The announcement Monday coincides with a power shortage warning by the government amid a heat wave that has stretched for 10 days.
Built in 1977, the Kori-1 reactor, in the southern port city of Busan, had been shut down since March after it briefly lost power in February during a safety check.
That blackout was covered up by officials for more than a month.
The International Atomic Energy Agency gave the go-ahead for the 578-megawatt reactor to restart following a safety check in June. While Korea's Nuclear Safety and Security Commission approved the restart on July 4, it has faced strong opposition from activists and residents.
Nature magazine reports Hiromitsu Ino, an emeritus professor of materials science at the University of Tokyo, as saying that Kori-1 isn't safe to operate because the weld material in the pressure vessel has degraded.
"Any 50 nuclear power plants in Japan are much better than Kori-1," he said.
And Hwang Il-soon, a nuclear scientist at Seoul National University, points to the cover-up of the February blackout as a cause for concern.
"The most serious issue is that staff in the control room decided not to report the more than 10 minutes of blackout and tried to hide this accident," Hwang said.
Since the Kori-1 incident, Korea's nuclear-safety authority hasn't been forthcoming enough about what is being done to ensure that another dangerous situation won't be repeated, he said.
While a government investigation has found that Kori-1's pressure vessel is safe, Hwang said, a more democratic process should be in place to get a consensus from local residents about reactor restarts.
"We keep talking with residents but reaching a consensus is expected to take time. There is no choice but to restart the operation of the Kori-1 reactor," Hong Suk-woo, minister of Knowledge Economy, said July 26, calling for the restart date to be no later than last Saturday.
In a statement Monday, Hong hinted at closer collaboration between the public and nuclear power plant operators, saying, "We have confirmed that the nuclear power plant and the community should develop together after the Kori-1 case."
The 30-year operational life span of the Kori-1 reactor finished in 2007 but after a 2008 inspection by the International Atomic Energy Agency, the reactor was declared safe for another 10 years.
South Korea has 23 nuclear reactors in operation, supplying about one-third of the country's electricity demand.
Seoul aims to increase the country's share of nuclear power to 40 percent by 2040.