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An inspection finding at Duke Energy’s Catawba nuclear power plant had low to moderate safety significance and as a result, the plant will be under increased oversight until the issue has been appropriately addressed, federal officials said.

In April, an electrical component on one of the Catawba Unit 2 emergency diesel generators failed during a scheduled test, said officials at the Nuclear Regulatory Commission (NRC).

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Inspectors reviewed the event and determined plant staff had failed to adequately develop and adjust preventive maintenance activities using operating experience, maintenance history and performance records. The two-unit Catawba plant is near York, SC.

The NRC preliminarily characterized the finding as white, or having low to moderate safety significance, and after reviewing Duke Energy’s response to the issue, the agency finalized the white finding.

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The increased oversight resulting from this finding will include a supplemental inspection to ensure Duke Energy fully understands the issue and has taken appropriate corrective actions.

The NRC said the two violations involved an incorrect modification to a Unit 1 generator protective relay that resulted in the loss of offsite power event. 

The chain of events started April 4, when the plant lost power coming into the plant. Backup generators kicked in and the one operating reactor shut down safely. Catawba’s second reactor was already down for refueling.

Catawba officials declared an “unusual event,” the lowest of four emergency levels and one not experienced at the plant since 2006.

Nuclear plants have multiple backup systems to keep them safe if there is some type of malfunction. But losing off-site power removes the primary energy source needed to operate the plant and keep vital cooling water in circulation.

The NRC investigation traced the April incident to a programming error when electrical modifications occurred on the plant’s Unit 1 last July and November. The error meant off-site power could inadvertently be lost anytime if the units’ generator shut down due to a power fluctuation.

That’s what happened April 4. A ground fault on a reactor coolant pump triggered the reactor, turbine and generator to stop. Because of the programming error, the unit also lost off-site power.

Duke corrected the problem and both Catawba reactors are now operating at full power.

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