The commission, however, said it will appeal the ruling as they underwent the proper decision-making process to extend the operation for 10 years.

Meanwhile, the reactor will be shut down after the Supreme Court makes the ruling sometime in the future, and the closure of the reactor will likely have little impact on the country's power supply, officials said.

"Power production of the Wolseong-1 reactor has been low, so its shutdown will have an insignificant influence," said an official from the Korea Hydro & Nuclear Power Co., a state-run electricity management firm. "We will wait and see for a while as the commission will file an appeal.

The reactor was shut down in 2012 after reaching the end of its 30-year commercial operation period. However, the commission lengthened the operation for another 10 years and restarted it in June 2015.

The decision sparked safety concerns in the wake of the 2011 Fukushima nuclear disaster in Japan.

South Korea has 23 nuclear reactors in operation from which it gets about 30 percent of its electricity supply.

Some 2,167 nearby residents participated in the collective suit, but the court recognized only those living within an 80-kilometer radius as qualified to file the suit. (Yonhap)