The move comes following concerns over the safety aspects of nuclear faciltiies across the country, which is at war with North Korea, reports Reuters.
The hacker, using account dubbed as "president of the anti-nuclear reactor group", uploaded the internal data, including blueprints of reactors and radiation exposure estimates on social media on Friday.
The hacker also warned people to stay away from nuclear reactors and promised to post further details if the three reactors were not closed from Christmas.
Nuclear reactor design specialist Suh Kune-yull of Seoul National University was quoted by Reuters as saying: "And a compromise of nuclear reactors’ safety pretty clearly means there is a gaping hole in national security."
However, the South Korean government was quoted by the BBC as saying that the reactors’ central operating systems had not been breached.
The company said in a statement that an investigation is currently underway into the hacking and internal documents leak.
Large-scale cyberattack drills are scheduled to be performed on Monday and Tuesday at four of its 23 nuclear power plants, the company said.
KHNP spokesman was quoted by The Guardian as saying: "A two-day drill is under way through simulators to ensure the safety of our nuclear power plants under cyber-attacks."
The company operates 23 nuclear reactors, catering to about 30% of electricity needs in the country.
Image: KHNP operates 23 nuclear reactors in South Korea. Photo: courtesy of xedos4/Freedigitalphotos.net.
