The accident at the construction site in the southeastern city of Ulsan came as the state-run Korea Hydro and Nuclear Power Co was on high alert over a series of threats by hackers who claim they can disable the control systems of its plants.
Choi Hee-ye, a company spokeswoman, said there was no reason to believe that today's accident was linked to the cyberattack threats.
The company has yet to determine the cause of the accident, although the leaking of nitrogen was suspected, she said, adding that it wasn't immediately clear how nitrogen was used in the construction process.
Construction of the plant is scheduled to be completed next year.
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Last week, documents including layouts of nuclear facilities and personal information of nuclear workers were published online.
Local media said the unidentified people who posted images of the stolen nuclear documents imprinted with "WHO AM I?" on a South Korean blog were hackers. The company said that the safety of the country's nuclear power plants was not affected.