South Korea's nuclear safety authority said Wednesday several engineers had covered up an electricity failure at an ageing atomic plant last month, and promised they would be punished.

The Nuclear Safety and Security Commission (NSSC) expressed regret that the incident had been covered up for more than a month until it came to light last week, creating a stir across the country.

"A chief engineeer and his colleagues had covered it up for more than a month," NSSC chairman Kang Shang-Sun told a televised press conference.

"They will be thoroughly held responsible, legally and administratively as well," he said.

The Gori-1 reactor near the southern port of Busan briefly lost mains power on February 9 and the emergency generator failed to kick in.

The power cut forced the cooling water to stop circulating.

But Kang said the outage was so short that the temperature of the fuel rods remained safely low and the incident resulted in no radioactive leaks.

The electricity supply resumed 12 minutes later but the Korea Hydro and Nuclear Power Co, operator of the country's atomic plants, failed to report the case to nuclear security authorities for more than a month.

Experts said the incident could have become more serious had the power not restarted.

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 heightened concern following last year's nuclear disaster at Japan's Fukushima.

South Korea will next week host a summit of more than 50 countries to discuss ways to safeguard atomic material worldwide and prevent acts of nuclear terrorism.