A South Korean nuclear power plant has launched a two-day safety drill after the company had its computer systems hacked.

Last week the designs and manuals of equipment from Korea Hydro & Nuclear Power’s (KHNP) plant were put online by an unknown group or individual who named themselves the “president of the anti-nuclear reactor group”.

According to the BBC, a threat was made to the nuclear power company that unless three of its 23 reactors were closed by Christmas, people should “stay away” from them.

The BBC reports the energy company said the leaked data did not undermine the safety of the reactors.

KHNP is a subsidiary of Korea Electric Power Corporation (KEPCO) and provides roughly 30% of power supply in Korea.