"We have monitoring capability here in the U.S. that is extraordinarily sensitive. We can detect radiation that is like a hundred-thousandth of what you get from a regular X-ray, and we don't expect to see even that.
"For the stuff to travel, it has to be picked up by the wind," she said, "higher-level winds that have global distribution. And that's just not happening. This is a little like a campfire -- the smoke is all near the ground."
The worry remained acute, of course, along the Japanese coast, where even outside the 12-mile evacuation zone, people were urged to stay indoors if they lived within 18 miles of the Daiichi plant.
"We are all-out urging the Japanese to get more people back in there to do emergency operation there, that the next 24 to 48 hours are critical," an American official told ABC News. If cesium and other radioactive elements with long half-lives get into the air, "that could be deadly for decades," the official said.
But Jeff Masters, a former meteorologist at the National Weather Service who now works at Wunderground.com, ran a computer model and concluded that radiation would not get very far.
"Ground-level releases of radioactivity are typically not able to be transported long distances in significant quantities, since most of the material settles to the ground a few kilometers from the source," he wrote.
"Given that the radioactivity has to travel 3,000 miles to reach Anchorage, Alaska, and 5,000 miles to reach California, a very large amount of dilution will occur, along with potential loss due to rain-out.
"Any radiation at current levels of emission that might reach these places may not even be detectable," he said, "much less be a threat to human health."