Minnesota's clean energy plan needs nuclear power, but aging plants have their own risks

When Xcel Energy's Monticello nuclear plant closed for scheduled maintenance this spring, the robots went to work.

One crawled along the bottom of the plant's thick steel reactor, probing for defects in metal bombarded by 52 years of radiation. The chance of potentially dangerous radiation-induced flaws increases with use.

The reactor got a clean bill of health.

But as Xcel joins a growing list of nuclear operators seeking to extend the lives of their plants for the second time, a question arises: How long can these old concrete-and-steel behemoths operate safely and how much will it cost to keep them running?

Xcel wants Monticello to run for up to 80 years, twice the life for which it was originally licensed. It also wants to extend the life of its plant near Red Wing.

Watchdog groups say the key is how well a plant is maintained.

"It really depends on the life history of the specific plant and the way it has been managed," said Edwin Lyman, a nuclear power expert at Union of Concerned Scientists.

Xcel has spent $2 billion maintaining the Monticello and Prairie Island plants since the early 2000s, the last time U.S. Nuclear Regulatory Commission (NRC) relicensed the facilities.

"We — and the industry — believe these plants have been well maintained and well invested in," said Christopher Clark, Xcel's Minnesota president. "They can be operated for another 20 years."

Xcel's reactors — and much of the nation's nuclear fleet — have become strategically more important, despite the industry's intractable problem of toxic radioactive waste. They don't emit carbon dioxide like coal or gas power plants. They also can provide constant electricity, unlike wind or solar farms.

Nuclear power "will be key to reach the state's new goal of 100 percent clean energy by 2040," Clark said, referring to a new Minnesota law requiring utilities to cut carbon emissions.

Nuclear energy key to Xcel's plans

The energy from the nuclear plants also would be hard to replace. Xcel's reactors generate more than 20% of Minnesota's electricity.

Nuclear plants are a special breed. They employ far more people than conventional or renewable power plants — Monticello has about 700 workers. And they are the only type of power plant where visitors are greeted by guards toting automatic rifles.

U.S. nuclear plants were originally licensed for 40 years, then relicensed for another 20. Xcel applied earlier this year to the NRC to extend Monticello's license for another 20 years after it expires in 2030. Xcel expects to do the same for its two Prairie Island reactors.

Regulatory records indicate Xcel has been largely free of substantial nuclear safety lapses, though there have been miscues. A recent radioactive water leak at Monticello did not threaten public safety, regulators say, but it was an embarrassing gaffe for Xcel.

Inside Monticello, a maze of pipes and cables connect hulking steel hardware in a complex system that harnesses heat from nuclear fission to create steam, which then spins a giant turbine.

"Many things degrade over time, so safety depends on how well aging is managed," said David Lochbaum, a nuclear safety engineer formerly with the Union of Concerned Scientists. "If you are replacing your parts that wear out, you can operate safely for 70 to 80 years."

Xcel has replaced most of the largest and most vital components at its nuclear plants. It has been an expensive undertaking, one borne by ratepayers — including a $400 million cost overrun at Monticello in the 2010s.

"We will continue to make substantial investments in both plants going forward," said Pam Gorman Prochaska, Xcel's director of nuclear regulatory policy.

Big repairs usually occur when the plants are idled for refueling, outages that last about a month. During an April refueling, more than 800 temporary trades workers swarmed Monticello, many clad head to toe in white or yellow radiation protection suits.

Several were replacing "control rod drivers" on one of the days. These rods decelerate a nuclear reaction or stop it altogether during a plant malfunction. The drivers — which move the rods — are replaced every decade due to wear and tear.

"You can't have an aging management program without having preventive maintenance," said Tom Conboy, general manager for Xcel's nuclear fleet operations. "But when you are talking about aging management, you are also talking about the long-term things you can't really see."

Keeping plants safe

The inside of a reactor vessel is one of the places where degradation is hard to see. Here, neutrons are constantly crashing into and splitting uranium atoms, releasing intense heat and radiation.

Those neutrons are also pelting the vessel's 6- to 10-inch-thick steel walls. Over time, tiny flaws can develop, embrittling metal and making it susceptible to cracking. The longer a plant runs, the greater the neutron bombardment and potential for embrittlement.

The Yankee Rowe nuclear plant in Massachusetts was closed in 1991 after 30 years of operation for that reason. Embrittlement has been an issue at about a half-dozen other plants over the past decade, none of them Xcel's.

"The most critical thing in terms of the safety of a nuclear power plant is the embrittlement of the reactor vessel," said Arthur Motta, a nuclear engineering professor at Pennsylvania State University.

"Because the consequences of a pressure vessel breaking are unthinkable," he said, "the reactor is way overdesigned and the safety requirements are very conservative to forestall even a small likelihood that this could occur."

Nuclear operators like Xcel periodically retrieve "capsules" of metal from inside reactor vessels, analyzing them for neutron irradiation.

A capsule pulled relatively recently from Prairie Island's Unit 1 reactor suggested that neutron-related wear and tear was more than anticipated, according to an NRC filing and Lochbaum, the safety consultant.

The capsule indicated that if the reactor keeps operating at the same rate, it may hit its end-of-life earlier than projected, Lochbaum said. However, Xcel can adjust the plant's operation to lower neutron wear-and-tear, he said. "It is a manageable issue."

Xcel said its "validated computations and measured data" will prove to the NRC that "there will be no challenge to safe operation out to 80 years."

Christopher Domingos, site vice president for Monticello and Prairie Island, said that all of Xcel's reactors "are in great condition."

Xcel has had violations, but not highest level

The same couldn't be said for a pipe at Monticello that failed last fall and leaked tritium, a mildly radioactive form of hydrogen.

About 400,000 gallons of water containing tritium — a reactor byproduct — seeped into the earth for about a month before Xcel fixed the leak in December. The fix didn't hold, and in March Xcel closed Monticello for a few days to make more extensive repairs.

The failed 3-inch diameter pipe, which was near ground level, dated to the plant's 1971 opening. Water had corroded it.

"We didn't fully appreciate the impact of the ground water below would have on causing the pipe to leak," Domingos said.

The leak has been confined to a plume beneath Xcel's property, and the company is cleaning it up.

Tritium leaks unfortunately have been relatively common in the nuclear industry, and the Monticello spill was among the nation's 10 largest. It didn't warrant a safety violation from the NRC; no tritium leaks have.

Over the past 25 years, the NRC has taken four "significant" safety enforcement actions at Monticello and 12 at Prairie Island, records show. The last one was at Monticello in 2016. Almost all of them were for low to moderate infractions.

Xcel has never received a "red" or "high" safety violation, the most severe of the NRC's four color-coded safety breaches. In 2013, Monticello got the second most serious citation — "yellow" or "substantial" — for failing to maintain an adequate flood plan.

Also, in 2015 the NRC alleged that Xcel didn't adequately monitor two contract workers at Monticello. The NRC concluded the contractors had willfully violated safety procedures and falsified reports about casks filled with high-level nuclear waste.

Xcel's nuclear plants have never had an accident that imperiled the public — and such incidents have been rare in the U.S.

The nation's worst nuclear accident was the 1979 partial meltdown of a reactor at Three Mile Island in Pennsylvania. Four years earlier, a major fire swept through the Browns Ferry nuclear plant in Alabama.

The Davis-Besse plant in Ohio came close to disaster in 2002 when deep corrosion was discovered in a reactor head. There have been other unsettling incidents since.

Still, "the data shows there have been so few events, and they have not been of the magnitude of Three Mile Island, Browns Ferry or even Davis-Besse," Lochbaum said. "I think that is more skill than luck."