U.S. officials in Washington, meanwhile, warned that the Fukushima Dai-ichi plant in northeastern Japan might be on the verge of spewing more radioactive material because water was gone from a storage pool for spent nuclear fuel rods.
The troubles at several of the plant's reactors were set off when last week's earthquake and tsunami knocked out power and ruined backup generators needed for their cooling systems, adding a major nuclear crisis for Japan as it dealt with twin natural disasters that killed more than 10,000 people and left hundreds of thousands homeless.
A Japanese military CH-47 Chinook helicopter began dumping seawater on the damaged reactor of Unit 3 at the Fukushima complex at 9:48 a.m., said defense ministry spokeswoman Kazumi Toyama. The aircraft dumped at least four loads on the reactor, though much of the water appeared to be dispersed into the air.
At least a dozen more loads were planned in the 40 minutes that each crew can operate before switching to limit radiation exposure, the ministry said.
The dumping was intended both to help cool the reactor and to replenish water in a pool holding spent fuel rods, Toyama said. The plant's owner, Tokyo Electric Power Co., said the pool was nearly empty, which might cause the rods to overheat.
Emergency workers were forced to retreat from the plant Wednesday when radiation levels soared. They resumed work after radiation levels dropped, but much of the monitoring equipment in the plant is inoperable, complicating efforts to assess the situation.
Japanese officials raised hopes of easing the crisis, saying early today that they might be close to bringing power back to the plant and restoring the reactors' cooling systems.
The new power line would revive electric-powered pumps, allowing the company to control the rising temperatures and pressure that have led to at least partial meltdowns in three reactors. The company is also trying to repair its existing disabled power line.
The nuclear crisis has partly overshadowed the human tragedy caused by Friday's massive earthquake, one of the strongest recorded in history.
More than 4,300 people are officially listed as dead, but officials believe the toll will climb to well over 10,000. Police say more than 452,000 people are staying in temporary shelters such as school gymnasiums.