The firm announced the 20-year deal would involve the restart of the Unit 1 reactor, "which operated at industry-leading levels of safety and reliability for decades before being shut down for economic reasons exactly five years ago today."

The Unit 1 reactor was not involved in the 1979 partial nuclear meltdown at the site in Pennsylvania.

US media reported that Microsoft will harness the energy to fuel its AI capabilities, a rapidly growing sector that requires large amounts of power.

Constellation Energy said the reactor would go back online in 2028.

"This agreement is a major milestone in Microsoft's efforts to help decarbonize the grid in support of our commitment to become carbon negative," said Bobby Hollis, vice president of energy at Microsoft.

Three Mile Island sent the United States into panic when, on March 28, 1979, its Unit 2 reactor partially melted down.

The Nuclear Regulatory Commission called it the "most serious accident in US commercial nuclear power plant operating history, although its small radioactive releases had no detectable health effects on plant workers or the public."

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MICROSOFT