(Bloomberg) -- Southern Co. expects further delays and cost overruns at its Vogtle nuclear facility in Georgia amid testing issues and other construction snags, the latest setbacks for the first new U.S. reactors in three decades.

The two planned generation units will each power up three or four months later than previously estimated in 2022 and 2023, according to a statement Thursday from the Atlanta-based utility’s Georgia Power unit. Southern also estimated its total capital costs will rise by $460 million to $9.2 billion.

The project is the only remaining U.S. effort to build large Nuclear reactors. Such plants are the country’s largest source of emission-free power, but they’ve struggled to compete economically with lower-cost natural gas generators, wind and solar. When finished, Vogtle may be the last of its kind.

The expansion is now six years behind schedule and costs have doubled after it endured setbacks from the bankruptcy of contractor Westinghouse Electric Co. and more recently from the pandemic and labor shortages.

“Our expectations have slipped just a quarter, but you have to think about that this is a construction we have been working on for a decade,” Chief Financial Officer Andrew Evans said Thursday in a phone interview. “Our No. 1 priority is to construct an asset to produce power beyond the year 2100.”

Plant Vogtle, near Waynesboro, began commercial operations in 1987. It has two working reactors and is one of Southern’s three nuclear facilities. Unit 3 is now projected to start service in the second quarter of 2022, followed by Unit 4 in the first quarter of 2023, according to the statement.

In the second quarter, Southern completed critical trials for Unit 3 that demonstrated its systems working properly. The so-called hot functional testing is the most important, and the company plans to add nuclear fuel to the reactor by the end of this year, Evans said.

The formal acknowledgment follows a warning last month by a Georgia Public Service Commission employee, who testified that the third unit would likely be delayed by seven to nine months.Georgia Power’s share of the costs are now 26% higher than the $7.3 billion originally approved by state regulators. The utility owns 45.7% of Vogtle. Other major owners include Oglethorpe Power Corp. and the Municipal Electric Authority of Georgia.

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