Officials at the Nuclear Safety and Security Commission also put off starting operations at two other reactors, Yonhap News reported.
The commission said it had learned substandard parts with fake quality warranties had been used when the two reactors, Shin Kori Reactor 2 and Shin Wolsong Reactor 1, were being built.
The commission's action also affected the Shin Kori Reactor 1, which had been undergoing regular maintenance and scheduled to resume operation, and the new Shin Wolsong Reactor 2, which had been under a review for the start of its commercial operation.
The commission said operations will not resume until all the substandard parts had been replaced.
Officials said there is no danger of a radiation leak.
However, Yonhap said the shutdown of the two reactors would affect supply of electricity, which reaches a peak during summer.
South Korea's 23 reactors currently account for 30 percent of total electricity consumption, and their suspension could sharply bring down power reserve levels, Yonhap said.
The development comes after a corruption scandal late last year involving more than 13,000 substandard parts supplied under fake quality warranties, the report said. Operations at two reactors at a plant 200 miles south of Seoul were suspended at the time.