Xcel Energy said its Prairie Island nuclear power plant is now back to full power despite an outage that dragged on for nearly two months longer than expected drew concern from state officials.

The Minnesota Department of Commerce was waiting on the company's two units that had been either fully or partly cold since October because of equipment problems and planned maintenance work. That left Xcel without a major source of carbon-free power and prompted the agency to seek an explanation for the delay March 8.

"The recent outage is unusual for Prairie Island, which has a 50-year track record of reliable operations," Xcel wrote in a filing Friday with the Public Utilities Commission.

On Monday morning, Unit 2 at Prairie Island was operating at 87% power, according a public report by the federal Nuclear Regulatory Commission. Xcel said later that afternoon the unit was back up to 100% power.

On Oct. 6, Xcel powered down Unit 2 for scheduled refueling and maintenance, an every-two-years process. Then Unit 1 shut down Oct. 19 after an issue between the turbine and the electric grid needed repairs.

At the time, Xcel said the refueling process at Unit 2 would have to wait until Unit 1 repairs were finished, lengthening the standard two-month timeline. But the utility said it expected both reactors to be up and running in January.

One nuclear safety engineer said the original equipment sounded like an "aging problem," but the safety systems were working as designed.

Unit 1 returned to service roughly on track, though another short outage occurred in early February after a high-pressure water leak created steam that shut down the facility for several days.

Unit 2 at Prairie Island remained largely down, however, and Xcel's second nuclear plant in Monticello even powered down briefly in a non-emergency incident during system testing.

The Commerce Department asked Xcel to provide a detailed explanation for why Prairie Island's Unit 2 wasn't operating at full steam yet and what it would take to make it operational as well for an estimated timeline to return to 100% power.

With less nuclear power, Xcel has had to turn to its other power sources or purchases on the open-energy market to make up for the lost electricity. That can have costs for customers depending on factors like the price of market purchases compared to running the plant.

Xcel's Friday filing said the equipment replacement "required that both units remain offline for an extended period." The company said it was rare to have both units offline at the same time, so Xcel did some of the maintenance it normally would have completed during a 2024 refueling of Unit 1.

"We were also able to conduct inspections and maintenance that cannot take place while both units are running," the Xcel response read. "This helps us plan for future maintenance needs. All this work will improve the performance of the plant and simplify future refueling outages."

The aging nuclear plants are a crucial part of Xcel's plans for a carbon-free energy grid by 2040. Nuclear makes up about 30% of the utility's power supply in the Upper Midwest.

Xcel wants to extend the life of its two units at Prairie Island so they can run until 2053 and 2054, 20 years beyond when their current licenses expire, something that requires state and federal approval. Xcel has also asked the Public Utilities Commission for permission to store more nuclear waste at the site in above-ground casks.