NEW YORK, March 26 (Reuters) - U.S. grid operator PJM has told Constellation Energy ( CEG ) that the former Three Mile Island nuclear power plant in Pennsylvania likely will not be able to connect to the grid until 2031, four years later than planned, the company said at the CERAWeek energy conference in Houston on Thursday.

Constellation is working to resume operations at the nuclear power plant, which is being renamed the Crane Clean Energy Center, to supply electricity to Microsoft ( MSFT ) data centers.

The company will be ready to produce electricity from the plant by its earlier goal, and it is speaking with grid operator PJM Interconnection to reduce the timeline, said David Dardis, chief external affairs and growth officer at Constellation. 

"We will have it ready to go in 2027," Dardis told Reuters.

PJM, in its initial feedback on the interconnection plan for the restarted plant, said it would take until 2031 to complete some of the transmission upgrades needed for the plant to connect to the grid. Constellation is in discussions with transmission owners to accelerate the timeline, Dardis said.

Shares of the company were down 3% following Reuters' report.

Constellation, the country's biggest independent power producer, announced in 2024 that it had contracted with Microsoft ( MSFT ) to reopen the nuclear power plant. No fully shut nuclear power plant has ever been restarted, but as demand rises from Big Tech data centers and the electrification of buildings and transportation, three U.S. plants are currently in that process.