The transformer is expected to be the biggest single piece of equipment that will need to be replaced for restart of the plant. It will cost about $100 million, said Constellation, the owner of the plant, which is investing $1.6 billion to revive the operation over the next four years.

When deciding whether to move forward with a restart, Constellation surveyed the site to determine the condition of essential infrastructure and equipment, much of which has sat idle since the plant shut in 2019.

"I have walked the facility top to bottom, every floor," Constellation Vice President of Generation Bryan Hanson said from inside the turbine deck at Three Mile Island, which is being renamed Crane Clean Energy Center. "The plant is in great condition."

Constellation announced last month that it signed a 20-year power contract with Microsoft (MSFT.O), to help restart the plant. The nuclear reactor, which is located on an island in the Susquehanna River in Pennsylvania, is expected to supply 835 megawatts of power to offset Microsoft's data center electricity consumption.

The sudden and voracious energy needs of Big Tech to power an expansion of artificial intelligence and cloud computing has doubled the country's projected electricity demand and led to a spate of unusual deals between power companies and the technology industry in the past year.