South Korea has shut down two nuclear reactors after it was revealed that some parts used had not been properly vetted, an official says.
Knowledge Economy Minister Hong Suk-woo said these were "non-core" parts and were not a safety threat.
They included fuses, cooling fans and power switches that did not have the required nuclear industry certificates.
The shutdown means there could be "unprecedented" power shortages in the next few months, Mr Hong said.
The more than 5,000 parts could be used in other industries but needed international certification for nuclear power plant usage, he said.
Almost all the parts were used at the Yeonggwang Nuclear Power Plant, in the south-west, where the two reactors were shut down.
"Comprehensive safety check-ups are necessary at these two reactors where the uncertified parts were used extensively," the minister said.
"It's inevitable that we will experience unprecedented power shortage during the coming winter with the two reactors shut."
He said the parts, worth 820m won ($750,000, £467,800), had been sourced from eight suppliers since 2003.
South Korea's 23 nuclear reactors, which supply 35% of the country's electricity, have experienced a series of malfunctions over the past few months.
While none have posed a public risk, opposition to the government's bid to vastly expand its nuclear industry has been growing, says the BBC's Lucy Williamson in Seoul.