Months-old safety problems could lead to extra inspections at the Catawba nuclear plant on Lake Wylie, SC, which lost off-site power in April, federal officials said.

Duke Energy, which runs the plant, received the Nuclear Regulatory Commission (NRC) findings last week. The company is still assessing the report, said Duke spokeswoman Mary Kathryn Green.

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It all started April 4, when the plant 18 miles south of Charlotte lost power coming into the plant. Backup generators kicked in and the one operating reactor shut down safely. Catawba’s second reactor was already shut down for refueling.

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

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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, and to Unit 2 in February. 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, Green said. Both Catawba reactors are now operating at full power.

The NRC initially assigned a “yellow” finding to the incident, meaning it had substantial safety significance. Duke will be given a chance to present its side at a public meeting.

If the NRC upholds the yellow finding, it would mean an additional 200 hours of inspections to ensure the problem has been resolved, said spokesman Roger Hannah.

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