Photo: formulanone, 2021, via Wikimedia Commons (CC BY-SA 2.0)
Forty-seven years after the worst accident in American commercial nuclear power, the name Three Mile Island is about to mean something new. The undamaged reactor on the island, Unit 1, ran for four decades before it was shut for economic reasons in 2019. Now it has a new name, the Crane Clean Energy Center, a 20-year power deal with Microsoft, a $1 billion federal loan and fresh fuel waiting on site. On November 9, 2026, Constellation Energy’s chief executive told investors the restart is on track for the second half of 2027.
The short version: Unit 1 is not the reactor that melted down in 1979. That was Unit 2, which never ran again and is being dismantled. Unit 1 sat cold from September 2019 with its fuel moved into dry storage casks. To bring it back, Constellation has to restore the plant’s operating license with the Nuclear Regulatory Commission, rebuild or replace worn equipment, load new fuel and reconnect to the regional grid. If it does, about 835 megawatts of carbon-free power will flow onto the grid that feeds Microsoft’s data centers in the Mid-Atlantic, and Three Mile Island will become one of the first shut-down American reactors ever brought back.
When will Three Mile Island restart?
The target is 2027, and the company now says the second half of the year. When Constellation and Microsoft announced the deal on September 20, 2024, the plant was expected online in 2028. The company later pulled that forward to 2027, and in September 2026 it told World Nuclear News the work "remains ahead of schedule." On the November 9 earnings call, chief executive Joe Dominguez said the restart is on track for the second half of 2027, with the first fuel assemblies and new transformers already on site, Utility Dive reported.
The regulator has not signed off yet. The NRC still has to rule on an exemption and three license amendments that would let the plant run again under its existing license, which expires on April 19, 2034. The environmental piece is done: in September 2026 the NRC and the Department of Energy issued a final environmental assessment and a finding of no significant impact, so no full environmental impact statement is required, according to the Federal Register notice. The safety reviews and an inspection program written specifically for reactors returning from retirement are still running.
The grid has its own clock. The regional grid operator, PJM, approved reconnecting the plant from 2027, but found that the transmission upgrades needed to deliver every megawatt could take until after 2030. To bridge that gap, federal regulators agreed in June 2026 to let Constellation move 760 megawatts of grid connection rights from two units at its Eddystone plant near Philadelphia, which FERC said could let Crane be "fully operational before December 31, 2030," as World Nuclear News reported.
Wait, isn’t this the reactor that melted down?
No, and the difference matters. Three Mile Island, on the Susquehanna River about 10 miles southeast of Harrisburg, had two pressurized water reactors side by side. Unit 1 started operating in 1974. Unit 2 came online at the end of 1978, and three months later it failed.
What went wrong in 1979. At about 4 a.m. on March 28, 1979, pumps feeding water to Unit 2’s steam generators stopped. The turbine and then the reactor shut down automatically, and a relief valve on top of the pressurizer opened to bleed off rising pressure, just as designed. Then it stuck open. A light in the control room told operators the valve had closed, so for hours nobody realized that cooling water was escaping as steam. Believing the system was too full of water, staff cut back the emergency cooling, the water level in the reactor vessel fell, and the top of the core was left uncovered and overheated, according to the NRC’s backgrounder on the accident.
Photo: U.S. Nuclear Regulatory Commission, 1979, via Wikimedia Commons (public domain)
What it did and didn’t do. The core partly melted, but the thick steel reactor vessel and the containment building held. The NRC estimates that the roughly 2 million people around the plant received an average extra dose of about 1 millirem, compared with about 6 millirem for a chest X-ray, and later studies found no health effects that could be traced to the accident. What it did change was everything about how American reactors are run: operator training, control-room design, emergency planning and resident NRC inspectors at every plant all trace back to Three Mile Island.
Image: U.S. Nuclear Regulatory Commission, via Wikimedia Commons (public domain)
Unit 1 kept going. Unit 1 was down for refueling when its neighbor failed. It stayed shut for years while regulators and the public argued over the site’s future, then returned to service in 1985 and ran for 34 more years. Constellation says that in its last year, the plant produced at full capacity 96.3% of the time, well above the industry average. It closed in September 2019 not because anything was wrong with it, but because it could not make money selling power into a market flooded with cheap natural gas.
Why Microsoft wants a 50-year-old reactor
Microsoft does not get a cable from the reactor to a server farm. Under the 20-year power purchase agreement, the plant’s output goes onto the PJM grid like any other power plant’s, and Microsoft buys the energy to match the electricity its data centers in that region use with carbon-free power, hour by hour. Neither company has disclosed the price or exactly how much of the output Microsoft takes.
The appeal is what nuclear does that wind and solar cannot do alone: run all day and all night, in every season. Modern AI data centers draw large, steady loads around the clock, and they also need serious cooling, something we covered in our guide to how data centers stay cool. When Constellation announced the deal, Dominguez made the pitch in one sentence.
Powering industries critical to our nation’s global economic and technological competitiveness, including data centers, requires an abundance of energy that is carbon-free and reliable every hour of every day, and nuclear plants are the only energy sources that can consistently deliver on that promise. (Joe Dominguez, Constellation CEO, September 20, 2024)
A restart is also faster than a new plant. A brand-new large reactor in the United States takes the better part of a decade to license and build, and the most recent ones ran billions of dollars over budget. A small modular reactor is closer to market, but still years away, as we explained when TVA won the first U.S. SMR construction permit. At Three Mile Island, the reactor vessel, the containment, the cooling towers, the switchyard and the grid connection already exist. What has to be rebuilt is the paperwork and the parts that wear out.
