A South Korean minister has sounded a warning about "unprecedented" power shortages after two nuclear reactors were shut down to replace components that had not been properly vetted.
The two units at the Yeonggwang nuclear complex were shut down on Monday and may remain offline until early January.
"It's inevitable that we will experience unprecedented power shortage during the coming winter with the two reactors shut," Hong Suk-Woo, the economy minister, said.
However, he said the "non-core" components posed no safety threat and were unrelated to a string of systems malfunctions at reactors this year that triggered calls for a safety review.
Last month, authorities temporarily shut down two 1,000-megawatt reactors at separate nuclear plants after system malfunctions which were also blamed for another reactor at Yeonggwang being tripped into automatic shutdown in July.
Engineers will replace more than 5,000 fuses in the units shut down, cooling fans and other parts for which suppliers had provided bogus quality certificates.
"Comprehensive safety check-ups are necessary at these two reactors where the uncertified parts were used extensively," Hong said.
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.
The government has pledged to continue using nuclear energy despite public concerns arising from last year's nuclear disaster in Japan.
If the two Yeonggwang reactors are not brought back online as scheduled, Hong warned of a "dramatic" drop in national power reserves to 300,000 kilowatts in January, compared to the government target of 4.5 million kilowatts.
"Energy authorities are preparing a super-intense power supply emergency plan, which will be carried out in mid-November," Hong said, without elaborating.
All parts supplied for use in South Korea's nuclear plants require quality and safety warranties from one of 12 international organisations designated by Seoul.
Eight suppliers cited by Hong faked 60 warranties covering nearly 7,700 items that had been provided at a cost of $750,000, Hong said.
Of the total, more than 5,200 parts have been used in five reactors - 99 per cent of them in the two Yeonggwang units closed on Monday.
Hong said prosecutors would investigate the suppliers as well as possible collusion by officials of the state-run Korea Hydro and Nuclear Power (KHNP).