Exelon Generation says one of two reactors at the company's Braidwood Nuclear Generating Station is back at full power after a refueling outage.
The reactor was off line for three weeks while technical workers replaced one-third of the unit's fuel and conduct inspections and maintenance.
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Unit 2 continued to generate electricity while the work was conducted.
The Will County 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 -- the most in any U.S. state.