Xcel Energy Inc. has shut down one of its two nuclear reactors in Red Wing, Minn., for refueling and to replace an original generator and transformer in a monthlong $68 million project that will employ about 1,000 specialized contractors, the utility said Monday.

The project at the Prairie Island nuclear power plant Unit 2 is part of more than $1 billion in investment Xcel is making in its two Minnesota nuclear power plants to extend their operating lives beyond the original 40-year licenses. In 2013, Xcel replaced Unit 2's steam generator at a cost of more than $280 million.

"The upgrades we will make while the unit is offline are investments in the future of Prairie Island, the Red Wing area and the Upper Midwest as a whole," Scott Sharp, director of site operations, said in a statement.

Sharp said the electricity from nuclear power plants is the backbone of Xcel Energy's strategy to generate 63 percent carbon-free electricity in the region by 2030.

Major upgrades to reactors usually occur during refueling outages, which happen every 18 months. About one-third of the fuel rods in the pressurized water reactor will be replaced during this refueling, Xcel said. Unit 1 is still operating.

The Minneapolis-based utility said Unit 2's main electrical generator, which has reached its 40-year operational life, will be replaced at a cost of $55 million. The new turbine weighing 1 million pounds was manufactured in Japan and shipped through the Panama Canal and up the Mississippi River, Xcel said. The plant is located on the banks of the river.

In a second upgrade, contractors also will replace a step-up transformer at a cost of $13 million, Xcel said. It converts the generator's output from 22,000 volts to 345,000 volts for the power grid.