America's fleet of aging nuclear power plants is rapidly becoming uneconomical to operate, argues a leading consumer advocate in a new report predicting the end of the nuclear era.

Mark Cooper, an independent economist and senior fellow for economic analysis at the Vermont Law School Institute for Energy and the Environment, bases his argument on another decade of continuing low natural gas prices to give new gas-fired power plants enough of an edge to squeeze out older nuclear plants already beset with rising maintenance costs.

"Renaissance in Reverse," a 40-page paper released Wednesday, notes that utilities are already beginning to close older nuclear plants that are too expensive to keep repaired as incomes shrink because of low power prices. Davis-Besse and Perry nuclear power plants are included among the reactors at risk.

Cooper, who thinks nuclear energy's cost overruns and frequent shutdowns have always made it more expensive than it appears, recommends that the industry develop an orderly closing plan over the next few years, avoiding the rate chaos that unplanned closings might create.

"In 2013, more (nuclear) capacity retired early than in any year of the U.S. commercial nuclear sector," he said in a press briefing. "In recent months, four reactors have been closed in early retirement, five major up-rates (increases in generating capacity) were cancelled.

"The bottom line is that the tough times the nuclear power industry faces today are only going to get tougher. Over three dozen reactors in almost two dozen states are at risk of early retirement. And a dozen face the greatest risk of being shut down," he said.

The Nuclear Energy Institute, an industry group, dismissed Cooper's arguments out of hand, citing the volatility of natural gas prices as a major reason that utilities will be reluctant to rely too heavily on it as a fuel.

"The fundamentals in the electricity sector continue to present a strong argument for the value of nuclear energy," said institute senior spokesman Steve Kerekes. "Natural gas prices are low now, but historically this fuel source has experienced major price volatility.

Kerekes said electric utilities historically have been reluctant to rely too heavily on any single energy resource and value diversity of supply. Nuclear energy facilities are by far the largest source of electricity that don't emit greenhouse gases and other air pollutants. They provide nearly two-thirds of low-carbon electricity production in the United States."

Cooper's report concludes that "economic reality has slammed the door on nuclear power.

"In the near-term old reactors are uneconomic because lower cost alternatives have squeezed their cash margins to the point where they no longer cover the cost of nuclear operation. In the mid-term, things get worse because the older reactors get, the less viable they become. In the long term new reactors are uneconomic because there are numerous low-carbon alternatives that are less costly and less risk."

Cooper dismisses a small but growing number of gas industry analysts who predict that shale gas fields are not long-lived and that today's below-$4 gas prices will disappear in a decade along with the shale boom.

Even if they are correct, he said in response to a question, gas prices will be low enough in the immediate future -- and that will be long enough for utilities that own gas-fired plants to force some reactor owners to close them.

Cooper put FirstEnergy's Davis-Besse east of Toledo and Perry nuclear power east of Cleveland on the list of endangered reactors because they are stand-alone reactors, which can be more expensive to operate.

Also, the two plants have had problems forcing long shutdowns in the past, especially Davis-Besse, which had to be shut down for two years at the beginning of the last decade.

Jennifer Young, FirstEnergy spokeswoman, said, "To suggest that our plants are at risk is a stretch. Davis-Besse and Perry are both economically viable power plants. We are committed to operating them safely and reliably. And if we do that, they will remain economical."

Cooper stressed that he is not in the business of predicting which power plants are vulnerable, only crafting an analysis revealing some of the risk factors that could lead to closures.

Cooper has been a frequent critic of the industry in recent years. Asked in an interview if he is ideologically opposed to nuclear energy, he responded, "I am not anti-nuclear. I am pro-consumer."