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* Cracks in 6 tunnels, but no leaks - operator
* Reactor closed for maintenance since Oct. 18
* May complicate power supply as peak winter season looms
By Ju-min Park
SEOUL, Nov 9 (Reuters) - South Korean nuclear regulators have found microscopic cracks in tunnels that guide control rods at a nuclear plant under maintenance, government officials said on Friday, raising new concerns over the country's nuclear power sector.
The discovery of the cracks at the reactor comes just days after two reactors at the same plant in Yeonggwang county, in the southwest of the country, were shut down to replace parts that had been provided with forged certificates.
South Korea is investigating how thousands of parts for its nuclear reactors were supplied using forged safety documents, with regulators set to inspect all 23 of the country's facilities - a move that could test public support for the industry and threaten billions of dollars worth of exports.
"There are cracks in six tunnels. The reactor has been halted since Oct. 18 for regular maintenance and now the process has been extended by a further 47 days for repair of the cracks," said a spokeswoman for the presidential Korea Nuclear Safety and Security Commission.
She said it was the first time cracks of this type had been found in South Korea's nuclear sector, but added the safety risk was not serious enough to require public disclosure.
The reactor affected by the cracks has a capacity of 1,000 megawatts, and a government official said the extended shutdown could complicate efforts to ensure steady supply of power through the peak winter season after the two other reactors had been stopped until the end of this year.
Asia's fourth-largest economy generates 30 percent of its electricity from 23 nuclear reactors at state-owned plants, and the government has warned of the potential for unprecedented power shortages due to the shutdowns as demand peaks in winter.
"This could affect power supplies, but we are preparing contingency plans," said a senior economy ministry official, who declined to be named as he is not authorized to speak to the media.
South Korea's state-run nuclear power utility said it was investigating the cause of the cracks, but said they had not caused any leaks.
"There are no penetrating cracks or leaks," Korea Hydro & Nuclear Power, a subsidiary of state utility Korea Electric Power Corp (KEPCO), said in a statement. It operates all of the country's nuclear power plants.
The country's power utility and nuclear regulators have come under heavy criticism this week after the disclosure that eight firms had used forged safety documents to supply parts to nuclear plants raising concern of broader potential problems in the large and growing nuclear programme.
A task force has been established with government and private sector experts to inspect all reactors to ensure their parts are properly certified. It will also inspect five reactors under construction to see if troubled parts with forged certificates have been provided. (Additional reporting by Meeyoung Cho; Editing by Jack Kim, Ed Davies and Ian Geoghegan)