Exelon Nuclear officials say there were no injuries, no release of radiation and no equipment damage at the LaSalle Generating Station in LaSalle County's Brookfield Township.
The plant is owned by Chicago-based Exelon Corporation. It was scheduled to shut down early Monday for a refueling outage.
But the Illinois Emergency Management Agency says the reactor did not shut down properly.
Company officials say instruments showed three of the 185 control rods failed to insert fully into the reactor core and operators declared a "site area emergency" at 12:28 a.m. The emergency was over about four hours later.
The plant is about 65 miles south of Rockford.