Xcel Energy initially pinpointed the cause of a lengthy outage at its Prairie Island nuclear power plant as an equipment issue between the turbine and the electric grid.

But that didn't quite tell the full story: Xcel workers at the plant actually cut a bundle of power cables when drilling sideways underground in October, interrupting power to some of the Red Wing plant's equipment and causing one of the two reactors to shut down.

Xcel told federal nuclear regulators last month it did not use ground radar in an area that would have shown the cables' location. The company also said its excavation planning and oversight was inadequate, admitting to "procedural weaknesses and poor communications" between departments.

"This resulted in work progressing in the field without all controls in place that would be expected for work at a nuclear plant," an Xcel filing with the U.S. Nuclear Regulatory Commission read.

Dave Lochbaum, a nuclear safety engineer formerly with the Union of Concerned Scientists, said the plant's systems worked as intended when the reactor safely shut down. But he said cutting into power cables could have been a safety risk for the workers, which Xcel acknowledged as well.

"We know that any electrical contact can potentially be hazardous," Xcel spokesman Kevin Coss said. "None of the workers operating the drilling equipment were hurt when it impacted the cables. To prevent similar issues in the future, we have made changes to how we approach excavation projects as well as our oversight of workers coming to the plant for projects."

In April, Xcel told the Minnesota Department of Commerce it realized the damaged cables were aging and at risk of water damage after inspecting them. That would have likely caused both units at Prairie Island to shut down in the future, Xcel said.

In March, Commerce also asked why Xcel customers should pay for the Unit 1 outage. In regulatory filings, the company said customers should cover the cost of fuel and power it bought on the energy market in place of nuclear energy during the Unit 1 outage because, despite causing the problem, Xcel was operating prudently.

"Prudence does not mean perfection, and even prudent decisions and operations can sometimes result in undesirable outcomes," Xcel wrote.

Unit 1 at Prairie Island went down on Oct. 19. That caused a longer-than-expected outage at the plant's second unit, too, which had powered down two weeks earlier for scheduled refueling and maintenance.

Xcel expected both reactors to be up and running in January, but the company did not begin running both units at the plant again consistently until mid March.

The Xcel workers drilling underground were helping to replace a different power cable between an electric substation and the plant. The company replaced the damaged cables rather than repair them. Xcel also took time when the plant was offline to do a more thorough plant inspection than it could have otherwise, among other maintenance, which will avoid the need to shut down both units in the future for that work.

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.