Washington — When does a nuclear plant become too old?

The nuclear industry is wrestling with that question as it tries to determine whether problems at reactors, all designed in the 1960s and 1970s, are middle-aged aches and pains or end-of-life crises.

This year, utilities have announced the retirement of four reactors, bringing the number remaining in the United States to 100. Three had expensive mechanical problems but one, Kewaunee in Wisconsin, was running well, and its owner, Dominion, had secured permission to run it an additional 20 years. But it was losing money, because of the low wholesale price of electricity.

“That’s the one that’s probably most ominous,” said Peter A. Bradford, a former member of the Nuclear Regulatory Commission and a former head of the Public Service Commission in New York. “It’s as much a function of the cost of the alternatives as it is the reactor itself.”

While the other three, San Onofre 2 and 3 near San Diego and Crystal River 3 in Florida, faced expensive repair bills because of botched maintenance projects, “Kewaunee not only didn’t have a major screw-up in repair work, it didn’t even seem to be confronting a major capital investment,” he said.