ATLANTA — A nuclear reactor at Plant Vogtle in eastern Georgia is back online after an unexpected shutdown Saturday.

Southern Co. officials said that the Unit 1 reactor was returned to service Monday afternoon after workers fixed a problem with a water pump.

Utility spokeswoman Amoi Geter said a problem was found in an electrical card in the control room that is connected to a feedwater pump. That pump provides water to steam generators. The card has now been replaced. The company said tests show that the electrical part and the pump are functioning as designed

The second reactor at the nuclear plant near Augusta was not affected and has continued producing electricity.