Exelon Generation says it will permanently shut down Three Mile Island Unit 1 at noon on Friday, closing the door on a nuclear power station that became infamous by association with the 1979 accident that crippled its neighboring twin reactor.

Exelon announced the closure of Unit 1 in May after Pennsylvania legislators balked at a proposed $500 million nuclear rescue. Exelon said that the power station, which began commercial operations 45 years ago, was losing money in competitive electricity markets dominated by low-cost natural gas.

The signature plumes of vapor from Unit 1′s cooling towers will soon disappear as the reactor shuts down and its core temperature drops, joining the two empty concrete towers of the damaged Unit 2 as giant memorials.

The company will retain about 300 of TMI’s 700 employees to conduct the first phase of decommissioning. Other employees have been reassigned within Exelon or decided to leave the company.

“Today we celebrate the proud legacy of TMI Unit 1 and the thousands of employees who shared our commitment to safety, operational excellence and environmental stewardship for nearly five decades,” Bryan Hanson, senior vice president and chief nuclear officer, said in a statement.

Exelon Generation, which is headquartered in Kennett Square, successfully engineered state rescues in Illinois, New York and New Jersey, but not in Pennsylvania, which hosts the nation’s second-largest nuclear fleet behind Illinois.

» UPDATE: ‘End of an era:’ After 45 years, Three Mile Island nuclear power plant is closed

“At a time when our communities are demanded more clean energy to address climate change, it’s regrettable that state law does not support the continued operation of this safe and reliable source of carbon-free power.”

The focus for nuclear power supporters is now likely to shift to Western Pennsylvania, where FirstEnergy Corp. plans to retire its two Beaver Valley reactors near Shippingport in 2021. Supporters plan to rally Friday at the Beaver County Courthouse to pressure legislators to revive a nuclear rescue that would keep the plants open.

The Ohio legislature and Gov. Mike DeWine in July approved a $1 billion bailout for FirstEnergy’s reactors in the Buckeye State. FirstEnergy has asked the Ohio Supreme Court to block a referendum petition drive that would overturn the rescue.

FirstEnergy, through a subsidiary, also owns TMI Unit 2, which was destroyed in a 1979 accident only a few months after it began commercial operations. An energy services company that specializes in dismantling old nuclear reactors, EnergySolutions Inc., announced in July that it is negotiating to acquire the damaged reactor to dismantle it under an accelerated schedule.

Eric Epstein, chairman of Three Mile Island Alert, a Harrisburg nuclear watchdog group, said attention will now be aimed at pressuring Three Mile Island’s two owners to accelerate decommissioning, rather than taking six decades to complete the task, as Exelon announced last month.

Exelon said the Unit 1 nuclear reactor decommissioning and shutdown will take $1.2 billion to complete.

Exelon’s decommissioning schedule, which was spelled out in a plan released in April, calls for the immediate removal of Unit 1′s nuclear fuel from the reactor after shutdown. The uranium fuel-rod assemblies would cool in spent fuel pools for three years until they are moved to above-ground sealed canisters in 2022.

But the reactor’s cooling towers and other large components would remain standing until 2074, according to Exelon’s Post-Shutdown Decommissioning Activities Report. All radioactive material would be safely stored or removed from the site by 2078.

The U.S. Nuclear Regulatory Commission would supervise the decommissioning.