japanese Embassy Thursday, Obama signed a condolence book and said: "We feel a great urgency to provide assistance to those ... who are suffering."
In the book he wrote, "My heart goes out to the people of Japan during this enormous tragedy. Please know that America will always stand by one of its greatest allies during this time of need."
White House spokesman Jay Carney said the fact that Obama had taken the rare step of asking the NRC _ an independent regulatory agency that is not under the president's control _ to undertake a review of U.S. reactor safety in light of the Japanese disaster "only adds to the urgency of that mission."
Representatives of the nuclear energy industry said Thursday that operators of U.S. reactors already had begun taking steps to better prepare for an emergency in this country.
While it will take some time to understand the true dimensions of the nuclear disaster in Japan, "we will learn from them, we will get that operating experience, we will apply it and try to make our units even safer than they are today," said Anthony Pietrangelo, senior vice president of the Nuclear Energy Institute, a Washington-based industry lobbying group.