WASHINGTON — The Nuclear Regulatory Commission voted 4-to-1 on Thursday to grant a license to build two reactors at a nuclear plant in Georgia — a milestone for an industry whose long-hoped-for renaissance has been smaller and later than anticipated.

It was the first time the commission decided to grant a license for a new reactor since 1978, a year before the Three Mile Island accident in Pennsylvania. In anticipation of approval, the plant’s owner, the Southern Company, has already dug a foundation and begun laying water pipes at its Alvin W. Vogtle plant near Augusta, Ga.

The sole vote against approval was cast by the commission’s chairman, Gregory B. Jaczko. He said the construction and operating license would not assure that all of the safety improvements mandated by the agency in response to Japan’s Fukushima disaster would be accomplished before the reactors begin operating in 2016 and 2017.

The plant, which already has two reactors dating from the 1980s, will be the largest nuclear complex in the United States.

Southern and its partners are swimming against the tide, betting that even with the project’s approximately $14 billion price tag, nuclear power will prove cheaper than using coal, natural gas or any other source over the estimated 60-year lifetime of the reactors. Few other companies are willing to take the gamble, given the current economic and political picture.

Natural gas prices are at low levels, and pressures to address global warming by putting a price on carbon emissions have faded, diminishing hopes that nuclear energy will be championed as a cleaner alternative to fossil fuels.

“In the current environment, we have the combination of low natural gas prices, abundant supplies of natural gas, relatively modest electricity demand growth coming out of the recession and the lack of a price on carbon,” said James K. Asselstine, managing director of Barclays Capital, who served on the regulatory commission from 1982 to 1987. “It’s probably unlikely we’ll see other plants move to construction in the very near term.”

Vogtle 3 and 4 were supposed to be the first in a wave of new projects after the Bush administration set aside $17.5 billion in loan guarantees for nuclear projects. But the nuclear rebirth has been so puny that much of that money is still available. The Energy Department has promised an $8.3 billion loan guarantee for the two reactors.

Mr. Asselstine and others say that the only other reactors that conceivably could be built in the next few years are two planned at the V.C. Summer plant in South Carolina.

Both Georgia and South Carolina regulate their utilities in the traditional way, allowing them to collect whatever they prudently invest, plus a profit margin, from their customers through electricity rates.

Georgia Power, the Southern Company subsidiary building the plant, said in a 2008 filing with the state Public Service Commission that electricity generated by Vogtle 3 and 4 could cost customers $6.5 billion less than power generated by coal or natural gas in the lifetime of the reactors.

In much of the Northeast and Midwest and in California, power plants are mostly built by third parties that sell the electricity in daily auctions on the open market, a setup that makes a long-term bet against natural gas difficult for a private company to swallow. For example, Constellation Energy, based in Baltimore, gave up on a third unit at its Calvert Cliffs site in 2010 after the federal government sized up the economics and demanded a huge fee in exchange for a loan guarantee.

Some industry experts and antinuclear groups argue that the Vogtle project poses major risks. “The potential is high for cost overruns, regulatory problems, outage issues, competing water needs in the state, drought situations, radioactive waste management issues and a range of ratepayer issues,” the Southern Alliance for Clean Energy, an advocacy group, said in a filing with the Georgia Public Service Commission.

The new reactors would produce electricity at a cost of 7 to 9 cents per kilowatt-hour when Southern instead could be reducing demand by investing in efficiency at a cost of 3 cents per kilowatt-hour, the group said.

Nine organizations, including the Southern Alliance, said this week that they would sue to try to block the license because the commission had not adequately analyzed the new reactors’ design for hazards in response to last year’s calamity at Japan’s Fukushima Daiichi plant. The commission has ordered existing plants to adopt some safeguards in response to the Fukushima calamity and is considering others.

But other analysts say the decision could look quite smart in retrospect, depending on market and regulatory factors. “What happens if we get fracking legislation?” said Eric Beaumont, an analyst at the firm Copia Capital, referring to pressure for tougher limits on a drilling technique that has depressed natural gas prices and greatly expanded reserves.

“Natural gas isn’t going to remain cheap forever,” he said.

Carbon policy is another wild card, Mr. Beaumont said, suggesting that the political pendulum could shift back toward imposing a tax on emissions of greenhouse gases. With a few changes, “lo and behold, it can make sense” to build the reactors, he said.

Aside from the 34-year hiatus in approval of construction, the commission’s action will be a milestone in another way.

The two reactors are of a kind that has never been built before, the AP1000, a model by Westinghouse that is supposed to withstand earthquakes and plane crashes and be less vulnerable to operator error or to a loss of all electricity, the last of which caused the triple meltdown at Fukushima.

Nuclear advocates point out that the design is virtually complete. By contrast, designers of reactors in the last round of construction, in the 1960s, ’70s and ’80s, were usually half a step ahead of the builders, resulting in repeated redesigns and cost increases.

Another plus is that China has four AP1000s ahead of Vogtle in the pipeline, enabling Southern to dispatch teams to watch those plants being built.

The company estimates that 3,400 workers will be on site in Waynesboro at the peak of construction, up from 1,500 now.