Radiation readings taken 20 minutes after self-defence force helicopters doused the plant's No 3 reactor remained unchanged, Tokyo Electric Power [Tepco] said, according to Kyodo news agcncy.
The use of Japanese military helicopters to drop seawater onto the plant's reactor marked the opening of a new front in the battle to avert nuclear meltdown.
TV footage showed the CH-47 Chinook helicopters dousing the No 3 reactor in an attempt to cool an overheating pool for spent fuel rods and prevent it from releasing dangerous radioactive steam.
Two helicopters, flying at less than 300 feet, dumped four loads of water on the reactor, although the footage suggested a significant quantity was missing the target.
The ministry said it planned to release at least 12 more loads in the 40 minutes that each crew can remain in the area before experiencing limited radiation exposure.
The aim of the operation is twofold: to cool the reactor and replenish a pool containing spent fuel rods. Although Tepco has been unable to take precise measurements, the pool is thought to be almost empty of water, raising the risk that the fuel rods will overheat and melt.
Earlier, Gregory Jackzo, chairman of the US nuclear regulatory commission told a congressional hearing in Washington that the storage pool at another reactor had lost all of its water and was in danger of spewing more radioactive material.
"We are afraid that the water level at [the No 4 reactor] is the lowest," said Hikaru Kuroda, a Tepco official. But he added, "Because we cannot get near it, the only way to monitor the situation is visually from far away."
At lunchtime on Thursday the police stood ready to spray the No 3 reactor from 11 water cannon trucks, as the focus of the crisis shifted from overheating reactors to the potentially more dangerous predicament of the storage pools.
The roofs of the No 3 and No 4 reactors were blown away by hydrogen explosions earlier this week, depriving them of a last line of defence against potentially dangerous radiation leaks.
In the worst-case scenario, overheating fuel rods could heat up to the point where they begin to melt and release high levels of radioactivity.
Tepco said it was attempting to open a temporary power line to the plant, 150 miles north of Tokyo, which would allow it to pump water directly into the storage pools and reactor cores.
Japan's nuclear safety agency said it hoped the power supply would be partially operational on Thursday afternoon.
"Once we establish the temporary power supply we will be able to pump seawater into the reactors," a Tepco spokesman said.
"We believe the operation will help cool down the fuel pools," the defence minister, Toshimi Kitazawa told reporters. "Ideally we want to repeat the exercise as many times as possible, but we also have to consider the health risks to our troops."
Each helicopter is capable of carrying 7.5 tonnes of water at a time, but the pools each hold 2,000 tonnes, an expert told public broadcaster NHK.
But he added: "It will be possible as long as the rods are fully submerged. That means the storage pool would need to be about a third full. But the dousing has to be done repeatedly."
About 70,000 people have been evacuated from a 20-kilomtre radius around Fukushima Daiichi, and another 140,000 living within a 20 to 30 kilometre radius ordered to stay indoors.
Japan's top spokesman, Yukio Edano, said there was no need to widen the exclusion zone, but signs are emerging that other countries are taking a more cautious approach.
The worsening situation prompted the US to ask citizens living within an 80-kilomtre radius to evacuate.
''We are recommending, as a precaution, that American citizens who live within 50 miles (80 kilometres) of the Fukushima nuclear power plant evacuate the area or to take shelter indoors if safe evacuation is not practical,'' the US embassy said in a statement.
The British embassy has since issued similar advice, and asked citizens living in Tokyo and northern Japan to consider leaving.
Elevated - though not hazardous - levels of radiation have been detected well outside the Fukushima evacuation zone. In Ibaraki prefecture to the south, officials said radiation levels were about 300 times normal levels by late Wednesday morning.
It would take three years of constant exposure to these higher levels to raise a person's risk of cancer.
http://www.guardian.co.uk/world/2011/mar/17/japan-nuclear-crisis-fukushima-seawater-reactors
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