Operators at the Younggwang Nuclear Power Plant in South Cholla Province said operations of Unit No. 1 were suspended after a malfunction was detected.
"There was no radiation leakage, and once experts determine the exact cause of the malfunction, operations will begin again," said a spokesman for the plant, located 322 kilometres south of Seoul.
Minor malfunctions that could lead to a shutdown are not reported to the International Atomic Energy Agency.
The generator is one of six units in the power plant. It went into commercial operation in August 1986 and has generates 950 megawatt of electricity.
In addition to the Younggwang plant, South Korea operates 20 nuclear reactors throughout the country that fuel 40 per cent of the country's electricity needs.