If the mother of all Tennessee Valley floods had occurred last year, results at Watts Bar or Sequoyah nuclear plants could have been catastrophic, nuclear regulators and TVA officials say.
This year, the plants have a better chance against rising water. And in the next two years, Tennessee Valley Authority officials say, the seven reactors at all three nuclear plants built on the Tennessee River should be monster-flood proof.
Come Monday, they have to convince the Nuclear Regulatory Commission of that.
"We're trying to show as much as possible that we're dead serious about eliminating this issue and fixing the problem and restoring confidence that the NRC can count on us to do the right thing," said Preston Swafford, TVA's chief nuclear officer and executive vice president for the nuclear power group.
The NRC last month cited TVA with six apparent violations that place both Sequoyah Nuclear Plant and Watts Bar Nuclear Plant under "yellow" safety flags, indicating what the commission calls "substantial safety significance."
The 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 in the event of a "probable maximum flood" -- one about twice the deluge that inundated Nashville in 2010, according to Swafford.
Utility officials are hoping that the $7 million they've spent on massive new generators -- backups for backups for backups -- and the more than $10 million spent at the plants to raise the level of crucial existing equipment will buy them some grace.
The utility still will spend more millions later this year and through 2015 to build F-5 tornado-proof, high-ground housing at all three of its nuclear plants for the seven new diesels -- one for each reactor.
TVA already has one plant, Browns Ferry, under a red finding.
Although utility officials insist their efforts are intended to protect the plants and the public, they acknowledge that they also hope to appease regulators nervous in the aftermath of Fukushima. Two years ago, three meltdowns in Fukushima, Japan, occurred after a 9.0 earthquake and tsunami wiped out the plant's power and back-up diesel generators. Thousands of people there still cannot return to their homes.
"Fukushima was a game changer," said John Carlin last week. Carlin is the site vice president at Sequoyah Nuclear Plant, where a probable maximum flood would raise water nearly two and a half feet higher than the plant was designed and licensed to handle.
Water would cover electrical connections and pumps and course through conduits not hardened for dampness to drown out power connections that control raw water intakes vital for cooling a reactor in shutdown.
The two things that can't be denied to a reactor are power and water, Carlin said.
So TVA engineers have made modifications -- adding special seals to conduits, and new sealing barriers to doors that might be susceptible to high water and the pressure behind it.
The same work has been going on at Watts Bar Nuclear Plant, where new flood calculations show water would rise more than four feet higher than what the plant was designed and licensed to handle.
What went wrong?
In March, the NRC charged that TVA has failed to adequately protect the Sequoyah and Watts Bar nuclear plants from the potential for failure of earthen dams upstream and flooding that would ensue in the event of the monster flood.
Part of the problem was NRC regulations that were toughened with adjustments for changing weather patterns. Some of the newest climate research shows this region -- in the coming 50 years -- could receive another 17 inches of rain a year on top of the normal average of 51 inches.
But TVA also -- decades ago -- had made mistakes in calculations for its own flooding models, and utility engineers had failed to factor into those models changes the utility had made in managing river levels through the years.
The sum of those differences put key cooling equipment at Sequoyah and Watts Bar under water -- at least according to models.
TVA already has made temporary changes at four East Tennessee dams, placing sand and gravel baskets along the earth support flanks of Fort Loudon, Tellico, Cherokee and Watts Bar dams.
The utility also has a detailed plan of sealing those temporary baskets across roadways and making other flood emergency preparations. And they have tested those efforts.
"We're doing drills we've never done before," Carlin said.
NRC spokesman Roger Hannah said that, in the post-Fukushima world, NRC inspectors are looking more carefully at potential flooding events, and NRC officials began publicly questioning TVA about its flood precautions in 2012.
In December, the NRC scheduled meetings with TVA officials, and TVA assured the regulator that it had the capability to monitor weather and implement a "27-hour-margin" plan to close gaps.
The NRC told TVA to test its margin plan. The utility did, twice.
The first test failed when TVA's effort took several hours longer. In the second test, TVA beat the deadline by a couple of hours, according to officials.
Contact staff writer Pam Sohn at psohn@timesfree press.com or 423-757-6346.