HARRISBURG — The final shutdown of Three Mile Island, where 1979’s terrifying partial meltdown became a landmark event in the life cycle of nuclear power in the United States, is imminent.

Exelon Corp. officials said the plant will stop producing electricity around noon Friday, following through on a decision the Chicago-based energy giant made in May after it became clear that it would not get a financial rescue from Pennsylvania.

The accident that destroyed Three Mile Island’s Unit 2 badly undermined public support for the energy source: No nuclear plant that was proposed after the accident has been successfully completed and put into operation in the United States.

Edwin Lyman, acting director the nuclear safety project for the Union of Concerned Scientists, said the steps to shut down a reactor are similar to when the plant temporarily shuts it down for a refueling outage.

The first step will be to insert control rods to stop the nuclear fission chain reaction in the reactor core. The control rods — made of minerals and soft metals — absorb neutrons and make it impossible for nuclear fission to continue.

The core remains hot, and heat removal systems must cool it down gradually to avoid cracking the reactor vessel before the fuel rods are removed.