TVA has failed to adequately protect Sequoyah and Watts Bar nuclear plants from the potential for the failure of dams and berms upstream and the flooding that would ensue, according to the Nuclear Regulatory Commission. NRC on Friday confirmed that NRC sent TVA six citations and placed both plants under new "yellow" safety flags, indicating what NRC calls "substantial safety significance."
TVA has failed to adequately protect Sequoyah and Watts Bar nuclear plants from the potential for failure of earthen dams upstream and flooding that would ensue in the event of what utility and nuclear regulators call a probable maximum flood - an event that would surpass any known local weather occurrence.
The Nuclear Regulatory Commission on Friday confirmed that its inspectors have cited the Tennessee Valley Authority with six violations and placed both Sequoyah and Watts Bar 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 and will continue to work with the NRC.
"The safety of our day-to-day nuclear operations has not been brought into question," said TVA spokesman Mike Bradley. "TVA takes its regulatory compliance seriously and has already responded to the extreme flooding issues identified by the Nuclear Regulatory Commission."
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to model potential flood problems three and four decades ago were both erroneous in their original calculations and outdated.
Weather patterns have since changed, and TVA also has since changed the way it manages river levels. When utility engineers applied correct figures and new technology to their calculations, they realized a catastrophic -- and unlikely -- probable maximum flood would raise water much higher than they thought.
Late last year in documents TVA filed with the NRC, engineers said new calculations show flooding would rise more than 4 feet higher than Watts Bar was licensed and designed to handle and 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 officials for discussions of how the plants would fare during a probable maximum flood.
TVA assured the NRC that it had the capability to monitor weather and implement a "27-hour-margin" plan to close any gaps in the sand and crushed stone baskets erected alongside the flanks of the dams several years ago at Fort Loudon, Tellico, Cherokee and Watts Bar dams to prepare for such a flood. All of those dams are upstream of Sequoyah Nuclear Plant and Chattanooga.
The large baskets -- all in place as temporary fixes until more permanent options are chosen -- have gaps if a road passes over the earthen flank.
NRC told TVA to test its margin plan. The utility did, twice.
The first test failed and TVA's effort took several hours longer. In the second test, TVA beat the deadline by a couple of hours, according to Bradley.
TVA also tested plant doors and seals at both Sequoyah and Watts Bar, and found some wanting -- a report NRC also heard in December. Some seals failed, meaning critical cooling equipment would have failed had there been a real monster flood.
On Monday, NRC spokesman Roger Hannah said TVA now will tell NRC how they will respond to the notices and a hearing will be scheduled.
Hannah said in the post-Fukushima world, NRC inspectors are looking more carefully at potential flooding events. Several reactors at a nuclear plant in Fukushima, Japan, melted down two years ago after the plant survived a 9.0 earthquake but the ensuing tsunami devastated the diesel backup generators that would have cooled the reactors after the power was disrupted.
Hannah acknowledged that a probable maximum flood might be unlikely, but he said the precautions are needed for Sequoyah and Watts Bar.
"Our inspectors found that their [TVA's] strategies were not adequate," he said.
Contact staff writer Pam Sohn at firstname.lastname@example.org or 423--757-6346.
with file art [both of the plants and sand baskets around the dams]
with documents online