Last week, designs and manuals of plant equipment owned by Korea Hydro and NuclearPower Co (KHNP) were put online by an unknown individual or group.
A threat was made that unless three reactors were closed by Christmas, people should "stay away" from them.
KHNP said the leaked data did not undermine the safety of the reactors.
KHNP, the sole nuclear operator in Korea, is part of the state-run utility Korea Electric Power Corp.
In a statement, the operator said it would conduct a series of large-scale drills at four nuclear power plant complexes on Monday and Tuesday.