WASHINGTON — The Energy Department is poised to approve $6.5 billion in federal loan guarantees for the first nuclear power plants built from scratch in this country in more than three decades.

Energy Secretary Ernest Moniz was expected to announce final approval of the deal at a speech on Wednesday, a day before he visits the $14 billion Vogtle nuclear plant now under construction in eastern Georgia.

Three government officials familiar with the deal confirmed its details Tuesday. They asked not to be identified because the deal has not been made public.

Atlanta-based Southern Co. is building the plant with several partners about 30 miles southeast of Augusta, Ga. The project is widely considered a major test of whether the industry can build nuclear plants without the endemic delays and cost overruns that plagued earlier rounds of building in the 1970s. Vogtle was originally estimated to cost around $14 billion, but government monitors have warned the final cost is likely to be higher.

The Energy Department tentatively approved an $8.3 billion loan guarantee for the project in 2010 as part of President Barack Obama’s pledge to expand nuclear power and other energy sources.

Obama and other proponents say greater use of nuclear power could cut the nation’s reliance on fossil fuels and create energy without producing greenhouse gas emissions blamed for global warming. More than two dozen nuclear reactors have been proposed in recent years, but experts now say it is likely that only five or six new reactors will be completed by the end of the decade. The once-expected nuclear power boom has been plagued by a series of problems, from the prolonged economic downturn to a sharp drop in natural gas prices and the 2011 Fukushima nuclear disaster in Japan.

Owners of at least four nuclear reactors have shuttered plants in recent months or announced plans to do so, including California’s troubled San Onofre nuclear plant. Utilities have decided it is cheaper to close plants rather than spend big money fixing them and risk the uncertainty of safety reviews.

Southern Co. officials told Wall Street analysts in late January that they had submitted the paperwork necessary to get the loan guarantees, but company spokesman Tim Leljedal would not discuss Tuesday whether a deal had been reached.

“We’re committed to enhancing the customer value of the project through various avenues including the pursuit of loan guarantees,” he said.

The company has said federal assistance was not needed to finish the project. The company was able to borrow $4.3 billion privately in 2012. Completion of the project is not expected until at least 2018. Georgia Public Service Commissioner Stan Wise said state officials have been invited to a signing ceremony Thursday. He had not been briefed on the terms.

Government officials familiar with the deal said two loan guarantees totaling $6.5 billion will be offered to Georgia Power, a Southern subsidiary, and Oglethorpe Power Co., a partner on the project. A separate agreement guaranteeing $1.8 billion was still being negotiated with a third Vogtle partner, the Municipal Electric Authority of Georgia.