Two South Korean 1,000-megawatt nuclear reactors were shut down on Tuesday at separate plants, both as a result of unrelated systems malfunctions, operators said.
The state-run Korea Hydro and Nuclear Power (KHNP) said there was no apparent danger of a radiation leak at either reactor - at Yeonggwang on the southwest coast and Shingori on the southeast coast.
"There is no correlation between the two incidents," a KHNP spokesman told AFP.
The Shingori reactor, near the city of Busan, was shut down after a warning signal at 8.10am (2310 GMT Monday).
"There was a malfunction in the reactor's control rod, but the reactor is now stable with no danger of a radiation leak," 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 plants which meet more than 35% of the country's electricity needs.
The other incident at the Yeonggwang plant saw an automatic reactor shutdown following the malfunction of a water supply pump, the spokesman said.
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 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.