TOKYO - As emergency workers prepared to return to a tsunami-stricken Japanese nuclear power plant after retreating from a spike in radiation, the plant operator said Thursday it has almost completed a new power line that could restore electricity to the complex and solve the crisis that has threatened a meltdown.
Meanwhile, the chief of the U.S. Nuclear Regulatory Commission said Wednesday that the water stopping one reactor's fuel rods from getting hotter and ultimately melting down is gone from one of the plant's spent fuel pools.
Japanese officials denied NRC Chairman Gregory Jaczko's comments that he made before a congressional committee.
If Jaczko is correct, the outer shell of the rods could ignite with enough force to propel the radioactive fuel inside over a wide area.
At the plant, there two key areas of concern, CBS News' Harry Smith reports from Tokyo: pumping enough water into the reactors to keep the nuclear fuel rods cool to avoid a potential meltdown and limiting damage to the all-important containment vessels surrounding the reactor cores.
There is already a breach in reactor No. 2, and there are fears of another breach in reactor No. 3. Wednesday brought new images of the damage in reactor No. 4, the result of Tuesday's fire.
Jaczko did not say Wednesday 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.
Jaczko said officials believe radiation levels are extremely high, and that could affect workers' ability to stop temperatures from escalating.
"We believe that around the reactor site there are high levels of radiation," Jaczko said told members of Congress. "It would be very difficult for emergency workers to get near the reactors. The doses they could experience would potentially be lethal doses in a very short period of time."
Japan's nuclear safety agency and Tokyo Electric Power Co., the plant operator also known as Tepco, deny water is gone from the pool. Utility spokesman Hajime Motojuku said the "condition is stable" at Unit 4.
As for the power line to the plant, Tepco spokesman Naoki Tsunoda said early Thursday it is almost complete. Officials plan to try it "as soon as possible" but he could not say when.
The new line would revive electric-powered pumps, allowing the company to maintain a steady water supply to troubled reactors and spent fuel storage ponds, keeping them cool. The 180 emergency workers have been working in shifts to manually pump seawater into the reactors because last week's earthquake and tsunami disabled main and backup power for electric-powered cooling pumps.
Earlier, Energy Secretary Steven Chu told members of a congressional committee that "we don't really know in detail what's happening" because of conflicting information coming from the area.
Soon after Chu's testimony, the U.S. embassy in Tokyo urged all Americans within 50 miles of the power plant to evacuate the area or stay indoors. The Reuters news agency later reported that the British government advised Britons in Tokyo and in areas north of the capital to consider leaving the area.
The workers' pullback Wednesday cost precious time in the fight to prevent a nuclear meltdown, further escalating a crisis spawned by last week's devastating earthquake and tsunami that pulverized Japan's northeastern coast and likely killed more than 10,000 people.
Tepco officials said at a late night press conference Wednesday that four out of the five vehicles pumping seawater into the plant's reactors are operational. One pump vehicle each has been assigned to reactors No. 1 and No. 3. Reactor No. 2 has two pump vehicles. As of midnight Thursday, a new pump vehicle was being prepared to start working on reactor No. 4.
Meanwhile, the U.N.'s nuclear watchdog confirmed core damage in reactors Nos. 1, 2 and 3 in a news conference Wednesday. International Atomic Energy Agency chief Yukiya Amano called the situation in Japan "very serious" but said the plant's reactor vessels seem intact, Reuters reports. Amano said he hoped to leave Vienna for Japan Thursday and meet with senior officials.
It was unclear what happened in the nuclear plant's overheating reactors after late morning, when the workers stopped pumping in seawater trying to cool their fuel rods.
Ryohei Shiomi, of Japan's Nuclear and Industrial Safety Agency, told CBS News in a phone interview that the plant was never completely abandoned. He would not say how many workers remained inside the Fukushima Dai-ichi plant after the government suggested an evacuation was taking place.