With power cable laid, will cooling system work?

Japanese authorities have informed the International Atomic Energy Agency that engineers were able to lay an external grid power line cable to reactor 2 at a tsunami-ravaged nuclear facility.

But now experts are asking that if the power comes on, will that be enough to solve the problem? On the CBS Evening News, physicist James Acton, with the Carnegie Endowment for International Peace, said it would be a "pivotal moment" it the cooling systems work.

"If the cooling systems in the reactors and fuel ponds are basically sound and then the power comes on, then we might look at that moment as the beginning of the end of this crisis," Acton told CBS Evening News anchor Katie Couric. "If, however, the cooling systems are damaged and the power comes on, they might not function correctly and this crisis could continue."

Authorities plan to reconnect power to unit 2 once the spraying of water on the unit 3 reactor building is completed.

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Emergency workers seemed to try everything they could think of Thursday to douse one of Japan's dangerously overheated nuclear reactors: helicopters, heavy-duty fire trucks, even water cannons normally used to quell rioters. But they couldn't be sure any of it was easing the peril at the tsunami-ravaged facility.

Three reactors have had at least partial meltdowns, but an even greater danger has emerged. Japanese and U.S. concerns were increasingly focusing on the pools used to store spent nuclear fuel: Some of the pools are dry or nearly empty and the rods could heat up and spew radiation.

In Washington, the head of the Nuclear Regulatory Commission said it could take weeks to get the reactors under control.

It could take days and "possibly weeks" to get the complex under control, U.S. Nuclear Regulatory Commission Chairman Gregory Jazcko said. He defended the U.S. decision to recommend a 50-mile evacuation zone for its citizens, a much stronger measure than Japan has taken.

A senior official with the U.N.'s nuclear safety agency said there had been "no significant worsening" at the Fukushima Dai-ichi nuclear plant but that the situation remained "very serious." Graham Andrew told reporters in Vienna that nuclear fuel rods in two reactors were only about half covered with water, and in a third they were also not completely submerged.

If the fuel is not fully covered, rising temperatures and pressure will increase the chances of complete meltdowns that would release much larger amounts of radioactive material than the failing plant has emitted so far.

Adm. Robert Willard, commander, U.S. Pacific Command is cautiously optimistic about progress on cooling the reactors.

"Based on the update I received at about 2 a.m. this morning I am cautiously optimistic that we're progressing in that regard based on what we've seen in the restoration of power and efforts that they've made to add water both from the outside and the top of these reactors, every effort is being made," said Willard.

Bill Whitaker reports it's clear that so much is riding on the workers who remain at the plant. Now more than 300, it's believed they can only work in shifts of minutes at time because of high levels of radiation. They wear cumbersome protective gear and must rely on flashlights in a hot, almost pitch-black plant.

And the conditions are starting to take their toll. Already roughly 20 workers have been exposed to radiation since the earthquake struck.

One former Fukushima Dai-ichi worker who wished to remain anonymous told Japanese television he's gravely concerned for those he was forced to leave behind.

"I am still in fear far from the plant after evacuation," he said. "But thinking about the staff who is still in and around the plant now, they should be in tremendous fear, I assume."

Japan's Prime Minister told the workers - many of whom volunteered to stay - that "retreat is unthinkable," leaving their families to wait - and worry.

President Barack Obama appeared on television to assure Americans that officials do not expect harmful amounts of radiation to reach the U.S. or its territories. He also said the U.S. was offering Japan any help it could provide, and said he was asking for comprehensive review of U.S. nuclear plant safety.

Japanese and American assessments of the crisis have differed, with the plant's owner denying Jazcko's report Wednesday that Unit 4's spent fuel pool was dry and that anyone who gets close to the plant could face potentially lethal doses of radiation. But a Tokyo Electric Power Co. executive moved closer to the U.S. position Thursday.

"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.

Two Japanese military CH-47 Chinook helicopters began dumping seawater on Unit 3 on Thursday morning, defense ministry spokeswoman Kazumi Toyama said. The choppers doused the reactor with at least four loads of water in just the first 10 minutes, though television footage showed much of it appearing to disperse in the wind.

Chopper crews flew missions of about 40 minutes each to limit their radiation exposure, passing over the reactor with loads of about 2,000 gallons of water. Another 9,000 gallons of water were blasted from military trucks with high-pressure sprayers used to extinguish fires at plane crashes, though the vehicles had to stay safely back from areas deemed to have too much radiation.

Special police units with water cannons were also tried, but they could not reach the targets from safe distances and had to pull back, said Yasuhiro Hashimoto, a spokesman for Japan's nuclear safety agency.

Unit 3's reactor uses a fuel that combines plutonium, better known as an ingredient in nuclear weapons, and reprocessed uranium. The presence of this mixed oxide fuel, or MOX, means potentially that two very harmful radioactive products could be released into the environment.

Tokyo Electric Power said it believed workers were making headway in staving off a catastrophe both with the spraying and, especially, with efforts to complete an emergency power line to restart the plant's own electric cooling systems.

"This is a first step toward recovery," said Teruaki Kobayashi, a facilities management official at the power company. He said radiation levels "have somewhat stabilized at their lows" and that some of the spraying had reached its target, with one reactor emitting steam.

"We are doing all we can as we pray for the situation to improve," Kobayashi said. Authorities planned to spray again Friday, and Kobayashi said: "Choices are limited. We just have to stick to what we can do most quickly and efficiently."