Why It Matters
Pennsylvania’s energy landscape could shift considerably when Three Mile Island — site of the United States’ most serious commercial nuclear accident — resumes generating electricity for the first time in years. The facility, located in Dauphin County’s Londonderry Township, would become only the second shuttered commercial nuclear plant in the country to return to service, supplying 835 megawatts of carbon-free power to meet surging demand from Microsoft’s artificial intelligence data centers.
What Happened
A 500,000-pound electrical transformer arrived at the plant’s north gate Monday morning, marking the first major equipment delivery in the yearslong effort to bring the reactor back online. The transformer, sourced from South Korea, completed a journey that spanned an ocean liner crossing, a rail segment, and final movement by a specialized heavy hauler before reaching the site.
Operator Constellation Energy — which also markets the facility under the name Crane Clean Energy Center — plans to receive two additional transformers as part of the same installation program. Each unit is designed to step up voltage output from the main generator tenfold, boosting the plant’s 23,000-volt generation to transmission-level power.
The delivery is the most visible milestone yet in a project that traces back to a 2024 agreement with Microsoft valued at $1.6 billion. Under that deal, electricity produced at Three Mile Island will flow to data centers supporting the company’s growing AI infrastructure.
By the Numbers
The scope of the project reflects both its cost and its ambition:
- $9.88 million — cost of each transformer
- $1.6 billion — total value of the Microsoft electricity agreement signed in 2024
- 835 megawatts — carbon-free generating capacity once the plant is operational
- 650 — permanent jobs the restart is expected to support
- Summer 2028 — Constellation’s current target date for full restart
Background
Three Mile Island’s Unit 2 reactor suffered a partial meltdown in 1979, an event that reshaped public debate over nuclear power in the United States for decades. Unit 1, a separate reactor on the same site, continued operating for another four decades before Constellation concluded in 2019 that it was no longer economically viable and moved toward decommissioning.
Bryan Hanson, Constellation’s chief generation officer, acknowledged the sharp reversal in course. “We made a decision, a tough decision, to shut the plant down. And we had no intention of restarting the plant,” Hanson told the Penn Capital-Star, as first reported by the Pennsylvania Capital-Star. The Microsoft agreement and broader shifts in energy demand — particularly from data centers — changed that calculus.
On the timeline, Hanson offered a clear commitment: “Our commitment still is the summer of 2028,” he said, noting that electricity provision could begin as early as 2027 depending on progress.
Zoom Out
Three Mile Island’s potential restart is part of a broader national reassessment of nuclear power’s role in the electricity grid. Demand growth driven by AI data centers, electric vehicle adoption, and domestic manufacturing has strained grid capacity in several regions, pushing utilities and technology companies toward long-term power purchase agreements with low-carbon generators. Nuclear energy, which produces no carbon emissions during operation, has attracted renewed interest from both federal policymakers and private industry after years of plant closures.
Pennsylvania lawmakers are separately weighing electricity grid investments, including a proposed battery storage mandate aimed at managing supply and demand as the state’s energy mix evolves.
If completed on schedule, Three Mile Island would join Palisades Nuclear Plant in Michigan as the only commercial nuclear reactors in U.S. history to restart after being taken offline — a distinction that underscores how unusual the project remains even as nuclear sentiment shifts nationally.
What’s Next
Two additional transformers are expected to follow the first delivery to the Londonderry Township site. Constellation must also complete federal regulatory requirements before any fuel loading or power generation can begin. The company’s stated goal is to begin supplying electricity to the grid in 2027, with full commercial restart targeted for summer 2028.