By Meeyoung Cho

SEOUL (Reuters) - A "safety-related action" automatically shut down one of South Korea's 23 nuclear reactors on Thursday, the country's nuclear operator said, bringing the tally of those closed to six and hiking the chances of power blackouts this winter.

Asia's fourth largest economy faces severe power shortages again this winter due to cuts in nuclear power use after a corruption scandal that started in late 2012.

News of the shutdown came the same day the South Korean cabinet was to meet to discuss power supply needs for the next few months. The meeting has been delayed to December because it's still not known when three reactors shut since late May by the scandal over fake safety documents will resume operation.

"We are now looking into the cause of (Thursday's) shutdown and it is not yet clear when the reactor will restart," said a spokesman at operator Korea Hydro & Nuclear Power.

Operation of the 587-megawatt Kori No. 1 reactor, over 300 km southeast of Seoul, was suspended early on Thursday morning.

The reactor, which started operation in 1978, had recently returned from nearly 180 days of scheduled maintenance through October 5, according to KHNP's website. (www.khnp.co.kr).

The unit's license to operate was extended another 10 years in end-2007, another KHNP spokesman noted.

South Korea has been striving to curb power demand ahead of winter amid the cutbacks in nuclear power, which provides about a third of the nation's electricity.

DELAYED SUPPLY PLAN MEETING

The South Korean cabinet had been set to discuss winter power supply measures on Thursday, but the meeting was delayed up to mid-December to give officials more time to draw up the plan, spokesmen in the prime minister's office said.

An energy ministry source with direct knowledge of the matter noted that the three reactors shut for cable replacement have a combined capacity of 3,000 megawatts.

"With no knowledge of the fate of the three, it is hard to plan winter power supply," said the ministry source who declined to be identified as he was not authorised to speak to media.

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Peak winter demand will only arrive in January, so the delay in the meeting is not critical, the source said.

Industry data shows South Korea's peak demand will mark a record high or more than 81,000 MW this winter, about 5,000 MW lower than total supply capacity.

Six nuclear reactors are now offline, including three shut because of control cables supplied with fake safety certificates, according to the KHNP website.

A fourth is awaiting an extension of its license after its 30-year life span expired in November of last year; a fifth is shut for scheduled maintenance through end-December.

To curb electricity demand, the government raised electricity tariffs earlier this month.

So far 100 people have been indicted in South Korea's nuclear safety scandals, while the government has come under pressure to rethink its reliance on nuclear power.

An international nuclear safety evaluation body, which completed a five-month review of the country's nuclear reactors, said earlier this month that the safety of the reactors "should be improved significantly," although it called for no immediate actions.

KHNP is fully owned by state-run Korea Electric Power Corp (KEPCO).

(Editing by Tom Hogue)