The Shocking Truth About Three Mile Island: Where It Is And Why It’s Restarting For Microsoft

The name "Three Mile Island" (TMI) is synonymous with the most serious commercial nuclear accident in U.S. history, but the facility's location and its current status are not what you might expect. As of late 2025, the narrative around TMI is undergoing a stunning reversal, moving from a symbol of nuclear disaster and decommissioning to a new beacon of clean energy, driven by the insatiable power demands of modern technology giants like Microsoft. This article provides the definitive, up-to-date information on exactly where Three Mile Island is situated and the extraordinary, multi-billion-dollar plan to resurrect one of its reactors.

The infamous nuclear power plant is located on an island of the same name in the Susquehanna River, situated near Middletown, Pennsylvania. It lies approximately 10 to 12 miles (16 to 19 km) southeast of the state capital, Harrisburg. While the entire facility was once slated for permanent shutdown and decommissioning, a massive new deal is bringing one of its units back online, cementing its place in the future of American energy, not just its past.

The True Location: A Pennsylvania Island with a Global Legacy

The Three Mile Island Nuclear Generating Station (TMI) sits squarely in the middle of the Susquehanna River, one of the longest rivers on the East Coast of the United States. The island itself is part of Londonderry Township in Dauphin County, Pennsylvania. For decades, the two prominent cooling towers of the facility have been a recognizable landmark for residents and travelers on Pennsylvania Route 441.

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The plant is not a single entity but was composed of two distinct Pressurized Water Reactors (PWR): Unit 1 and Unit 2. The physical location is a key geographical entity in the history of nuclear power, but the distinction between the two units is crucial to understanding the facility’s current status.

• State: Pennsylvania, USA
• County: Dauphin County
• Nearest City: Middletown, PA
• Proximity to Capital: ~10-12 miles southeast of Harrisburg, PA
• Body of Water: Susquehanna River

TMI Unit 1 vs. Unit 2: Why Only One Is Restarting

The world remembers the name Three Mile Island because of the catastrophic event that occurred on March 28, 1979. However, that accident involved only one of the two reactors, Unit 2. The two units have had drastically different histories and operational statuses ever since.

Unit 2: The Permanent Shutdown

Unit 2 (TMI-2) was the site of the partial meltdown in 1979, the most significant accident in U.S. commercial nuclear power history. While the radioactive releases were deemed minimal and had no detectable long-term health effects on the public, the reactor core was severely damaged. The cleanup process, which included removing 99% of the fuel, was a massive, decades-long effort that officially concluded in December 1993, costing approximately $1 billion (over $2 billion in today's value). TMI-2 is permanently shut down and is currently owned by EnergySolutions, which is responsible for the remaining decommissioning activities.

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Unit 1: The Phoenix Rises

Unit 1 (TMI-1) was operating at the time of the accident but was completely separate and unaffected. It was a well-regarded, high-performing reactor that continued to operate safely for decades after the Unit 2 incident. However, due to unfavorable economic conditions in the competitive power market, its then-owner, Exelon, shut it down permanently in September 2019. The plan was to eventually decommission it, with an estimated completion date as far out as 2079.

That plan has now been scrapped entirely. The current owner, Constellation Energy (CEG), announced in late 2024 a groundbreaking plan to bring TMI-1 back online. This reversal is a direct response to the massive, unforeseen surge in electricity demand from AI data centers across the country.

The Stunning Restart: Powering Microsoft's AI Future

The most compelling and up-to-date story about Three Mile Island is its future. The key driver for the restart of Unit 1 is a 20-year Power Purchase Agreement (PPA) signed between Constellation Energy and Microsoft. This deal is a landmark moment, showcasing how the burgeoning demand for Artificial Intelligence (AI) computing power is altering the energy landscape.

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The Unit 1 reactor, which has a capacity of 871 MW, will provide a significant amount of carbon-free, 24/7 power directly to Microsoft’s regional data center operations. This move highlights the growing recognition of nuclear power as a vital source of clean energy needed to meet the intense, reliable power requirements of modern technology infrastructure.

The restart project is progressing rapidly, with Constellation Energy anticipating the reactor will be operational ahead of its initial 2028 timeline, possibly as early as 2027.

A New Identity: The Christopher M. Crane Clean Energy Center

To further signal this dramatic shift from a troubled past to a clean energy future, Constellation Energy has sought and received approval for a name change for Unit 1. In May 2025, the Nuclear Regulatory Commission (NRC) approved the request to officially rename the facility from "Three Mile Island Nuclear Station, Unit 1" to the Christopher M. Crane Clean Energy Center (CCEC).

This new name honors the former CEO of Constellation's parent company, Chris Crane, a key figure in the nuclear energy sector. The change is more than symbolic; it is a clear statement that the facility is being rebranded as a critical asset in the nation's push toward a decarbonized grid and a reliable power source for the future of computing.

The restart of the CCEC (formerly TMI Unit 1) is a powerful example of the global reconsideration of nuclear power. Once viewed as a relic of a bygone era, the need for stable, high-capacity, emission-free electricity to power the AI revolution and combat climate change has propelled this Pennsylvania island back into the forefront of the energy discussion.