How to Restart a Nuclear Reactor

Data centers’ need for clean electricity is so ravenous that a shuttered nuclear reactor at Three Mile Island—the site of the worst nuclear disaster in U.S. history—may be revived just to power them.

Constellation Energy in September announced that it plans to restart Unit 1 at Three Mile Island, which will supply 835 megawatts to grid operator PJM. Tech giant Microsoft will buy the power produced at Three Mile Island to help match the power consumed by its data centers in the PJM Interconnection, according to a 20-year power purchase agreement it made with Constellation.

The Unit 1 reactor was shuttered in 2019 for economic reasons and sits adjacent to Unit 2, which was destroyed in 1979 in a partial meltdown. Restarting Unit 1 will require Constellation to surmount engineering and regulatory hurdles, and address the public relations challenge of overcoming the site’s historical link to nuclear disaster.

If the plan comes to fruition, Three Mile Island Unit 1 could be the second nuclear reactor in the world to be resurrected. The first—the Palisades Nuclear Plant in Covert, Mich., which shuttered in May 2022—is currently on track to return to operations in 2025. According to media reports, the single nuclear reactor at the Duane Arnold Energy Center near Palo, Iowa, which ceased operations in 2020, might also be brought back online.

The burgeoning trend could birth a new area of expertise: How to restart a nuclear reactor.

Reviving Three Mile Island for Data Centers

Constellation aims to have Unit 1 at Three Mile Island up and running in 2028. To prepare for the restart, significant investments will be made to restore the plant, including its turbine, generator, main power transformer, and cooling and control systems, according to Constellation.

The rationale for restarting a nuclear plant rather than building one from scratch is easy to see, says Kathryn Huff, an associate professor of nuclear, plasma, and radiological engineering at the University of Illinois, and a former assistant secretary of nuclear energy at the U.S. Department of Energy. For example, developing a new nuclear site would involve figuring out how to dissipate the reactor’s heat. That’s already been sorted at Three Mile Island, located near Middletown, Penn., which has an existing ecosystem in place.

“I think one of the most important features is that the Susquehanna River’s right there. They have the ultimate heat sink already set up,” says Huff. Crucially, the island also has high-voltage transmission lines in place, along with all the other infrastructure necessary to connect the plant to the power grid.

It helps that the recently closed reactor operated for decades under a license from the U.S. Nuclear Regulatory Commission, which oversees the industry. The most straightforward way to get the reactor back online, Huff says, is for Constellation to seek to restore the old license and to bring the plant up to the standard at which it ran until 2019. That would negate the need to start the permitting process from zero.

Exactly how this process will unfold has not been made public, and Constellation did not respond to interview requests. But there are a few key considerations and steps, Huff says. The first thing would be to gather all the spent nuclear fuel that had been in wet storage where it sits in pools for a period of years, and move it to on-site dry storage where it will remain for about a decade before going to a permanent home. That’s helpful for a potential restart, since it clears out the pools for a new round of fuel.

It’s likely that workers at Three Mile Island also would’ve taken the top off of the reactor pressure vessel to remove the spent fuel, Huff says.That equipment, which was exposed to extreme temperatures and pressures during operation, will be evaluated to determine whether it can endure another two or three decades of irradiation, she says. Such equipment has proven to endure for many decades, so some parts may not need to be replaced.

Still, all the pipes and pumps required to operate the plant will need to be checked for degradation and wear. This amounts to a litany of inspections and possible upgrades. “The whole place is pipes and pumps,” Huff says.

Decommissioned nuclear plants often sell off still-viable components like steam turbine generators to other facilities, so Constellation may need to replace some machinery. Overall, though, “my expectation is that it probably looks a little dusty” from sitting unused, Huff says. Aside from that, TM1 probably looks a lot like it did when it was operational.

