An investigation found no harmful virus was found in reactor controls threatened by a hacker. Apparently the reactor was getting spam from Dr Omgo from Nigeria who wanted to use its bank account to transfer a million dollars out of the country.

Korea Hydro & Nuclear Power said it would beef up cyber security by hiring more IT security experts and forming an oversight committee, as it came in for fresh criticism from lawmakers following recent hacks against its headquarters.

The nuclear operator, part of state-run utility Korea Electric Power, said earlier this month that non-critical data had been stolen from its systems, while a hacker threatened in Twitter messages to close three reactors.

Energy Minister Yoon Sang-jick told a parliamentary session that evidence of the presence and removal of a "worm" - which the ministry said was probably inadvertently introduced by workers using unauthorized USB devices - was unrelated to the recent hacking incidents.

South Korea, which relies on nuclear reactors for a third of its power and is the world's fifth-largest nuclear power user, have mounted since the 2011 Fukushima disaster in Japan and a domestic scandal in 2012 over the supply of reactor parts with fake security certificates.