South Korea's nuclear safety watchdog said Friday it has launched an investigation at two power plants after a corruption scandal involving locally-made copies of foreign components.

The Nuclear Safety and Security Commission (NSSC) is conducting probes at plants in Gori near the southeastern city of Ulsan and in the southwestern county of Yeonggwang, a spokeswoman said.

The Korea Hydro and Nuclear Power Corp (KHNP), which operates the nation's atomic plants, said the two plants were using components developed by a local company but based on illegally obtained French technology.

Manuals on the French components produced by Areva were "illegally leaked" in the course of finding local suppliers for key parts, KHNP said in a statement.

The components are seals used to prevent neutrons from leaking, it said.

The case was uncovered last week during an investigation by prosecutors into corruption at the plants.

"Our investigation began in late March and four KHNP officials have been arrested so far for taking bribes from (local) suppliers, along with a broker," a prosecutor in Ulsan told AFP on condition of anonymity.

An official identified only as Heo, in charge of managing component suppliers at Gori, is accused of giving a technology manual on the original component to a local manufacturer in 2009.

Prosecutors said the manufacturer delivered copied parts after being selected as a supplier a year later. In return, Heo received 80 million won ($70,547).

A KHNP spokesman said the local manufacturer had now acquired patent rights in Korea and the safety of the parts it supplied had been verified.

"The units are not counterfeit and they are not so crucial to the safety of reactors," he said. "However... there is no excuse for illegally leaking (the manual) to the local company."

Dong-A Ilbo newspaper called the incident a "red light" for the safety of nuclear plants, which generate about a third of the country's electricity.

And the Korea JoongAng Daily, in an editorial headed "Nuclear warning bell", said there had been previous similar cases of corruption involving parts.

Jan Vande Putte, Greenpeace International nuclear expert who is currently visiting the city of Busan near Gori, said the incident shows "that the Korean nuclear industry is out of control.

"The illegally copied components were not certified by anybody," Vande Putte said in an emailed comment to AFP.

"We have no idea what's happening inside Korean reactors as we cannot trust the Korean regulator, the NSSC, to have a proper grip on this industry."

Concern grew about nuclear safety after engineers covered up for one month an electricity failure at the ageing Gori-1 reactor.

The reactor briefly lost mains power in February and the emergency generator failed to kick in. The power cut forced cooling water to stop circulating.

The incident resulted in no radioactive leaks but sparked an uproar over safety.

South Korea relies on 20 nuclear reactors to meet about 35 percent of its electricity needs. The government has vowed to stick to the programme despite last year's nuclear disaster in Japan.