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Microsoft has agreed to buy power for 20 years from Constellation Energy, contingent on restarting Three Mile Island Unit 1 in Pennsylvania. The deal is intended to help match electricity used by Microsoft data centers in the PJM regional grid with carbon-free generation; it does not mean the reactor will send electricity directly to a named Microsoft or AI data center. The latest target in the cited project updates is a 2027 restart, but the plant had not been cleared to resume operations in the Nuclear Regulatory Commission’s September 2026 notice.
Is Three Mile Island reopening?
Constellation’s proposal is to restore and restart Unit 1, which has been shut down since September 20, 2019. The company said it closed for economic reasons. The project is now called the Crane Clean Energy Center, named for former Constellation CEO Chris Crane.
Unit 1 is not the reactor involved in the 1979 partial meltdown. That incident occurred at Unit 2, a separate facility under separate ownership that remains in long-term cleanup. The Microsoft agreement concerns the proposed restart of Unit 1 only.
What the restoration involves
Constellation has described work on the turbine, generator, main power transformer, cooling systems, and control systems. In its 2024 announcement, the company estimated the project investment at about $1.6 billion; that is a company estimate, not a verified final project cost.
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Will Three Mile Island power Microsoft’s AI data centers?
The agreement is a 20-year power purchase agreement between Microsoft and Constellation. Microsoft says the purchase will support matching the electricity used by its data centers in the PJM grid with carbon-free energy. The public information described here does not identify individual Microsoft or AI data centers that will receive electricity directly from the reactor.
That distinction matters: this is a grid supply and energy-matching arrangement, not evidence of a dedicated cable from the plant to a particular data center. The sources cited do not disclose the contract price or all of its commercial terms, so claims about what Microsoft pays or how project risks are allocated are not established.
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How much electricity could the plant provide?
The Nuclear Regulatory Commission’s 2026 notice says the proposed actions would make 835 megawatts-electric of baseload generation capability available to the PJM grid. Constellation’s 2024 announcement described Unit 1 as having 837 megawatts of generating capacity before retirement. Those are figures from different sources and are not interchangeable measurements.
Constellation also said Unit 1 produced at maximum capacity 96.3 percent of the time in its final operating year. That is the company’s account of the plant’s past performance, not a guarantee of future output after a restart.
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The most recent target in the project updates cited here is 2027. Constellation announced the project in September 2024 with an expected 2028 online date, then said in September 2025 that it had accelerated its target to 2027. A company target is not a confirmed operating date, and regulatory approvals and restoration work remain relevant to the schedule.
| Date | Milestone | What it establishes |
|---|---|---|
| September 20, 2019 | Unit 1 shut down. | The reactor has not operated since that date. |
| September 20, 2024 | Constellation announced the Microsoft agreement and an expected 2028 online date. | The company’s original project target and the announced 20-year agreement. |
| September 23, 2025 | Constellation announced a 2027 target and reported progress on staffing, inspections, grid interconnection, and regulatory work. | A revised company target and reported progress, not approval to operate. |
| September 25, 2026 | The NRC made its final environmental assessment and finding of no significant impact available. | An environmental review milestone; the NRC’s decision on the requested exemption and license amendments was still pending in its notice. |
Has the NRC approved the restart?
No. In its September 25, 2026 notice, the NRC said it had completed an environmental assessment and found no significant impact from the federal actions it assessed. The agency was still considering an exemption and three license amendments needed for the proposed return to power operations. The environmental finding is not itself operational authorization.
The NRC’s environmental review considered land use, air, water, ecological and cultural resources, health, waste, transportation, accidents, and climate. Its conclusion was that impacts would not be significant for the assessed resource areas; this is the NRC staff’s environmental determination, not a finding that every project risk is absent.
State environmental review also matters
Pennsylvania’s Department of Environmental Protection oversees applicable state requirements involving areas such as air, water, waste, and drinking water. DEP’s project page described a draft NPDES permit issued April 2, 2026, with a comment period through May 18, 2026, and discussed PFAS in site wells and corrective actions. Those permit details are time-sensitive; the information cited here does not establish the permit’s status after that comment period.
What license term would a restart use?
The NRC notice says Unit 1’s existing renewed operating license expires April 19, 2034. The requests discussed in that notice support possible operation through the end of that existing license term; they do not establish that a later license renewal has been granted. The NRC’s planning table lists a subsequent license renewal application for 2029, which is a future application milestone rather than an approval.
What this deal does—and does not—show
The agreement is a notable example of a technology company contracting for carbon-free generation tied to a grid where its data centers use electricity. It also depends on the proposed return of a retired nuclear unit, with restoration work and regulatory steps still ahead as of the NRC’s September 2026 notice. The available information does not establish a direct physical supply to specific AI facilities, the deal’s price, or that the 2027 target will be met.
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