Xcel Energy says it needs to store far more nuclear waste at its Prairie Island facility to extend the plant's life by 20 years, as it has proposed in its long-range plan.

Keeping the plant's two units running until 2053 and 2054 is a crucial part of Xcel's blueprint for a carbon-free energy grid. Nuclear provides about a third of the energy on the Minneapolis-based company's upper Midwest system.

The waste also has caused controversy. The plant on the banks of the Mississippi River is next to the Prairie Island Indian Community.

On Wednesday, Xcel filed an application with the Minnesota Public Utilities Commission for permission to expand its storage capacity. Xcel also needs approval from the PUC and federal regulators to extend its license.

Xcel currently has 50 casks of solid spent fuel, which sit on three reinforced concrete pads. Xcel already has expansion plans under its current operating license to add another 15 casks, some of which are slightly smaller and are made with a new technology.

But to operate another 20 years, Xcel says it will need roughly 34 more casks, as well as a fourth and potentially fifth concrete pad to store waste.

In its filing, Xcel said the federal government is legally and contractually required to get rid of spent fuel but hasn't done so.

Because of the conundrum about spent fuel across the U.S., the issue of how to deal with it has been a source of debate. It is no different in Minnesota as lawmakers have grappled with whether to study emerging nuclear technology or lift a moratorium on new nuclear plants.

Last year, state lawmakers approved a deal between Xcel and the tribe that will increase annual $2.5 million payments to $10 million and create a $50,000-per-cask storage fee every year as a requirement for a 20-year extension.

Xcel also pays fees to the state for the waste at Prairie Island and the company's Monticello plant. That money is used to pay for clean energy initiatives.

"While we have 50 years of experience safely storing spent fuel at our Prairie Island and Monticello nuclear plants, we agree with the Prairie Island Indian Community that it is the federal government's legal obligation to provide a storage solution for nuclear fuel once it has been used," Xcel spokesperson Theo Keith said.

Five Prairie Island tribal council leaders including President Grant Johnson on Thursday said in a written statement that the community and Xcel "have agreed to work constructively regarding the approval process for Xcel's request" for an extension of the nuclear plant.

"We remain steadfast in our efforts to require the Federal government to fulfill its legal obligation under the Nuclear Waste Policy Act to remove the spent fuel currently sitting less than 700 yards from our Community," the statement says. "In addition, we continue to work on ensuring that our Community has alternative Reservation land and resources at a safe distance from the plant."

Last year, Allison Macfarlane, former chair of the U.S. Nuclear Regulatory Commission, and Rodney Ewing, former chair of the U.S. Nuclear Waste Technical Review Board, wrote in Scientific American that storing nuclear waste in pools or dry casks at reactor sites rather than repositories built deep underground is a "temporary solution" that is safe for decades but "not the millennia needed to isolate this radioactive material from the environment."

Edwin Lyman, director of nuclear power safety at the Union of Concerned Scientists, said there are risks to running aging nuclear plants. But when it comes to waste, he said above-ground storage in casks much is less of a danger than spent fuel pools on site. But he said the fuel waste is still vulnerable in some circumstances, like terrorist attacks.

"Obviously, the longer you operate, the more waste you're going to accumulate," Lyman said. "Waste has nowhere to go because the federal government reneged on its commitment. You're going to be getting larger and larger dry-cask storage facilities."

An equipment problem in October shut down the Prairie Island plant for several months. Only one of the plant's two units is back operating, according to federal nuclear regulators.