Quake Brings Questions on Nuke Design

Friday, September 2, 2011 @ 04:09 PM gHale

The risk that an earthquake could cause a severe accident at a U.S. nuclear plant is greater than previously thought, according preliminary government data.

With that in mind, the Nuclear Regulatory Commission (NRC) said 25 percent of America’s reactors may need modifications to make them safer.

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The threat came into sharp focus when shaking from the largest earthquake to hit Virginia in 117 years exceeded what the North Anna nuclear power plant northwest of Richmond was built to sustain.

The two North Anna reactors are among 27 in the eastern and central U.S. that a preliminary NRC review has said may need upgrades. The upgrades could happen because those plants are more likely to suffer an earthquake larger than the one they originally based their design upon. Just how many nuclear power plants are more vulnerable won’t be available until all operators recalculate their own seismic risk based on new assessments by geologists, something the agency plans to request later this year.

The NRC and the industry said reactors are safe as they are, for now. The average risk to U.S. reactors of core damage from a quake remains low, at one accident every 500 years, according NRC data.

But emails obtained in a more than 11,000-page records request by The Associated Press show NRC experts worry plants needed stronger safeguards to account for the higher risk assessments.

The nuclear industry said last week’s quake proved reactors are robust. When the rumbling knocked out off-site power to the North Anna plant in Mineral, Va., the reactors shut down and cooled successfully, and the plant’s four locomotive-sized diesel generators turned on. The quake also shifted about two dozen spent fuel containers, but Dominion Virginia Power said all remain intact.

Still, based on the AP analysis of NRC data, the plant is 38 percent more likely to suffer core damage from a rare, massive earthquake than it appeared in an analysis 20 years ago.

That increased risk comes from an even bigger earthquake than the one last week. Richard Zuercher, a spokesman for Dominion, the plant operator, said the earlier estimate “remains sound because additional safety margin was built into the design when the station was built.”

Federal scientists update seismic assessments every five to six years to revise building codes for some structures. But no similar system is in place for all but two of the nation’s 104 reactors — even though improving earthquake science has revealed greater risks than previously realized.

The exception is Diablo Canyon in earthquake-prone California, which must review the risk of an earthquake routinely since 1985. The NRC does not require plants to re-examine their seismic risks to renew operating licenses for 20 years.

The NRC flagged the 27 plants for possible upgrades by calculating the likelihood of a severe accident based on 2008 hazard maps from the U.S. Geological Survey and comparing it to the seismic risk estimated in 1989 or 1994. Those data saw use the last time existing reactors evaluated their earthquake hazards.

The NRC identified the 27 reactors with the greatest risk increase but did not provide the risk numbers. The AP used the NRC’s data and methodology to calculate the risk increase for each reactor.

The Perry 1 reactor in Ohio tops the list with the steepest rise in the chance of core damage: 24 times as high as thought in 1989. The four other plants with the largest increases include River Bend 1 in Louisiana, up nine times; Dresden 2-3 in Illinois, eight times; Farley 1-2 in Alabama, seven times, and Wolf Creek 1 in Kansas, also seven times. The smallest increase was the 38 percent at North Anna.

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