CHATTANOOGA, Tenn. (AP) — The Nuclear Regulatory Commission has found substantial safety issues at two Tennessee Valley Authority nuclear plants involving flooding risks.
NRC found a total of six apparent violations at the Sequoyah and Watts Bar plants on the Tennessee River.
The federal regulatory agency said earthen dams upstream could be overtopped during a catastrophic flood, or what the report calls a "probable maximum flood." That would cause flooding at the plants that would submerge emergency generators and cut off the power needed to prevent damage to the nuclear cores.
The NRC report said TVA recognized the problem in 2009 when it discovered outdated calculations for flooding and has since protected the dams.
TVA spokesman Mike Bradley said Monday that the scenario outlined in the NRC report is extremely rare. "Such a flood has never occurred," he said.
A written statement from TVA said, "The safety of our day-to-day nuclear operations has not been brought into question. We will continue to work cooperatively with the NRC in developing plans to be prepared for these highly unlikely events."
NRC has stepped up safety enforcement after the 2011 earthquake and tsunami that crippled Japan's Fukushima Dai-ichi nuclear plant. It conducted inspections at the two TVA plants in February to review procedures, observe activities and interview workers.
The NRC evaluates nuclear power plants with a color-coded system which classifies findings as green, white, yellow or red in increasing order of safety significance.
NRC coded four of the apparent violations at TVA as yellow, which indicates substantial safety significance that may require more NRC inspections, but it said they were not immediate safety concerns.
Beyond the flooding dangers, the NRC found apparent violations because TVA didn't report the problems it found in 2009 until early last month. It also said it found problems in TVA's procedures for reconfiguring and realigning the plant within 27 hours of notification of a significant flooding event.
Bradley said the next step will be a meeting, not yet scheduled, where TVA presents more information about how it has addressed the NRC's findings. He said TVA has enhanced barriers at the dams, taken steps to protect equipment at the plants and updated emergency procedures.
The planning for a flood of this magnitude strengthens TVA preparations for smaller-scale events, Bradley said.