The Nuclear Regulatory Commission says TVA has failed to adequately protect two nuclear plant sites from the potential for the failure of earthen dams upstream and the flooding that would ensue.
NRC on Friday cited TVA with six violations and placed both Sequoyah Nuclear Plant and Watts Bar Nuclear Plant under "yellow" safety flags, indicating what NRC calls "substantial safety significance."
NRC ranks safety findings as white, yellow and red, in increasing order of significance. If a plant is deemed completely safe, it is rated as green.
TVA officials said there is no immediate safety concern because the utility has implemented interim measures to protect the plants against dam failure.
TVA discovered the potential for flooding several years ago when preparing to seek new construction licensing for the mothballed Bellefonte Nuclear Plant.
TVA officials realized the assumptions they used to model potential flood problems three and four decades ago were both erroneous and outdated. When they applied correct figures and new technology to their calculations, they realized a catastrophic flood would raise water much higher than they thought.
New calculations show flooding would rise more than 4 feet higher than Watts Bar was licensed and designed to handle.
Such a flood would raise water nearly two and a half feet higher at Sequoyah than that plant was designed and licensed to handle.
NRC began publicly questioning TVA about its precautions in 2012, and in December NRC scheduled meetings with TVA for discussions of how the plants would fare during what the nuclear industry refers to as the "probable maximum flood" - a hypothetical flood that would surpass any known local weather occurrence.
Read more in Tuesday's Chattanooga Times Free Press.