South Korea on Thursday fired the head of the state-run company that oversees the country's 23 nuclear reactors over a forged documentation scandal that has shut a host of those reactors down.

Korea Hydro and Nuclear Power president Kim Kyun-Seop was dismissed from his post for the scandal involving parts provided with fake safety certificates, the Ministry of Trade, Industry and Energy said in a statement.

It added that An Seung-Kyoo, CEO of KEPCO Engineering and Construction, which is responsible for nuclear power plant design and technology, would also be sacked at a board meeting on Friday.

The ministry had vowed stern punitive action against any senior officials of the two companies if they were found involved in the scandal.

The move came after President Park Geun-Hye demanded action over what she called "unpardonable" corruption in the nuclear power sector.

State prosecutors have launched an extensive probe into the case which forced the shutdown of two reactors on May 28 and delayed the scheduled start of operations at two more.

At proper capacity, South Korea's nuclear reactors supply more than 35 percent of national electricity needs.

But 10 of 23 reactors are currently offline for various reasons, prompting government warnings of serious power shortages.

All parts supplied for use in the reactors require quality and safety warranties from one of 12 international organisations designated by Seoul.

Last year, officials said eight suppliers were found to have faked warranties covering thousands of items used in a number of reactors. In early May six nuclear engineers and suppliers were jailed for their part in the scandal.

South Korea's nuclear sector has been dogged by a series of malfunctions, forced shutdowns and corruption scandals that have undermined public confidence already shaken by the 2011 Fukushima nuclear disaster in Japan.

Despite increasing public concern, the government has vowed to push ahead with its nuclear power programme and plans to build an additional 16 reactors by 2030.