How you bring a nuclear plant back from retirement
There is no single "on" switch. According to Constellation’s presentation to the NRC and the agency’s project page, the restart is a long list of jobs, and many of them have never been done in the United States before.
Undo the retirement on paper
When a U.S. plant retires, its owner tells the NRC it has permanently stopped operating and removed the fuel, and from then on the license no longer allows it to run the reactor or put fuel in the vessel. Constellation asked for an exemption to rescind those 2017 and 2019 certifications, plus amendments to restore its operating license and technical specifications, its emergency plan and its security plan. The plant was formally renamed in May 2025.
Wake up the machinery
Most major systems were drained and powered down, but no major components were removed. The steam generators, the giant heat exchangers between the reactor’s radioactive water and the clean steam that spins the turbine, were replaced in 2009 and passed inspection again in spring 2024. The main generator was inspected and put into long-term storage. The main power transformers, which step up the generator’s voltage for the transmission lines, are being replaced with new ones. Constellation says it is also restoring the turbine, the cooling systems and the control systems.
Bring back the people and the fuel
A running reactor needs a licensed crew, and Constellation says the plant will support more than 600 full-time jobs. Fuel is the other piece. When the plant retired, all of its used fuel was moved from the pool into dry casks and the spent fuel pool was drained and covered. A restart means a working fuel pool again and a full core of new fuel assemblies, the first of which are now on the island.
Photo: Constellation Energy, 2019, via Wikimedia Commons (CC BY-SA 4.0)
A little physics on the numbers. The NRC licenses Unit 1 for 2,568 megawatts of heat, and the plant turns that into about 835 megawatts of electricity. That is roughly 33% efficiency, typical for a pressurized water reactor, whose steam runs cooler than the steam in a coal or gas plant. The rest of the heat leaves through the condenser and those famous hyperboloid cooling towers. The white plumes rising from them are water vapor condensing into cloud, not smoke and not radiation. DOE says 835 megawatts is enough to power the equivalent of about 800,000 homes.
Three Mile Island is not the only restart
Three American reactors that were shut for good are now trying to come back, each with federal loans and, in two cases, a tech company buying the power.
Palisades, Michigan. Holtec International’s 800-megawatt plant on Lake Michigan, closed in 2022, is the furthest along and has $1.5 billion in federal backing. It began loading fuel in late August 2026, but a fuel assembly tipped inside the reactor and took weeks to recover. Fuel loading resumed in late October, and Holtec has not named a startup date. Energy Secretary Chris Wright said on October 29 that it could restart by the end of November, a date the company did not confirm, Engineering News-Record reported.
Three Mile Island, Pennsylvania. The Crane Clean Energy Center is second in line, with the Microsoft deal and a $1 billion loan from DOE closed on November 18, 2025, according to the Department of Energy.
Duane Arnold, Iowa. NextEra Energy’s 615-megawatt plant, Iowa’s only nuclear station, closed in 2020. Google signed a 25-year agreement in October 2025 to buy its power for its cloud and AI work in Iowa, and DOE closed a loan of up to $1.9 billion in September 2026. NextEra’s target is no later than the first quarter of 2029, according to the company.
What we know and what’s still unclear
What we know
The plant is Unit 1, not the reactor that failed in 1979. Microsoft has a 20-year deal for its power. DOE has lent $1 billion. The environmental review is finished with no significant impact found, the first fuel assemblies and new transformers are on site, and Constellation says the plant is on track for the second half of 2027. It also plans to seek a license renewal to keep running until at least 2054.
What’s still unclear
The NRC has not yet approved the license changes the plant needs to run, and its inspectors still have to confirm the restored equipment works. A hearing request filed by a member of the public was referred to an NRC licensing board in May 2026. Nobody has said how much of the plant’s output will reach the grid before PJM’s transmission upgrades are finished. The price Microsoft pays has never been published. And a restart that began on time can still slip: Palisades, which once aimed to run by late 2025, is now about a year behind.
The bottom line
Three Mile Island is becoming a test of whether the country can get more out of the nuclear plants it already has, faster and cheaper than it can build new ones. If the Crane Clean Energy Center starts up in 2027 as planned, the island whose name became shorthand for nuclear fear will be powering the AI boom, and its cooling towers will be steaming for a new reason. For more on where clean, round-the-clock power might come from next, see our look at Helion’s Polaris fusion machine.
Related on aglena: Form Energy’s Iron-Air Batteries: Reversible Rusting for 100-Hour Grid Storage (2026), Sunlight on Demand: The Fight Over Reflect Orbital’s Space Mirrors
Further reading
Three Mile Island: A Nuclear Crisis in Historical Perspective by J. Samuel Walker
The NRC’s former historian tells the hour-by-hour story of the 1979 accident at Unit 2 and how it reshaped nuclear regulation, from the stuck valve to the hydrogen bubble scare.
Atomic Accidents: A History of Nuclear Meltdowns and Disasters by James Mahaffey
A nuclear engineer’s readable tour of what has gone wrong with reactors and radiation, from early experiments to Three Mile Island, Chernobyl and Fukushima, and what each failure taught.
Snap Circuits Explorer SC-100 Electronics Kit
More than 100 snap-together projects for ages 8 and up. A hands-on way to learn how switches, motors and circuits move power around, the same ideas that run a grid.
Kasa Matter Smart Plug with Energy Monitoring, 2-Pack (KP125M)
Plug-in smart plugs that show how many watts and kilowatt-hours a lamp, PC or space heater really uses, a small-scale look at the demand that utilities and data centers plan around.
