The system that controls the two giant Peach Bottom reactors in York County is largely analog, installed when the company then known as Philadelphia Electric Co. — now Peco Energy — launched the units in 1974. The current owner, Exelon Generation, says it expects to fully digitize the control room in the next decade, part of an ongoing effort to modernize a plant that supplies electricity to 2.7 million homes.

The Peach Bottom plant, 60 miles west of Philadelphia near the Maryland border, is operating under a 20-year extension from its original 40-year license, like many of America’s aging fleet of nuclear reactors. Last year it became one of the first plants to apply for what the Nuclear Regulatory Commission calls a “Subsequent License Renewal” — that would permit the reactors to run through 2053 and 2054, when they turn 80 years old.