ZAO, Japan —
Chopper crews were flying missions of about 40 minutes each to limit their radiation exposure, passing over the reactor with loads of about 7,500 liters (about 2,000 gallons) of water.
The dousing is aimed at cooling the Unit 3 reactor, as well as replenishing water in that unit's cooling pool, where used fuel rods are stored, Toyama said. The plant's owner, Tokyo Electric Power Co., said earlier that pool was nearly empty, which would cause the rods to overheat and emit even more radiation.
Defense Minister Toshifumi Kitazawa told reporters that emergency workers had no choice but to try the water dumps before it was too late.
U.S. officials, meanwhile, said Unit 4 also was seriously at risk.
U.S. Nuclear Regulatory Commission Chairman Gregory Jaczko said at a congressional hearing in Washington that all the water was gone from that unit's spent fuel pool. Jaczko said anyone who gets close to the plant could face potentially lethal doses of radiation.
"We believe radiation levels are extremely high," he said.
Tokyo Electric executives said Thursday that they believed the rods in that pool were covered with water, but an official with Japan's nuclear safety agency later expressed skepticism about that and moved closer to the U.S. position.
"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.
Chief Cabinet Secretary Yukio Edano said that along with the helicopter water drops, special police units would use water cannons — normally used to quell rioters — to spray water onto the Unit 3 storage pool. The high-pressure water cannons will allow emergency workers to stay farther away.
Military vehicles designed to extinguish fires at plane crashes will also be used, said Gen. Ryoichi Oriki.