As Xcel Energy and other U.S. electricity producers strive to reach 100% carbon-free power goals by midcentury, nuclear power is critical to their plans.

Wind and solar power will continue expanding, but by nature their output is variable. Nuclear power, though freighted with its own longstanding problems, is one of the only constant sources of carbon-free electricity.

Xcel has already announced plans to extend the life of its Monticello nuclear plant by at least 10 years, and Xcel CEO Ben Fowke said the utility would like to do the same at Prairie Island.

The company also is keeping a close eye on a new breed of smaller, advanced nuclear reactors.

Xcel aims for 100% carbon-free power by 2050 and an 80% carbon-emission cut by 2030 from 2005 levels. "When I look beyond 2030, that last 20% will take different technology and it could be the next generation of nuclear," Fowke said.

It also could include grid batteries to store solar and wind power, or technologies now just in an incubation stage, he said.

The answers will depend on cost.

Advanced reactors won't come with the crushing multibillion-dollar price tags of large nuclear plants being built today. But it's not clear how competitive they will be with other types of power generation.

"Can they be priced at the right point?" Fowke asked.

Coal-fired and nuclear power — the historic workhorses of Minnesota's electricity production — are "dispatchable," meaning they can be provided on demand.

Xcel plans to close all four of its Minnesota coal plants by 2030 as it adds more renewables. It will also build a big natural gas power plant in the mid-2020s. Gas-fired plants are dispatchable and emit just half the greenhouse gases as coal generators. But they don't solve the climate-change challenge.

Gas-fired power is in a golden age as fracking has freed up enormous and cheap supplies of the fuel. Low-cost gas is putting coal out of business, and it's threatening nuclear economics, too. Nine U.S. nuclear power plants have closed since 2013.

"The reason new nuclear has to be cheaper is because it has to compete with natural gas — unless we start taxing carbon or giving nuclear [tax] credits like we do with renewables," said Michael Corradini, professor emeritus of engineering physics at the University of Wisconsin.

Advanced reactors

Recent large-scale U.S. nuclear power projects have been a fiasco. Two reactors being built in Georgia are expected to cost $27 billion — $13 billion over budget — and aren't slated to start up until 2021 and 2022, five years late.

Now, several companies worldwide are drawing up plans for smaller, cheaper and innovative reactors.

The closest to deployment is Oregon-based NuScale Power, backed with at least $475 million in funding from engineering giant Fluor and more than $300 million from the U.S. Department of Energy. NuScale expects to have a commercial project running in Idaho later this decade.

The company's "small modular reactor" is water-cooled, like conventional reactors. But it would produce 50 megawatts of power, less than 10% of the capacity of Xcel's reactors. NuScale's reactor will be built on a factory line.

Nuclear-industry analysts said that should cut costs. Several modules could be located at one site.

NuScale's design is simpler and relies on "passive safety features," which is common for advanced reactors. Minimal electricity is required to run cooling systems if an accident occurs.

"The plant basically takes care of it itself," said Jacopo Buongiorno, a nuclear science and engineering professor at the Massachusetts Institute of Technology. "They have exceptionally robust safety practices."

Still, advanced nuclear reactors aren't foolproof. Passive safety systems give a nuclear operator more time before active systems must kick in, said David Lochbaum, former nuclear safety director of the Union of Concerned Scientists. "Passive safety systems are not indefinitely passive."

Most advanced reactors employ different coolants than water, including helium, liquid metal and molten salt. They run at higher temperatures — particularly molten salt — making them more thermally efficient and therefore more cost-effective than water reactors.

That extra heat could also be stored and used to generate revenue beyond electricity sales — say for industrial heating — or the production of hydrogen, which has several industrial applications.

Xcel is participating in an $11 million project, mostly funded by the federal energy department, looking at whether Prairie Island and other U.S. nuclear plants can cost effectively produce hydrogen.

'Wear and tear' on equipment

Advanced reactors would have another advantage: Their power levels can be flexed up and down more easily than today's nuclear fleet. "They can essentially ramp up as fast if not faster than a gas plant," said Tim O'Connor, Xcel's chief generation officer.

Such ramping ability is becoming more important with the surge of wind and solar power. Renewables are dispatched first on the electric grid because they have the lowest fuel cost — zero — in wholesale power markets.

So, coal and nuclear plants must increasingly vary output.

Xcel has shown it can "flex" its three Minnesota nuclear reactors, gradually ramping down output as much as 25% to respond to renewable power flows.

But by lowering output, nuclear plants lose revenue — hence the attraction of other sources of income like hydrogen production.

Flexible nuclear generation "is not outside the design of the reactors," O'Connor said. Nuclear plants in France have been doing it for years.

Still, Lochbaum said that "flexible operation can increase the wear and tear on equipment."

Last month, the U.S. Nuclear Regulatory Commission (NRC) found an issue at Prairie Island connected to flexible operation.

The finding was of "very low safety significance." Still, the deficiency was "more than minor because if left uncorrected it would have the potential to lead to a more significant safety concern," according to an NRC document.

Essentially, Xcel's procedures didn't fully account for potential ramping-related wear on the control rods at one of Prairie Island's reactors, the NRC concluded. (Control rods moderate the rate of a nuclear reaction, allowing power to be lowered or stopped.) The company promptly fixed the issue.

Waste-storage plans

Xcel's nuclear plants at Monticello and Prairie Island near Red Wing are critical for their low-carbon energy plans. With nuclear and renewables, Xcel expects to be generating 71% carbon-free power by 2030. (A downward revision from its original goal of 80%.)

Xcel is seeking to extend Monticello's life — its current federal license expires in 2030 — by at least 10 years.

Xcel's projections for the lowest-cost, long-term power would entail also extending Prairie Island's life, the company said in filings with the Minnesota Public Utilities Commission (PUC). Licenses for Prairie Island's two reactors expire in 2033 and 2034.

But the nation's lack of long-term storage for spent fuel particularly looms over any extension at Prairie Island. That's because it's next-door to the Prairie Island Indian Community — the closest U.S. community to a nuclear plant and its on-site waste casks.

The federal government's decades-old plan to build a permanent U.S. nuclear waste repository in Nevada is comatose. However, two plans for "interim" waste-storage facilities in New Mexico and Texas are progressing through the NRC's regulatory process.

Spent fuel currently stored at Prairie Island and other U.S. power plants could be transported to those storage sites if they get approval.

In 2019, informal sessions were held at the Prairie Island plant on transporting spent nuclear fuel to such interim storage sites.

The sessions were organized by the Nuclear Energy Institute, a trade group; Xcel was a major sponsor and participant.

Shelley Buck, president of the Prairie Island Indian Community, attended and addressed the gathering.

Buck said in a statement that for the tribe, it's critical that the federal government solve the waste issue. She said the tribe will address the re-licensing of Prairie Island when and if it becomes an issue.

"Like Xcel, we continue to have concerns with the long-term storage of nuclear waste on Prairie Island," she said.

Fowke said he expects that Xcel will eventually look to extend Prairie Island's licenses. "But we haven't crossed that bridge. We certainly recognize the issues the community has."

Mike Hughlett • 612-673-7003