(Reuters) - Duke Energy Corp's (DUK.N) 846-megawatt Unit 1 at the Oconee nuclear power station in South Carolina shut by early Monday, the U.S. Nuclear Regulatory Commission said in a report.

Electricity traders guessed the unit shut for a planned seven-week refueling and maintenance outage expected to start in mid October.

The unit last shut for refueling from about April 12-June 3, 2008. It is on an 18-month refueling cycle.

The 2,538 MW Oconee station, which entered service in 1973-74, is located in Seneca in Oconee County about 145 miles northwest of Columbia, South Carolina. There are three 846 MW Units 1-3 at the station.

The NRC renewed the plant's original 40-year operating licenses in 2000 for another 20 years until 2033-2034.

All of the other units continued to operate at full power.

One MW powers about 700 homes in the Carolinas.

Duke, of Charlotte, North Carolina, owns and operates about 39,000 MW of generating capacity in North America and Latin America, markets energy commodities, and transmits and distributes electricity to about 4 million U.S. customers in the Carolinas and the Midwest. (Reporting by Scott DiSavino; Editing by John Picinich)