The state-run company said it has also completely divided its nuclear plant control systems from its internal computer networks and restricted both systems' access to the Internet.
USB ports of the nuclear power plant control systems have also been sealed, it added.
The network separation is the safest way of protecting the nuclear power plants from hacking attacks from the outside, the company explained.
South Korea has 23 reactors nationwide, which generate around 35 percent of the country's electricity.
Last week, South Korea's government announced that North Korea was behind the massive hacking attack that paralyzed the networks of local financial firms and broadcasters last month, though the North later denied its involvement.
Three South Korean banks -- Shinhan, NongHyup and Jeju -- and their insurance affiliates as well as three TV broadcasters -- KBS, MBC and YTN -- were hit by the cyber attack as malicious code infected some 48,000 computers in their networks on March 20.
North Korea is known to operate a cyber warfare unit of 3,000 elite hackers who are trained to break into computer networks to steal information and distribute malware.
In addition to the hacking attacks, the North has been interfering with global positioning system signals since 2010, disrupting hundreds of South Korean commercial ships and flights in the border area.
Korean Language
사이버테러NO! 한수원 내외부전산 분리
한국수력원자력은 업무용 전산망과 인터넷을 물리적으로 완전히 분리했다고 14일 밝혔다.
사이버 테러의 위협으로부터 안전성을 확보하려는 조치라고 설명했다.
원자력 발전소의 운영에 관여하는 원전제어시스템을 인터넷은 물론 업무용 전산망과도 분리된 폐쇄 시스템으로 돼 있다. USB 포트도 봉쇄돼 있다.
한수원은 전산망 분리가 지능화된 공격으로부터 시스템을 보호할 수 있는 가장 안전한 방식이라고 소개했다.