Federal inspectors found months-old safety problems that could lead to extra inspections at the Catawba nuclear plant on Lake Wylie, which lost off-site power in April.
The Nuclear Regulatory Commission reported its initial findings in a letter sent to Duke Energy, which runs the plant, on Wednesday. Duke spokeswoman Mary Kathryn Green said the company is still assessing the report.
On April 4, 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. Catawbas 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.
Nuclear plants are designed with multiple backup systems to keep them safe in malfunctions. 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 were made on the plants Unit 1 last July and November, and to Unit 2 in February. The error meant that off-site power would be inadvertently lost anytime the units generator shut down due to a power fluctuation.
Thats 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.
The problem has since been corrected, 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, said spokesman Roger Hannah, it would mean an additional 200 hours of inspections to ensure the problem has been resolved.