A South Korean nuclear reactor went into automatic shutdown on Monday apparently after a malfunction, plant operators said, while ruling out a possible radiation leak.

The 1,000-megawatt reactor at Yeonggwang some 260 kilometres (156 miles) south of Seoul halted operations after warning signals around 3:00 pm (0600 GMT), the state-run Korea Hydro and Nuclear Power said.

An investigation was under way to determine the cause of the shutdown, it said, adding there was no danger of a radiation leak.

A warning signal and automatic shutdown can be triggered by a number of factors including malfunctions in the cooling pump, Yonhap news agency said.

South Korea operates 22 reactors, which meet about 35 percent of its electricity needs.

In February the country's oldest nuclear plant at Gori, built in 1978 near the southern city of Busan, briefly lost mains power and the emergency generator failed to kick in.

The incident resulted in no radioactive leaks but sparked an extensive probe amid concerns over nuclear safety following last year's atomic crisis in Japan.

In May, five engineers at Gori were charged with trying to cover up the potentially dangerous power failure and in July 32 people were charged with corruption involving the state nuclear power agency and its contracts with suppliers.

A nuclear safety agency approved the restart of the Gori plant this month, but it has yet to resume operations due to protests by civic groups.