Xcel's nuclear generating plant in Monticello automatically shut down late Thursday night, the second unplanned shutdown in two years.
A breaker failure at a substation prompted the shutdown and the plant switched to off-site power sources, Xcel officials announced early Friday morning.
Plant personnel worked Friday to fix the equipment and said they expect to restart the plant "in the near future." There were no injuries. No radiation was released. Officials emphasized there was no threat to the public.
In January 2007, the Monticello plant closed for a week after a 35,000-pound control box broke loose from its support beams and fell onto a large pipe carrying radioactive steam. The pipe did not rupture or leak.
In August, Xcel shut down one of its two Prairie Island reactors because of elevated hydrazine levels found in the turbine building. Levels of the anti-corrosive, ammonia-like chemical stayed elevated for about six hours before being corrected. The plant was back to full power the next day.
The problems at both nuclear plants concerned some Minnesota environmentalists and Indian tribes who worry about plant safety and excessive radioactive waste storage.
Concern has heightened since Xcel applied to extend operating licenses for both of its Minnesota-based nuclear plants.
In February, Xcel asked the Minnesota Public Utilities Commission (PUC) to approve a $135 million expansion in Monticello that would add 70 megawatts to the plant's current 600-megawatt capacity. The plant generates about 10 percent of Xcel's power. Last year, Xcel received a license from the Nuclear Regulatory Commission (NRC) to extend operations until 2030.
