Operators shut down one of Prairie Island nuclear plant's reactors just before midnight Tuesday as part of a regularly scheduled refueling outage.
Operators gradually reduced power in Unit 2 and the reactor was completely and safely shut down at 11:42 p.m., a statement from Xcel Energy said.
"All systems operated as designed," the statement says, adding that there was no release of radioactive material and no danger to the public or plant employees.
During the outage, operators will replace about one-third of the unit's fuel, as well as complete several maintenance tasks that cannot be done while the unit is operating.
There will be several hundred contractors, including electricians, welders and pipe-fitters, brought in to help with the maintenance work.
Each of the plant's two reactors is refueled about every 18 months. During that time, Xcel Energy buys power from other sources, and there is no disruption of service for Xcel customers.