ATLANTA (AP) — The second of two new nuclear reactors in Georgia has entered commercial operation, capping a project that cost billions more and took years longer than originally projected.

Georgia Power Co. and fellow owners announced the milestone Monday for Plant Vogtle’s Unit 4, which joins an earlier new reactor southeast of Augusta in splitting atoms to make carbon-free electricity.

Unit 3 began commercial operation last summer, joining two older reactors that have stood on the site for decades. They’re the first two nuclear reactors built in the United States in decades.

The new Vogtle reactors are currently projected to cost Georgia Power and three other owners $31 billion, according to calculations by The Associated Press. Add in $3.7 billion that original contractor Westinghouse paid Vogtle owners to walk away from construction, and the total nears $35 billion.