Microsoft Will Pay to Restart a Reactor at Three Mile Island to Power Its AI Data Centers

The reactor that shut down in 2019 for lack of business — next to the one that partially melted down in 1979 — will come back online by 2028 under a 20-year deal. Microsoft will buy all of its electricity.

Constellation Energy announced today that it will restart Unit 1 at Three Mile Island, the Pennsylvania nuclear plant, and that Microsoft has agreed to buy all the electricity it produces for twenty years. The reactor closed in 2019 because it couldn't compete with cheap natural gas. It sits beside Unit 2, which partially melted down in 1979 in the worst nuclear accident in American history and has been sealed ever since. Unit 1 was not involved in the accident and ran safely for another forty years.

The reason a software company is reviving a nuclear plant is the electricity that AI consumes. Training and running the programs happens in data centers filled with chips that draw as much power as small cities, and Microsoft, Google, and Amazon are building them as fast as they can. Their promises to run on clean energy are colliding with the arithmetic: solar and wind don't run all night, and a data center does. Nuclear does too. Amazon bought a data center attached to a nuclear plant in March; Google is reported to be negotiating similar deals.

Restarting the reactor will cost about $1.6 billion and take until 2028, and requires federal approval that has never before been granted for a plant that was shut down. Constellation says it will create 3,400 jobs. Pennsylvania's governor supports it.

For most of the past decade, the argument about AI was about what the programs could do. This deal is about what they cost — in electricity, in water, in the land the data centers sit on — and who pays. That argument is just beginning, and its first landmark is a reactor most Americans over fifty remember for a different reason.