The federal government says nuclear plants throughout the central and eastern U.S. must be reassessed within four years to determine how well they might withstand earthquakes.
The Nuclear Regulatory Commission this week released an updated seismic risk model that plant operators must use to recalculate risks.
Officials say the model has been in development for four years but took on a greater urgency after last year's nuclear disaster in Japan following an earthquake and tsunami.
The NRC says plants in Illinois and Iowa are in areas where new geological data suggest earthquakes could be more frequent than previously believed.
Exelon Energy's Dresden Nuclear Power Plant southwest of Chicago and NextEra Energy's Duane Arnold Energy Center outside Cedar Rapids, Iowa, also use the same type nuclear reactor that failed in Japan.