Last week, designs and manuals of plant equipment owned by the state-run Korea Hydro and Nuclear Power company (KHNP) were leaked online.
A threat was made that unless three reactors were closed by Christmas, people should “stay away” from them.
The cyber-attacks come amid concerns that North Korea may attempt hacking industrial and social targets after accusations by the United States that Pyongyang was responsible for hacking Sony Pictures.
South Korea is still technically at war with the North.
South Korea‘s energy ministry said it was confident that its nuclear plants could block any infiltration by cyber attackers that could compromise the safety of the reactors.
“It’s our judgment that the control system itself is designed in such a way and there is no risk whatsoever,” the deputy energy minister Chung Yang-ho, said.
An official at KHNP said: “It is 100 per cent impossible that a hacker can stop nuclear power plants by attacking them because the control monitoring system is totally independent and closed.”
Both the government and KHNP said they could not verify messages posted by a Twitter user claiming responsibility for the attacks and demanding the shutdown of three aging nuclear reactors by Thursday.