The Vogtle Unit 3 near Waynesboro, in the southeastern state of Georgia, has begun serving some 500,000 homes and businesses, operator Georgia Power said in a statement.

"The new unit represents a long-term investment in the state's clean energy future and will provide reliable, emissions-free energy to customers for decades to come," the company said.

The project comes online seven years after it was supposed to start producing power.

The costs for Unit 3, and a fourth reactor anticipated to enter operation in late 2023 or early 2024, have topped $30 billion, according to an estimate by the Municipal Electric Authority of Georgia (MEAP).

That is more than double the budget of $14 billion announced at the start of the project.

Cost overruns pushed original investor Westinghouse, a subsidiary of Japan's Toshiba, to file for bankruptcy in 2017, and it withdrew from the project.

Once all four units are in operation, Vogtle will become the largest generator of clean energy in the nation, according to Georgia Power.

Vogtle Units 3 and 4 were the first new reactor projects approved by US authorities since 1979 and an incident at Pennsylvania's Three Mile Island, the most serious nuclear accident in US history.

The most recent prior commission of a nuclear reactor was Unit 2 at Watts Bar Nuclear Plant in Tennessee, in 2016.

Construction for that reactor had begun in 1973. Work was suspended for more than two decades before the project was revived.

Low-carbon nuclear power has been hailed as a climate-friendly energy source as the world struggles to combat global warming.

Environmentalists, however, worry about safety and warn that disposing radioactive waste carries huge risks.

Since 1990 only three reactors have entered into operation in the United States: the two units at Watts Bar, in 1996 and 2016, and Vogtle Unit 3 on Monday.

No other conventional reactor project is underway.

Construction of two reactors at the Virgil Summer Nuclear Station in South Carolina was abandoned in 2017, despite $9 billion already invested in the projects.

Manufacturers have now reoriented themselves towards smaller, new-generation facilities known as small modular reactors, or SMRs.

The newer designs -- none of which have yet to come into operation in the United States -- are expected to be less expensive, take less time to build and are considered safer than conventional power plants.