South Korea has been forced to shut down two nuclear reactors to replace components provided with fake quality certificates, a minister said, warning of "unprecedented" power shortages.

Knowledge economy minister Hong Suk-Woo stressed the "non-core" components posed no safety threat and were unrelated to a string of systems malfunctions at reactors this year which triggered calls for a safety review.

The two affected reactors at the Yeonggwang nuclear complex in the south west may have to remain offline until early January, as engineers replace more than 5,000 fuses, cooling fans and other parts provided by eight suppliers.

"Comprehensive safety check-ups are necessary at these two reactors where the uncertified parts were used extensively," Mr Hong said.

"It's inevitable that we will experience unprecedented power shortage during the coming winter with the two reactors shut."

Mr Hong said prosecutors had been asked to investigate the suppliers who faked the quality certificates as well as possible collusion by officials of the state-run Korea Hydro and Nuclear Power.

South Korea operates 23 nuclear power reactors which meet more than 35 per cent of the country's electricity needs.

It plans to build an additional 16 reactors by 2030.

AFP