Engineers still struggled today with Japan's crippled Fukushima Daiichi nuclear plant, and engineers warned that time may be running out before melting fuel rods spread dangerous radiation along the Japanese coastline.
The radiation could be deadly in the areas surrounding the plant. The Japanese government has ordered people to stay at least 12 miles away; the U.S. Nuclear Regulatory Commission cautiously urged Americans in Japan to stay at least 50 miles away.
"This is very, very radioactive material," said Kenneth Bergeron, a physicist who has worked at Sandia National Laboratories. "If there is core on the floor and containment penetration, there will be significant public health consequences."
But could the danger spread to American shores? Nuclear engineers and meteorologists said the U.S., including Alaska and Hawaii, is safe.
"These releases from the plant, because they're not elevated, because they're not getting up high in the atmosphere, they won't travel very far," said Kathryn Higley, director of the department of nuclear engineering at Oregon State University. "There are so many factors in our favor. Rain will knock it down. There are 5,000 miles of ocean between us and Japan. It will be diluted, it will mix with sea spray, long before it gets remotely close to us."
President Obama, in his first statement on the nuclear crisis, also said Americans should not worry.
"I want to be very clear," he said at the White House Thursday. "We do not expect harmful levels of radiation to reach the West Coast, Hawaii, Alaska or U.S. territories in the Pacific."
Nevertheless, the Environmental Protection Agency said that it was adding several monitors to its RadNet system of radiation detectors. The small monitors are scattered around the country and send their readings to EPA computers. The EPA said it was setting up two new ones in Hawaii, two in Guam, and three in Alaska.
The agency said it acted "in an abundance of caution," to gather new data. "As the Nuclear Regulatory Commission has said, we do not expect to see radiation at harmful levels reaching the U.S. from damaged Japanese nuclear power plants."
The high-altitude winds over Japan are primarily out of the west -- good news for Japan in a worst-case scenario, if there were a large release of radiation into the air.
And if there is a worst case, with radioactive particles carried long-distance by upper-level winds?
In that case, "we will get some fallout on the West Coast 2-3 days after its release in Japan," said Edward Morse, a nuclear engineer at the University of California, Berkeley, in an e-mail to ABC News. "The levels will not be threatening to life and health but they will be observable."
At Texas A&M University, Prof. Ken Bowman and graduate student Cameron Homeyer put together a computer projection showing that if any radiation from the March 14 explosion at the plant reached altitudes of 4-5 miles (6 km), it might pass over northern Alaska on Friday. But Bowman said his model was not designed to show the intensity of any plume getting that far.