Bae Kyong-min, an official at the Korea Hydro and Nuclear Power Co., said nitrogen gas leaked from an underground cable is believed to have caused the deaths of the three workers at the reactor construction site in Ulsan, about 410 kilometers southeast of Seoul.
The New Gori No. 3 reactor with a rated capacity of 1,400 megawatts was 99 percent complete for full operation scheduled for next June, according to the company.
The accident came days after the state nuclear power operator suffered a recent series of cyberattacks that raised a safety concern in a country that relies on 23 nuclear reactors for one-third of its energy needs.
Over the past week, an anonymous anti-nuclear group had posted a series of documents and operating manuals for some South Korean nuclear reactors on the Internet.
On Wednesday, a joint investigation team of government and prosecution officials said a suspect in the hacking attack was found to have used multiple Internet protocol addresses — the online equivalent of a street address or a phone number — based in China.
South Korean officials have not ruled out the possible involvement of North Korea, which has a track record of waging cyberattacks on major financial institutions, government web sites and media organizations in South Korea. North Korea has flatly denied its involvement.