Last month's hacking of South Korea's nuclear operator means the country's second-oldest reactor may be shut permanently due to safety concerns, said several nuclear watchdog commissioners, raising the risk that other ageing reactors may also be closed. "The operator failed to prevent [the hack], and they don't know how much data has been leaked. If the old reactor is still allowed to continue to run, it will just hike risks," said Kim Hye-jung, one of the nine commissioners who will this month review an application to restart the Wolsong No1 reactor. The future of Wolsong No 1, shut in 2012 after reaching its 30-year lifespan, is seen as critical to the fate of other reactors, including the oldest Kori No 1, which had its lifespan extended by 10 years to 2017. Nuclear power accounts for about a third of South Korea's electricity supply. Korea Hydro and Nuclear Power Co, which operates the small 679-megawatt Wolsong No 1, has said its headquarters computer system was hacked last month, but that all reactors remain safe. Responsibility for the hacking was claimed by unidentified anti-nuclear activists. The operator, part of state-run utility Korea Electric Power Corp, has been seeking to restart Wolsong No.1 and runs 23 reactors, producing a third of South Korea's power. The Nuclear Safety and Security Commission will start on January 15 at the earliest to review whether to approve to extend Wolsong's lifespan by 10 years to 2022. Five of nine commissioners said they held doubts over the viability of Wolsong No 1. "The reactor has completed its designed lifespan. Even if modified, still it is not safe," said commissioner Kim Ik-jung. If five vote against a restart, the reactor will be shut permanently. The other commissioners were not reachable or declined to comment. Korea Hydro and Nuclear Power argues that its reactors are safe as they are inaccessible from external networks, such as the company's hacked computers or the internet. A spokesman at Korea Hydro and Nuclear Power said the company would accept whatever the nuclear watchdog decides. Even those commissioners who deem the reactor safe think that other issues such as public pressure will likely influence their decision, which is expected next month at the earliest. Concerns over nuclear power have grown since the Fukushima disaster in Japan in 2011 and revelations in 2012 of fake certificates for reactor parts in South Korea. More nuclear closures would boost South Korea's fuel imports, which had soared since late 2012 after some reactor closures forced Asia's fourth-largest economy to replace nuclear power with liquefied natural gas and thermal coal.