YAMAGATA, Japan — Emergency workers seemed to try everything they could think of Thursday to douse Japan's most dangerously overheated nuclear reactors: helicopters, heavy-duty fire trucks, even water cannons normally used to quell rioters. But they couldn't be sure any of it was easing the peril at the tsunami-ravaged facility.
Three reactors have had at least partial meltdowns at the Fukushima Dai-ichi nuclear plant, where wisps of white steam rose from the stricken units this morning. But Japanese and U.S. officials believe a greater danger exists in the pools used to store spent nuclear fuel: Fuel rods in one pool were believed to be at least partially exposed, if not dry, and others were in danger. Without water, the rods could heat up and spew radiation.
It could take days and "possibly weeks" to get the complex under control, U.S. Nuclear Regulatory Commission Chairman Gregory Jazcko said. He defended the U.S. decision to recommend a 50-mile evacuation zone for its citizens, a much stronger measure than Japan has taken.
CNN reported that radiation levels at the plant peaked today at 20 millisieverts per hour near where workers were trying to reestablish electrical power, "the highest registered so far," a TEPCO spokesman said. Radiation levels Thursday morning at the plant were nearly 3.8 millisieverts per hour — more than a typical resident of a developed country receives in a year.
The first readings from U.S. data-collection flights over the stricken nuclear plant show that the worst of the contamination has not spewed beyond the 19-mile range of highest concern established by Japanese authorities, the New York Times reported.
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 the nuclear crisis. 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.
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 pods, which were flown into Japan by the Air Force over the past day, are critical to efforts to identify hot-spots inside the three reactors and at the spent fuel pools.
The nuclear crisis triggered by last week's earthquake and tsunami has forced thousands to evacuate. The official death toll from the disasters stood at 5,692 as of this morning, with 9,522 missing, the national police agency said.
President Barack Obama appeared on television to assure Americans that officials do not expect harmful amounts of radiation to reach the United States or its territories.
Japanese and American assessments of the crisis have differed, with the plant's owner denying Jazcko's report Wednesday that Unit 4's spent fuel pool was dry and that anyone who gets close to the plant could face potentially lethal doses of radiation. But a Tokyo Electric Power Co. executive moved closer to the U.S. position Thursday.
"Considering the amount of radiation released in the area, the fuel rods are more likely to be exposed than to be covered," Yuichi Sato said.
Another utility official said Wednesday that the company has been unable to get information such as water levels and temperatures from any of the spent fuel pools in the four most troubled reactors.
Workers have been dumping seawater when possible to control temperatures at the plant since the quake and tsunami knocked out power to its cooling systems, but they tried even more desperate measures on Units 3 and 4.
Two Japanese military CH-47 Chinook helicopters began dumping seawater on Unit 3 on Thursday morning, defense ministry spokeswoman Kazumi Toyama said. The choppers doused the reactor with at least four loads of water in just the first 10 minutes, though television footage showed much of it appearing to disperse in the wind.
Chopper crews flew missions of about 40 minutes each to limit their radiation exposure, passing over the reactor with loads of about 2,000 gallons of water. Another 9,000 gallons of water were blasted from military trucks with high-pressure sprayers used to extinguish fires at plane crashes, though the vehicles had to stay safely back from areas deemed to have too much radiation.
Special police units with water cannons were also tried, but they could not reach the targets from safe distances and had to pull back, said Yasuhiro Hashimoto, a spokesman for Japan's nuclear safety agency.
Tokyo Electric Power said it believed workers were making headway in staving off a catastrophe both with the spraying and, especially, with efforts to complete an emergency power line to restart the plant's own electric cooling systems.
"This is a first step toward recovery," said Teruaki Kobayashi, a facilities management official at the power company. He said radiation levels "have somewhat stabilized at their lows" and that some of the spraying had reached its target, with one reactor emitting steam.
"We are doing all we can as we pray for the situation to improve," Kobayashi said.
Authorities planned to spray again today, and Kobayashi said: "Choices are limited. We just have to stick to what we can do most quickly and efficiently."
Work on connecting the new power line to the plant was expected to begin this morning and take 10 to 15 hours, said nuclear safety agency spokesman Minoru Ohgoda.
But the utility is not sure the cooling systems will still function. If they don't, electricity won't help.
Four of the plant's six reactors have seen fires, explosions, damage to the structures housing reactor cores, partial meltdowns or rising temperatures. Officials said temperatures Wednesday are rising even in the spent fuel pools of the other two reactors.
Unit 3's reactor uses a fuel that combines plutonium, better known as the fuel in nuclear weapons, and uranium. The other reactors do not use the same mixed oxide fuel, or MOX, but they also contain both uranium and plutonium because the latter is a byproduct of generating nuclear energy.
Plutonium is more toxic than uranium and is especially harmful to lungs and kidneys, so Unit 3 may be somewhat more dangerous.