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updated 4/28/2011 6:06:15 PM ET 2011-04-28T22:06:15

The nation's top nuclear regulator cast doubt Thursday on whether reactors in the U.S. are prepared for the type of days-long power outage that struck a nuclear power plant in Japan.

The Nuclear Regulatory Commission has only required plants in this country to cope without power for four to eight hours. After that time, it assumes some electrical power will be restored.

NRC chairman Gregory Jaczko on Thursday questioned whether four to eight hours is enough time, even though it's unlikely a nuclear power plant would lose power from both the grid and emergency diesel generators as the Japan plant did. Requirements put in place after the September 11 terrorist attacks could lengthen plants' ability to withstand a blackout.

"Four hours doesn't seem to be a reasonable time to restore offsite power if you lost the diesels immediately," Jaczko said at a commission meeting at the NRC's Rockville, Md., headquarters. "In the event there is a station blackout that is externally driven, I'm not convinced that in that situation four hours" is enough time to restore offsite power.

A nuclear power plant in Alabama that lost power after violent thunderstorms and tornadoes on Wednesday will be down for days and possibly weeks, but the backup power systems worked as designed to prevent a partial meltdown like the disaster in Japan.

The Browns Ferry nuclear power plant, one of the biggest in the country, provides power to 2.6 million homes. It has three reactors that are similar in design to the Fukushima Dai-ichi reactors crippled by the earthquake and tsunami in Japan on March 11.

"The reactors will remain shut until we have restored the reliability of the transmission system," said Ray Golden, spokesman for the Tennessee Valley Authority, which owns and operates the 3,274-megawatt Alabama plant.

An Associated Press investigation last month examined the risk to the nation's 104 nuclear reactors to a complete loss of electrical power. In the U.S., such a "station blackout" has only happened once, at the Vogtle Electric Generating Plant in eastern Georgia in 1990. There, power was restored in 55 minutes.

The Japan disaster showed that it could be days before the electricity needed to pump water and keep the radioactive core from melting can be turned back on. In Japan's case, the plant operator found other ways to cool the cores without onsite or offsite power.

Of the 104 nuclear reactors in the U.S., 87 can cope for four hours in a blackout. Another 14 can cope for eight hours, and three can last for 16 hours.

As part of a review initiated after the Japan incident, the commission is looking at whether the blackout rule needs to be updated. At the time the rule was written in the 1980s, the commission assumed electrical power could be restored in 50 minutes to 2 hours. The NRC added an additional two hours to that time as a safety buffer.

Since then, plants have lost offsite power for longer periods of time. In every case, diesel generators kicked on and supplied electrical power, sometimes for days. There also are agreements with power grid operators that nuclear power plants get first priority as power is restored.

"We have a high expectation you will restore offsite power, restore emergency diesels or use alternate sources," said Pat Hiland, director of the NRC's reactor regulation engineering division.

But Jaczko pointed out that the blackout regulation is designed to deal with a situation where even diesel generators don't work, as in the case of the Fukushima Dai-ichi plant in Japan.

A top staffer told NRC commissioners Thursday that the Japan situation "has definitely improved" in recent weeks.

Bill Borchardt, NRC's executive director for operations, said that while there are still many unanswered questions about equipment failures and other problems at the facility, the situation is "certainly not as highly dynamic" as it was.

Overall, Japan is "making progress," he said. "They have a road map and certainly a good start toward long-term restoration."

Copyright 2011 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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