(Recasts, updates with company comment)
NEW YORK, Sept 8 Exelon Corp (EXC.N) reduced the 867-megawatt Unit 2 at the Dresden nuclear power station in Illinois over the weekend for maintenance, a spokesman for the plant said on Monday.
He could not say when the unit would return to full power due to competitive reasons.
On Sunday, with the unit at 67 percent power, Exelon told the U.S. Nuclear Regulatory Commission the isolation condenser was inoperable following surveillance testing. In the report, the company said it would have to fix the system within 14 days or shut the unit.
The spokesman said workers were performing maintenance to adjust the cooling flow valve stroke time to retest the system.
The 1,734 MW Dresden station is located in Morris in Grundy County about 60 miles southeast of Chicago. The plant's two 867 MW Units 2 and 3 entered service in 1970 and 1971.
Unit 3 ramped up to 99 percent Monday from 96 percent Friday.
Electricity traders noted Unit 3 would likely shut in early November for a refueling outage.
One MW powers about 800 homes in Illinois.
In 2004, the NRC renewed the plant's original 40-year operating licenses for both units for another 20 years until 2029 and 2031.
Exelon, of Chicago, owns and operates more than 38,000 MW of generating capacity, markets energy commodities, and transmits and distributes electricity to 5.4 million customers and natural gas to 480,000 customers in Illinois and Pennsylvania. (Reporting by Scott DiSavino; Editing by David Gregorio)