Exelon Generation officials say one of two reactors at the company's Braidwood Nuclear Generating Station is shut down for scheduled refueling and maintenance.
The Unit 1 outage began just after midnight on Monday and will continue for about six weeks. Unit 2 will continue to generate electricity during the Unit 1 outage.
Officials say the operation involves 2,000 technical workers who'll replace one-third of the unit's fuel and conduct inspections and maintenance. The reactors are refueled every 18 months.
The plant is about 60 miles southwest of Chicago.
Exelon recently applied to the Nuclear Regulatory Commission for a 20-year license extension at Braidwood, which began commercial operation in 1988.
Exelon owns six nuclear plants in Illinois, with a total of 11 reactors -- more than any other U.S. state.