Forty years after the worst commercial nuclear power plant accident in United States history unfolded on Three Mile Island in Pennsylvania, the only nuclear power reactor still operating there is preparing to shut down.

The facility, which is in Londonderry Township, Pa., has been losing money, and in a statement on Wednesday, Exelon Generation, the company that owns the plant, said it would be closed by Sept. 30. The company and its employees had been hoping for a subsidy from the state, and when that fell through, a shutdown was the only option, that statement said.

“Today is a difficult day for our employees, who were hopeful that state policymakers would support valuing carbon-free nuclear energy the same way they value other forms of clean energy in time to save TMI from a premature closure,” Bryan Hanson, Exelon’s senior vice president and chief nuclear officer, said in the statement.

A company spokesman said the cost of decommissioning the site was estimated at around $1.2 billion.

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