The first readings from American data-collection flights over the stricken Fukushima Daiichi nuclear plant in Japan show that the worst of the contamination has not spewed beyond the 18-mile range of highest concern established by Japanese authorities, but there is also no indication that another day of frantic efforts to cool nuclear fuel in the reactors and spent fuel pools has yielded any progress, according United States government officials.
The data was collected in the first use of 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 the top American nuclear official said Thursday could go on for “possibly weeks.”
The data show ground-level fallout of harmful radioactive pollution in the immediate vicinity of the stricken plant — a different standard than the trace amounts of radioactive particles in an atmospheric plume now projected to cover a much broader 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, a far larger perimeter than the Japanese government has established.
President Obama affirmed the warning, saying in the Rose Garden on Thursday afternoon, that the decision was based on “a careful scientific evaluation” of the “substantial risk” to those near the plant. He also repeated that there was no expectation that the radioactive plume emitted by the plant would bring harmful levels of radiation to any part of the United States, including its territories in the Pacific.
But he also called for a “comprehensive review” of the country’s nuclear plants.
In interviews, American officials said their biggest worry about the Japanese plant was that a frenetic series of efforts by the Japanese military to get water into the four reactors there — including water cannons and fire-fighting helicopters that dumped water but appeared to largely miss their targets — showed few signs of working.
Another effort by the Japanese, to hook electric power back up to the plant, only began on 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 the tsunami and then torn apart by hydrogen explosions, would work, if at all.
“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 had radically different assessments of the danger of what is spewing from the plants, the two governments attempted 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. Officials say they suspect that company has consistently underestimated the risk and moved too slowing 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 quake, the tsunami 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.”
But before the recovery can begin, the nuclear plants must be brought under control. And 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 they were decreasing. So far they saw none, but on the Web site of the International Atomic Energy Agency, the United Nations nuclear watchdog, it was clear that there were no readings at all from some critical areas. Part of the American effort, by satellites and aircraft, is to identify the hot-spots, something the Japanese have not been able to do in some cases.
Critical to that effort are the “pods” flown into Japan by the Air Force over the past day. Made for quick assessments of radiation emergencies, the Aerial Measuring System is an instrument pod that fits on a helicopter or fixed-wing aircraft to sample air and survey the land below. The information is used to produce colored maps of radiation exposure and contamination.
The sensors on the instrument pod are good at mapping radioactive isotopes, like Cesium-137, which has been detected around the stricken Japanese complex and has a half-life of 30 years. Its radiation can alter cellular function, leading to an increased risk of cancer.
Cesium-137 mixes easily with water and is chemically similar to potassium. It thus mimics how potassium gets metabolized in the body and can enter through many foods, including milk. Cesium gets widely distributed in the body and its concentrations are said to be higher in muscle tissues and lower in bones.
On Wednesday when the American Embassy in Tokyo, on advice from the Nuclear Regulatory Commission, told Americans to evacuate a radius of “approximately 50 miles” from the Fukushima plant, the recommendation was based on a specific calculation of risk of radioactive fallout in the affected area.
In a statement, the commission said the advice grew out of its assessment that projected radiation doses within the evacuation zone might exceed 1 rem to the body or 5 rem to the thyroid gland. It is extremely sensitive to Iodine-131 — another of the deadly radioactive byproducts of nuclear fuel that causes thyroid cancer.
A rem is a standard measure of radiation dose.
David E. Sanger reported from Washington and William J. Broad from New York.