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The Nuclear Regulatory Commission reported that a valve malfunction at a Browns Ferry reactor left the unit susceptible to core damage in some highly unlikely, but possible, accident scenarios. By categorizing the safety issue as “red” – the agency’s highest level of significance – the NRC will require more intensive oversight at the plant.

On Oct. 23, unit 1 was undergoing refueling when a low-pressure coolant injection valve failed to open while operators tried to use the reactor’s residual heat removal shutdown cooling loop, according to a release issued Tuesday by the NRC. The last time the valve was used successfully was in March of 2009.

“The valve was repaired prior to returning the unit to service and Browns Ferry continued to operate safely,” NRC Region II administrator Victor McCree said in the release. “However, significant problems involving key safety systems warrant more extensive NRC inspection and oversight.”

That oversight will include an extra inspection of the plant’s safety, organizational issues and culture, as well as extensive reviews of programs and processes that are not part of regular NRC inspections.

The NRC issued its finding after disagreeing with the Tennessee Valley Authority's response to the regulator's investigation of the incident. For its part, TVA maintains that the valve problem was caused by a manufacturing defect, and that the valve would have worked in an actual emergency.

"TVA found the problem and fixed it," TVA’s Chief Nuclear Officer Preston Swafford said in a release. "The valve stem threads, which are designed to screw into the valve disc, were determined to be too small.  We made repairs and reinstalled the valve. It has worked properly during tests, and it worked properly again last week when the plant really counted on it after the severe weather that recently hit Alabama."

Three reactors at the plant tripped last Wednesday when dozens of tornados whipped across several southern states. The storm disabled some of the lines feeding external power to the plant. The reactors shut down safety and diesel generators provided backup power. TVA indicated the NRC’s finding will not keep the units from being placed back into service once repairs to power lines are finished.