Nuclear Plants Getting a Second Life

The less time that has passed since a nuclear plant shuttered, the easier it is to reopen. That’s certainly the case with the Palisades plant in Michigan, which sat dormant for only a year before it was decided to bring it back to life, says Patrick O’Brien, the director of government affairs and communications for Holtec International, the firm in charge of the decommissioning and recommissioningit. When Palisades plant operator Entergy closed the plant in 2022 because it decided it couldn’t compete economically with cheap natural gas, it asked Holtec whether the firm could preserve Palisades in stasis, O’Brien says. “They asked us: Could you bring the plant back online? And we said we could keep it in a condition that wouldn’t preclude us from doing that,” he says.

To that end, Holtec began last year to move Palisades’ spent nuclear fuel. But it planned to wait 10 years before beginning demolition so the reactor could be brought back online. As a result, most of the plant remains in place, putting Holtec on a path toward a reactor restart in late 2025. The project also just secured a US $1.5 billion loan from the U.S. Department of Energy.

Three Mile Island’s reactor has been mothballed for five years, not just one. And although Constellation CEO Joseph Dominguez has said the reactor is in “excellent shape” and on track to restart within a few years, the company has not publicly specified how many decommissioning activities happened in the past half decade, and how much infrastructure would need to be rebuilt.

“What’s going to [matter],” O’Brien says, “is how well were the systems laid up for the last five years? How’s the material condition of your plants? I don’t think [Constellation] would’ve made some of these decisions and announced a restart if they hadn’t done a lot of work ahead of time, like we did, to evaluate the likelihood of success on this path.”

Beyond mechanical repairs, there’s the murky question of regulation. Although nuclear bureaucracy moves at a famously glacial pace, there is reason for Constellation and Microsoft to be optimistic they can quickly get a green light to get up and running, even though the NRC says it has not yet received Constellation’s application to restart Unit 1. Pennsylvania Governor Josh Shapiro recently spotlighted the resurrection of Three Mile Island as the kind of “shovel-ready” project that ought to be fast-tracked through approvals and has lobbied for the reactor to be back online in 2028.

Finally, Constellation and Microsoft must also contend with the legacy of Three Mile Island. Although it resulted in no direct deaths or injuries, the partial meltdown of the Unit 2 reactor in March 1979 was the most severe nuclear accident in American history, a disaster that dogged nuclear power for decades. (Perhaps not coincidentally, Constellation has formally rebranded the revived reactor as the Crane Clean Energy Center.)

There is no danger of reopening Unit 1 right next to its doomed counterpart, Huff says. Unit 2 has been safely entombed for decades. The two reactors and all their ancillary buildings and equipment are entirely separate. “[Unit 1] operated and had workers on site—[who] conducted all of its maintenance and refueling—right next to this sarcophagus of Unit 2 ever since 1979,” she says.

Data Centers Secure Nuclear Deals

Microsoft isn’t the only data center operator aggressively looking for creative ways to power its operations. Amazon Web Services earlier this year paid $650 million for a data center that would draw its power directly from a neighboring nuclear power plant, and is now in a regulatory battle over it.

So Three Mile Island may not be the last reactor to come back from the dead, given the growing demand for constant, clean energy to support hungry data centers and meet emissions reduction targets. Indeed, U.S. Energy Secretary Jennifer Granholm told Reuters in July that she would be “surprised” if the federal government wasn’t talking to owners of other shuttered nuclear reactors about potential restarts.

Most other shuttered nuclear plants in the United States and globally have been offline a lot longer and decommissioned more aggressively than Palisades and Three Mile Island, making the financial case for salvaging them harder to justify. But if nuclear reactors themselves can’t be resuscitated, their sites can still be primed for a second life. These sites tend to have the right infrastructure, a public attitude that’s more positive toward nuclear power, and a workforce with experience in energy, O’Brien says.

“Who supports nuclear the most? It’s usually the communities that live around and understand nuclear plants,” he says. Even coal plants slated to go offline around the world in the coming years would be ideal locations for new nuclear plants—if a nuclear power renaissance truly has arrived, he says.