The unit was shut on Friday, but a company spokesman at that time declined to discuss the cause of the outage or its expected length.

Electricity traders on Friday heard that the problem may be a sequencer and that the unit could return in a few days.

The 1,096 MW Prairie Island station, which entered service in 1974, is located in Red Wing in Goodhue County about 55 miles southeast of Minneapolis. There are two units at the station, the 551 MW Unit 1 and the 545 MW Unit 2.

Unit 2 continued to operate at full power.

An affiliate of Xcel, Nuclear Management Co, of Hudson, Wisconsin, operates Xcel's reactors in Minnesota.

One MW powers about 800 homes in Minneapolis.