The prospects for building new nuclear reactors may be sharply limited, but the owners of seven old ones, in Pennsylvania, Virginia and South Carolina, are preparing to ask for permission to run them until they are 80 years old.

Nuclear proponents say that extending plants’ lifetimes is more economical — and a better way to hold down carbon dioxide emissions — than building new plants, although it will require extensive monitoring of steel, concrete, cable insulation and other components. But the idea is striking even to some members of the nuclear establishment.

At a meeting of the Nuclear Regulatory Commission in May, George Apostolakis, a risk expert who was then one of the five commissioners, pointed out that if operation were allowed until age 80, some reactors would be using designs substantially older than that.

“I don’t know how we would explain to the public that these designs, 90-year-old designs, 100-year-old designs, are still safe to operate,” he said. “Don’t we need more convincing arguments than just ‘We’re managing aging effects’?”