One day before the nightmare began at the Fukushima Daiichi nuclear plant in Japan, a reactor in Vermont with the same decades-old design was getting a big thumbs-up from U.S. regulators.

The 38-year-old Vermont Yankee plant, which state lawmakers say is well past its prime, has an operating license that is set to expire next year. Last Thursday, the Nuclear Regulatory Commission (NRC) agreed to add 20 more years to the life of the plant.

This is the state of nuclear energy in this country, where the average plant was built in 1980 and the cost of launching new reactors - and, industry executives say, safer ones - remains prohibitively high. The United States, which relies on nuclear power for 20 percent of its electricity, is leaning more heavily than ever on the first generation of plants built decades ago, even as critics worry that aging reactors have some dangerous weaknesses.

Aging plants are not necessarily failing plants. U.S. nuclear facilities have less down time than they did a decade or two ago.

GE defends its model, calling the Boiling Water Reactor Mark 1 "the industry's workhorse." Out of 105 reactors in the United States, 23 are BWR Mark 1s. The two oldest - Oyster Creek in New Jersey and Nine Mile Point in New York - began operating in 1969. Utility companies running the reactors with the Mark 1 design insist that they are built to last and that many components have been replaced over the years.

The NRC has renewed licenses for 17 of these reactors and 62 altogether; it has rejected none. All reactors were originally granted 40-year licenses when they began operating, and the renewals are for 20 years.

'Arbitrary' time frame

"There was nothing magic about the 40-year span," said Peter Bradford, a former NRC commissioner. "It wasn't as though somebody said, from an engineering standpoint, 'What's the year after which the plants will start to fall apart?' The 40 years was arbitrary to begin with."

Scott Burnell, a spokesman for the NRC, said that when Congress passed the Atomic Energy Act of 1954, which gave the commission authority to hand out licenses, lawmakers were less concerned with engineering than with blocking companies from developing monopolies in their markets.

The NRC said that regulators are constantly monitoring the plants and that a renewal does not give a reactor free rein to operate more loosely.

"If it's capable of running for 40, we can tack on 20 and then consider things from there," Burnell said.

The NRC said renewal hinges on how a plant affects its surrounding environment and on the condition of its aging equipment.