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 near Middletown, 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.

The reactor where the accident occurred 40 years ago is not owned by Exelon, and it was dormant long before the company began operating a nearby reactor around two decades ago. But for many residents of the area around Harrisburg, Pa., the words “Three Mile Island” will always evoke memories of that panic.

It was, as The New York Times reported in the aftermath, “an accident that would generate a week of doomsday fear, panicky flight, conflicting statements, noisy demonstrations and intense confusion.”

Disaster struck in the early morning hours of March 28, 1979, when water-coolant pumps failed and a reactor overheated. The temperature kept rising after a stuck valve misled the operators into stopping the flow of emergency cooling water. There was a partial meltdown, and the plant was in crisis for several days. Radiation was purposefully released into the air to relieve pressure within the system.