The Obama administration signed off Thursday on a $6.5 billion loan guarantee to help Georgia utilities build the nation’s first new nuclear reactors in more than three decades.

Energy Secretary Ernest Moniz traveled to the sprawling Plant Vogtle site in east Georgia to announce the loan aid for Southern Co. and Oglethorpe Power. Another $1.8 billion loan for a third company involved, MEAG, is still in the works.

“This plant will be the economic engine for the state of Georgia,” said Georgia Power chief executive Paul Bowers. “We are proud to be the first utility to restart the nuclear renaissance here in America.”

The two nuclear reactors underway at Vogtle were supposed to be the first in a wave of new nuclear construction as part of a push toward cleaner energy when the Obama administration announced the aid in February 2010. But the boom never materialized.

Stalled efforts to limit carbon emissions, plunging natural gas prices and other stumbling blocks threatened the broader revival. And four years of behind-the-scenes wrangling imperiled the loans for the Vogtle plant. Moniz said an agreement was only reached when he and executives huddled in a “small room and said, ‘Dammit it’s time to get this thing done.’ ”

With the announcement, Vogtle became the first project to receive the federal loan guarantee since Congress established the program in 2005. Bowers said it would help save Georgia Power customers $250 million.

Critics cited safety concerns surrounding nuclear power plants, and pointed to the disaster at Japan’s Fukushima nuclear plant after an epic tsunami there. “Fewer than three years have passed since the tragedy at Fukushima demonstrated that nuclear reactors can never be safe,” said Katherine Fuchs of Friends of the Earth. “Yet the president and energy secretary are ignoring its lessons.”

But Moniz said the federal guarantee is part of a broader “all of the above” effort to offer a range of alternative energy sources that could lower carbon emissions.

“If we don’t move out with these kinds of projects, we won’t be ahead of the train,” said Moniz. “And we in the United States shouldn’t be running to catch up with the caboose.”