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WASHINGTON (Reuters) - A historic earthquake that rocked the U.S. East Coast last week underscores the need to analyze new seismic risks for nuclear plants and invest in upgraded defenses where needed, the chairman of the U.S. nuclear regulator said on Thursday.
Since 1991, the Nuclear Regulatory Commission has been working toward requiring plants to more regularly model quake risks using new seismic data -- a long and detailed process that has not uncovered imminent risks but may eventually result in some changes to power plants.
"It's something we're working through, and I certainly expect that in some way, shape, or form, that (some) plants will need to make modifications as a result," Chairman Gregory Jaczko told Reuters.
Last week, a 5.8-magnitude quake shut down Dominion Resources Inc's North Anna nuclear power plant, which is only about 10 miles from the earthquake's epicenter near Mineral, Virginia.
It's too soon to say what kind of changes the utility may need to make for the NRC to allow it to reopen, Jaczko said in his first interview since the quake.
The regulator is combing the plant for damage after the quake's ground force appears to have exceeded the plant's design rating -- the first time that has ever happened in the United States for an operating reactor.
"While in some regards the earthquake may have been larger than what we anticipated, at this point we haven't seen any indications of equipment that has shown a performance problem as a result of that," Jaczko said.
The ability of the 104 aging U.S. reactors to withstand natural disasters has been under scrutiny since March, when an earthquake and tsunami wrecked Japan's Fukushima Daiichi plant, causing the world's worst nuclear accident in 25 years.
Plants in the United States must be built to withstand the worst quakes on record for their sites.
"This is something that we need to really look at and update periodically because we get better information. We think we have predicted what are the worst earthquakes that can happen in an area, but sometimes our knowledge is limited," Jaczko said.
The next stage of earthquake assessment will probably take a couple of years, the NRC said in a news release on Thursday.
The agency will then determine whether changes are needed. Those backfits could cost hundreds of millions of dollars for the industry, including operators such as Exelon, Entergy and PG&E.
For the North Anna plant, it's unclear whether the NRC will require an upgrade to its "design basis," Jaczko said, and how long the plant may be required to stay closed.
"Because this really is the first time we've done this with an operating plant, we are working through exactly what they will need to demonstrate. The timeframe will be dictated really more by what, if any, adverse conditions we find at the site, and what, if anything, you would need to do to address those," he said.
Much will depend on the analysis of the different rattles, jolts and vibrations felt during the quake. Preliminary data shows that at some of the higher frequencies, the quake's forces may have exceeded the plant's design rating, while other frequencies did not, Jaczko said.
"Those (higher frequencies) are the kinds of things that tend to affect instrumentation and some electrical equipment, and don't necessarily tend to affect the big structures," he explained.
The quake shocked the East Coast, which is not used to feeling the ground shake. But Jaczko said he was not surprised when the NRC learned the quake exceeded the plant's design basis.
"You can never in this job believe that something won't happen. You always have to believe that however unlikely, that certain things can happen, and you have to be prepared for that," he said.
Editing by Phil Berlowitz