South Korea shut down two 1,000-megawatt nuclear reactors at separate plants Tuesday, following apparently unrelated systems malfunctions that triggered calls for a safety review.

"Both power plants remain stable and pose no threat of radiation leakage," the state-run Korea Hydro and Nuclear Power (KHNP) said in a statement, adding that investigations were under way to determine the cause of the malfunctions.

A KHNP spokesman told AFP there was "no correlation" between the two incidents -- at Yeonggwang on the southwest coast and Shingori on the southeast coast.

The Shingori reactor, near the city of Busan, was shut down after a warning signal at 8:10 am (2310 GMT Monday).

"There was a malfunction in the reactor's control rod, but the reactor is now stable," the spokesman said.

It is the first time the reactor has been shut down since it began operations in February last year.

South Korea operates 23 nuclear power reactors which meet more than 35 percent of the country's electricity needs. It plans to build an additional 16 reactors by 2030.

In the other incident at the Yeonggwang plant, a reactor automatically shut down around 10:45 am after its steam generator showed a low water level.

In July, a different 1,000-megawatt reactor at Yeonggwang -- some 260 kilometres (156 miles) south of Seoul -- went into automatic shutdown after a malfunction.

The South Korean government has vowed to stick to its nuclear power programme despite public concerns arising from last year's nuclear disaster in Japan.

Doubts over safety standards were fuelled in May when five senior engineers were charged with trying to cover up a potentially dangerous power failure at South Korea's oldest nuclear plant.

The five, including a 55-year-old chief engineer at the Gori-1 reactor, were accused of violating a law on nuclear safety.

The reactor, built in 1978 near the southern city of Busan, briefly lost mains power on February 9 and the emergency generator failed to kick in. The power cut caused cooling water to stop circulating.

Following Tuesday's shutdowns, the Korean Federation of Environment Movement, a coalition of civic environmental groups, called for a further review of safety standards.

"By good fortune, there was no radiation leak, but the frequent stoppage of nuclear power plants is a bad sign," the group said in a press release.

"Problems in the control rod -- a key safety feature to control nuclear fission -- could lead to major accidents when coupled with other natural disasters or malfunctions.

"The KHNP and the Nuclear Safety and Security Commission must identify the fundamental problem and resolve the people's fundamental concerns," it said.