Story highlights Microscopic cracks are found in six control rod tunnels at a reactor

The unit will be offline for an extra 47 days, a nuclear regulator says

It comes just days after two other reactors were shut down over a parts scandal

A nuclear expert says he is "very concerned" about the risk of winter power shortages

Tiny cracks have been found in tunnels at a nuclear plant in South Korea, increasing concerns about nuclear safety in the country following a recent scandal involving the use of unverified parts.

The reactor where the cracks were found will remain offline for weeks as regulators investigate the problem, putting extra strain on South Korea's already stretched power supply going into the winter months.

The utility Korean Hydro & Nuclear Power Co. (KHNP) said it detected microscopic cracks in six control rod tunnels at Unit 3 of its Yonggwang nuclear plant in the southwest of the country. Control rods are used to regulate the speed of nuclear reactions taking place inside reactors.

"The cracks are not serious and there is no risk of radiation leakage," said Jang Yong-jin, head of the mechanics department at KHNP.

The problem was discovered while the reactor was switched off for a regular 36-day maintenance period. But it will now stay out of service for a further 47 days as inspectors seek to determine the cause of the cracks, the South Korean Nuclear Safety and Security Commission said.

That deprives the national power grid of another source after operations were halted this month at two other reactors at the same complex to replace thousands of parts that were supplied with forged quality certificates.

Authorities warned at the time that halting those two reactors, Units 5 and 6, may result in "an unprecedented level" of strain on the nation's power supply. They account for about 5% of South Korea's total supply, according to the government.

Now, the situation appears even bleaker.

"Winter here is brutal, and I am now very concerned that the unexpected shutdowns of three nuclear units will cause power shortages," said Huh Kyun-young, a nuclear engineering professor at Kyung Hee University.

Experts have been warning about insufficient power supplies in South Korea for years, according to Huh.

"I just bought an oil heater because I have kids in my home," he said.

It remains unclear how much power supply will be affected by the extended shutdown of Unit 3, said Jang of KHNP.

"Relevant departments are mapping contingency plans," he said.

The cracks themselves are not a serious issue and have been found at reactors in such other countries as the United States and Japan, said Jae Moo-sung, a professor in the nuclear engineering department of Hanyang University.

But Jae warned that the news could hurt South Korea's efforts to export its nuclear power technology to other countries.

The problems at the South Korean reactors come amid increased scrutiny of nuclear power worldwide following the crisis at the Fukushima Daiichi nuclear plant in Japan during the aftermath of the earthquake and tsunami that hit the country in March 2011.