The BBC's Andrew Wood reports: "There was no release of radiation outside of the plant"

The accident at the Wolsung plant, south-east of Seoul, happened at 7pm local time (1000GMT) on Monday during maintenance and safety checks, the Science and Technology Ministry said. The nuclear reactors were not in operation at the time.

Ministry officials said the exposure - the first-ever case of group contamination in South Korea - was "not that severe".

"All those exposed to radiation have now turned out to be safe," a ministry spokesman said. "None of them were hospitalised."

About 45 litres of so-called "heavy 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 were employees of Korea Electric Power Corp, which runs one of the three nuclear power plants in Wolsung.

The state-owned company operates a total of 14 nuclear power plants.

Nuclear power dependency

Several minor incidents, including leaks and small fires, have occurred at the Wolsung complex in the past,.

Environmental groups have been calling for tighter controls and checks for several years.

The plant where the accident happened went live in July last year.

South Korea relies heavily on nuclear power for its electricity - 40% is produced by nuclear power plants.

Echoes of Japan accident

The accident came five days after Japan's biggest nuclear accident happened at Tokaimura, north east of Tokyo.

That accident happened when workers mixing a uranium solution triggered a nuclear chain reaction at a uranium processing plant.

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.