Since Friday's earthquake and tsunami, three different reactors have seen explosions, and in reactor 4, there have been two different fires.
"There is no water in the spent fuel pool and we believe thatradiation levels are extremely high, which could possibly impact the ability to take corrective measures," said NRC Chairman Gregory Jaczko.
If Jaczko is correct about the depleted pool of water, it would mean there's nothing to stop the fuel rods from getting hotter and ultimately melting down. The outer shell of the rods could also ignite with enough force to propel the radioactive fuel inside over a wide area.
Jaczko did not say how the information was obtained, but the NRC and U.S. Department of Energy both have experts on site at the Fukushima Dai-ichi complex of six reactors. He said the spent fuel pool of the complex's Unit 4 reactor has lost water, but Japan's nuclear safety agency and Tokyo Electric Power Co., which operates the complex, deny the report that water is gone from the pool.
In testimony before a House Energy and Commerce subcommittee, Jaczko did not say how the information was obtained, but the NRC and U.S. Department of Energy both have experts in Japan.
Utility spokesman Hajime Motojuku said the "condition is stable" at Unit 4.
Jaczko said officials believe radiation levels are extremely high, and that could affect workers' ability to stop temperatures from escalating.
- The U.S. on Wednesday authorized the first evacuations of Americans out of Japan as the nuclear crisis worsens. U.S. officials also warned its citizens to defer all non-essential travel to any part of Japan as unpredictable weather and wind conditions risked spreading radioactive contamination.
- Nuclear plant operators trying to avoid complete reactor meltdowns said Thursday that they were close to finishing a new power line that could end Japan's crisis. The new power line would bring power back to the plant and revive the reactors' cooling systems, allowing the company to control the rising temperatures and pressure that have led to at least partial meltdowns in three reactors.
- Japanese military helicopters dumped loads of seawater onto a damaged nuclear reactor, trying to avoid full meltdowns.
- Workers had been dousing nuclear reactors with seawater to cool them when they were quickly pulled out of the plant due to spiking radiation levels. With the workers evacuated, the Japanese military attempted to drop water into the reactors from the air, but then aborted their mission. The government said it's a complicated process.
- It's believed that three different containment vessels are leaking dangerous radiation levels.
- There were reports of two different fires inside Unit 4, which still could be burning.
- The situation at the nuclear plant has led to widespread fear of radiation exposure. Some people were tested and decontaminated in Koriyama near the plant. A 12-mile radius around the plant remains under evacuation orders. Those in an 18-mile radius were told to stay inside and try to prevent outside air from entering their homes.
Even in Tokyo, nearly 175 miles from the plant, people were seen wearing masks and testing for radiation. There were new worries that changing winds could increase the risk for Tokyo.
"I think people are worried because we don't really understand the radioactivity," said one man in Japan. "You have to give faith in the scientists that study this kind of thing, and hopefully they're telling the truth."
Lack of communication has been a widespread complaint. One woman said she's hearing both positive and negative news, so she doesn't know what to believe.
International Atomic Energy Agency Chief Yukiya Amano plans to head to Japan on Thursday. Amano will meet with senior officials, saying he hopes to come back with firsthand information on the situation and address the issue of improving the flow of information to the IAEA.
Amano earlier urged the Japanese government to provide better information to the agency about the nuclear crisis.
More than 5,100 people are officially listed as dead, but officials believe the toll will climb to well over 10,000. About 8,600 were reported missing.
The Associated Press contributed to this story