The Nuclear Regulatory Commission last week told TVA and other nuclear plant operators to reassess the earthquake risks at each of their reactors.
Initial reviews by NRC and the nuclear industry indicate there are increased risks at some plants, including Sequoyah near Soddy-Daisy and 17 miles north of Chattanooga.
The news comes during the same week NRC told the Tennessee Valley Authority that the sand baskets on three dams above Sequoyah could fail during a massive flood, putting the nuclear plant and its diesel generators at risk.
The baskets were placed along the top and side edges of Cherokee, Fort Loudon, Tellico and Watts Bar dams in 2008 when TVA discovered problems with flood calculations. The data TVA used for its calculations was decades old and outdated, agency officials said.
NRC began work on the new quake rule last year after Japan's 9.0 earthquake, tsunami and subsequent triple meltdown at the Fukushima Dai-ichi nuclear plant. The U.S. regulatory agency had been gathering data for years before the Japanese accident, and in 2008 had begun work to build a new quake modeling tool using newer geology data than what was available when most of the U.S. plants were designed and built.
NRC partnered with the Electric Power Research Institute and the Department of Energy to develop the model. The project is called CEUS-SSC, standing for Central and Eastern United States Seismic Source Characterization for Nuclear Facilities.
Jeff Hamel, the program manager for Electric Power Research Institute, said Chattanooga was chosen as one of seven geographic sites to test the model, in part because of new information about the East Tennessee seismic zone, the second most active quake zone east of the Mississippi River, behind the New Madrid fault area in Northwest Tennessee and Southwest Missouri.
One Chattanooga test found that the single-worst earthquake likely to happen in a 10,000-year period here would be nearly twice as damaging to structures as previously calculated with a model from the 1980s. Hamel stressed that there are many scenarios for earthquakes, and that calculation represents only one example.
The NRC on Tuesday instructed nuclear plant operators to begin using the new model to reassess their specific quake risks.
"We expect broadly across the region [the central and eastern U.S.] that the risks will go up," Hamel said.
TVA spokesman Ray Golden said it is too early to know what the new testing will mean for Sequoyah and TVA's other nuclear plants -- Watts Bar in Spring City, Tenn.; Browns Ferry in Athens, Ala.; and the half-finished but idled Bellefonte in Hollywood, Ala.
"NRC has told us we'll have about four years to assess and make changes," he said.
NRC spokesman Scott Burnell called the new model "a starting point for more analysis."
Risks and estimates
When NRC first obtained the new earthquake data from the U.S. Geological Survey, engineers made some initial reassessments at the nation's 104 commercial nuclear plants. Those estimates, when sorted and analyzed by msnbc.com last spring, showed Sequoyah with the nation's fourth-highest earthquake risk.
The chances of an earthquake causing core damage at each of Sequoyah's two reactors were 1 in 19,608, according to the analysis.
For local residents, those core odds are far greater than the chance of being struck by lightning, which the National Weather Service says is about 1 in 500,000.
The now-completed new seismic model redefines the earthquake motion that could affect a plant, Burnell said. If the new motion is greater than what the plant was designed for, more calculations will be made to determine what the plant, as built, can withstand.
Most plant builders already had added "safety margins" to their seismic designs, he said.
"So this additional analysis will determine how much margin is left. If the NRC determines the new margin is too small, we'll require a plant to take whatever steps are necessary to restore the margin."
Generally, however, the nuclear industry has said it expects the cost of retrofits to meet the new quake models will be high.
Burnell said utilities must complete the first analysis within two years and, if a second round is needed, the plants with the highest apparent risk would have several additional years to do more work. Plants with less-apparent risk would have even more time, he said.
"In the meantime, every plant also will have taken qualified engineers through their buildings to check the existing plant against current seismic hazards, and they'll fix any issues they find," Burnell said.
As for the flood concerns, an NRC letter to TVA dated Jan. 25 said the sand baskets are not capable of withstanding the impact of debris barreling down the Tennessee River in a worst-case flood.
"There is potential for this debris to damage the baskets or push the individual baskets apart, causing a breach," the letter states. "There would be no time to repair the baskets because the flood would already be in progress."
The problem with TVA's flood calculations came to light in 2008 when TVA was doing preliminary work toward completing Bellefonte Nuclear Plant.
Mike Eiffe, TVA's hydrology program manager, said TVA in 2009 had NRC buy-in on the sand baskets as a temporary fix, and the utility had reviewed strength tests on the baskets to make sure they were sturdy.
"We respectfully disagree" that they are not sturdy enough. "And 'temporary' is exactly what TVA intended for them," he said.
The utility is completing required environmental reviews for a permanent fix and expects that final upgrade to be complete by December 2015, said Eiffe and TVA spokesman Travis Brickey.
NRC spokesman Joey Ledbetter told The Tennessean newspaper in Nashville that the baskets are acceptable as a temporary fix.