South Korean officials say minor cracks were found in control rod tubes in a nuclear reactor that is undergoing maintenance.
Friday's announcement comes five days after two other reactors at the same plant in Yeonggwang county were shut down to replace parts that had been provided with fake safety certificates.
A spokeswoman for the Korea Nuclear Safety and and Security Commission said it is the first time such cracks have been found in a reactor, but there is no danger of leakage.
The country's nuclear watchdog said half of Yeonggwang's six reactors will now be offline until January at the earliest, raising concerns of power shortages during the harsh winter months.
South Korea's 23 nuclear reactors supply more than one-third of electricity in the country.
After the meltdown of three reactors in Fukushima, Japan in March of 2011, many people in resource-poor South Korea are questioning whether the risks of nuclear power outweigh the benefits.