Federal regulators are increasing scrutiny of a South Carolina nuclear power plant after finding an equipment problem deemed to be of "substantial safety significance."

The Nuclear Regulatory Commission said faulty circuit breakers at Duke Energy Corp.'s Oconee power plant near Seneca could have prevented a backup shutdown system from operating during fires and storms.

The equipment is needed to safely shut down the plant and cool its three reactors, NRC said. The nuclear plant is at Lake Keowee, about 30 miles west of Greenville, and can generate about 2,538 megawatts.

"The facility was vulnerable to core damage if an accident involving a series of unlikely events occurred," NRC said in a statement.

Plant workers identified the issue in June, and the commission held a follow-up hearing on Nov. 16. NRC said the public was never in danger and no actual disaster ever occurred.

Duke argued that the breaker problem was, at worst, a "low to moderate safety" violation that did not represent the plant's current performance, but NRC disagreed and found the problem to be more severe.

Duke spokeswoman Sandra Magee said the backup shutdown system has not been used since the plant started operating in 1973 and that Duke replaced the breakers with fuses in August.

NRC said inspectors will review the breaker problem and notify Duke of its findings. The utility will have a month to respond.