** FILE ** In this May 26, 2011, file photo, the Nuclear Regulatory Commission (NRC) headquarters is seen in Rockville, Md. The risk of an earthquake causing a severe accident at a nuclear power plant is up to 24 times greater than previously believed, according to an AP analysis of preliminary government data, and the nation’s nuclear regulator believes that a quarter of reactors may need modifications to make them safer. (AP Photo/Pablo Martinez Monsivais, File)