WASHINGTON (AP) -- Earthquakes pose more of a threat to U.S. nuclear plants than officials believed earlier. That's according to an AP analysis of preliminary government data.

It's an issue that came into sharp focus last week, when the shaking from the strongest quake to hit Virginia in 117 years appeared to exceed what a plant near Richmond was designed to take.

At one plant, according to the data, the risk of a severe accident is 24 times higher than had been thought. The nation's nuclear regulator believes a quarter of America's reactors may need changes to make them safer.

The review was launched well before the Virginia quake and the Japanese nuclear disaster in March.

The nuclear industry says last week's quake proved reactors are sturdy. When the rumbling knocked out off-site power to the Virginia plant, the reactors shut down and cooled successfully, and the plant's four diesel generators turned on.  But a Virginia utility says the earthquake shifted about two dozen massive containers for spent nuclear fuel but that they remained safe and intact.

Dominion Virginia Power said that 25 containers moved between one and 4.5 inches. The containers are at the North Anna Power Station, about 13 miles from the epicenter of the 5.8-magnitude quake on Aug. 23.

The containers weigh about 115 tons when full. Dominion said they are still intact and kept the used fuel inside in a safe condition. The utility is evaluating whether to move the containers back into their original positions.

The tremor shut down both reactors at the plant after they lost power.

(Copyright 2011 by The Associated Press. All Rights Reserved.)

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