WASHINGTON — The first readings from American data-collection flights over the stricken Fukushima Daiichi nuclear plant in northeastern Japan show that the worst contamination has not spread beyond the 19-mile range of highest concern established by Japanese authorities.
But another day of frantic efforts to cool nuclear fuel in the stricken reactors and the plant’s spent-fuel pools resulted in little or no progress, according to United States government officials.
Japanese officials said they would continue those efforts, but were also racing to restore electric power to the site to get equipment going again, leaving open the question of why that effort did not begin days ago, at the first signs that the critical backup cooling systems for the reactors had failed.
The data was collected by the Aerial Measurement System, among the most sophisticated devices rushed to Japan by the Obama administration in an effort to help contain a nuclear crisis that a top American nuclear official said Thursday could go on for weeks. Strapped onto a plane and a helicopter that the United States flew over the site, with Japanese permission, the equipment took measurements that showed harmful radiation in the immediate vicinity of the plant — a much heavier dose than the trace levels of radioactive particles that make up the atmospheric plume covering a much wider area.
While the findings were reassuring in the short term, the United States declined to back away from its warning to Americans to stay at least 50 miles from the plant, setting up a far larger perimeter than the Japanese government had established. American officials did not release specific radiation readings.
American officials said their biggest worry was that a frenetic series of efforts by the Japanese military to get water into four of the plant’s six reactors — including water cannons and firefighting helicopters that dropped water but appeared to largely miss their targets — showed few signs of working.
“This is something that will likely take some time to work through, possibly weeks, as eventually you remove the majority of the heat from the reactors and then the spent fuel pool,” said Gregory Jaczko, the chairman of the United States Nuclear Regulatory Commission, briefing reporters at the White House.
The effort by the Japanese to hook electric power back up to the plant did not begin until Thursday and was likely to take several days to complete — and even then it was unclear how the cooling systems, in reactor buildings battered by a tsunami and then torn apart by hydrogen explosions, would help end the crisis.
“What you are seeing are desperate efforts — just throwing everything at it in hopes something will work,” said one American official with long nuclear experience who would not speak for attribution. “Right now this is more prayer than plan.”
After a day in which American and Japanese officials gave radically different assessments of the danger from the nuclear plant, the two governments tried on Thursday to join forces.
Experts met in Tokyo to compare notes. The United States, with Japanese permission, began to put the intelligence-collection aircraft over the site, in hopes of gaining a view for Washington as well as its allies in Tokyo that did not rely on the announcements of officials from the Tokyo Electric Power Company, which operates Fukushima Daiichi.
American officials say they suspect that the company has consistently underestimated the risk and moved too slowly to contain the damage.
Aircraft normally used to monitor North Korea’s nuclear weapons activities — a Global Hawk drone and U-2 spy planes — were flying missions over the reactor, trying to help the Japanese government map out its response to the last week’s 9.0-magnitude earthquake, the tsunami that followed and now the nuclear disaster.
President Obama made an unscheduled stop at the Japanese Embassy to sign a condolence book, writing, “My heart goes out to the people of Japan during this enormous tragedy.” He added, “Because of the strength and wisdom of its people, we know that Japan will recover, and indeed will emerge stronger than ever.”
Later he appeared in the Rose Garden at the White House to offer continued American support for the earthquake and tsunami victims, and technical help at the nuclear site.
But before the recovery can begin, the nuclear plant must be brought under control. So American officials were fixated on the temperature readings inside the three reactors that had been operating until the earthquake shut them down, and at the spent fuel pools, looking for any signs that their high levels of heat were going down. If they are uncovered and exposed to air, the fuel rods in those pools heat up and can burst into flame, spewing radioactive elements.
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