The accident occurred at 7 p.m. Monday during repair work on a cooling water pump at Wolsung nuclear power plant, the science and technology ministry said in a statement. About 12 gallons of radioactive water leaked, the ministry said.
"It did not leak outside of the building. It did not affect the environment," the ministry said in the statement.
Those who were exposed to the radiation included employees of Korea Electric Power Corp., which runs the plant.
A statement from South Korean officials says the leak was contained inside a building, and did not affect the environment.
It says the leak took place Monday evening during repair work on a cooling water pump.
The leak follows Japan's worst-ever nuclear accident last Thursday when workers mixing a uranium solution triggered a nuclear chain reaction at a uranium processing plant in Tokaimura, about 90 miles northeast of Tokyo.
Fifty-five people, mainly plant workers and emergency personnel who responded to the Tokaimura accident, were exposed to the radiation and three remain in serious condition. South Korean government officials said after Japan's incident that they did not anticipate similar accidents occurring in Korea.
"The amount leaked was small and not harmful," the spokesman told Reuters by telephone.
The spokesman said the leaked heavy water was about 11.7 gallons and the leak was contained within the plant. The level of radiation was 440 millirem, 9 percent of legally allowed radiation exposure, he added.