Last October, Xcel used a piece of equipment owned by Westinghouse to inspect the fuel assemblies in a pool of spent nuclear fuel at the Prairie Island plant.
When the job was done, workers packed the 13-foot-long canister into a metal box and put it on a semi bound for a Westinghouse plant in Pennsylvania.
When it was unloaded there, workers measured radiation levels eight-times the permitted level.
Scott Northard, plant manager at Prairie Island, says the contents apparently shifted during the trip, and a microscopic radioactive particle ended up close to the surface of the container.
"Our survey of the container and package before it was placed in the box should have been more detailed and should have identified this discrete radioactive particle and the potential for exceeding the limit," Northard said.
Xcel notified the Nuclear Regulatory Commission.
After an investigation, just last week the NRC issued a so-called preliminary yellow finding.
That's third most serious violation in a scale of four.
In written communications with Xcel Energy, the NRC said workers who packed the box weren't qualified and had not been properly trained.
The agency said it was only a matter of chance that the public wasn't exposed to radiation, partly because the particle ended up at the bottom of the container, not on the side. There it would have been closer to people in nearby vehicles.
Edwin Lyman, senior scientist at the Union of Concerned Scientists, a nuclear safety watchdog group, agrees.
"It was fortuitous in that if the particle had shifted to one of the other walls, it could well have been an issue for someone on road driving next to this vehicle," Lyman said.
Prairie Island ships this type of equipment about once every four or five years, and the company has set up new procedures, including training, to prevent another episode like this.
The company is applying for a 20-year license extension for the two units at Prairie Island. Xcel also wants to expand production at both its Prairie Island and Monticello plants.
Later this week, representatives from the Nuclear Regulatory Commission will visit the Prairie Island plant as they consider granting the company's request.