Workers gather near the No. 2 reactor that is undergoing refueling during a tour Friday of the Browns Ferry Nuclear Plant in Athens, Ala. AP Photo/Dave Martin

ATHENS, Ala.-What if a million-year flood lapped at the doors of Brown's Ferry Nuclear Plant and TVA's sister plants Sequoyah and Watts Bar?

What if an F-5 tornado hit Brown's Ferry in Athens, Ala., at the same time?

With the double assault of a 9.0 magnitude earthquake and tsunami that left six reactors of a Japanese nuclear plant in various stages of radioactive crisis, those "what if's" and others have the Tennessee Valley Authority taking extra precautions and playing out tabletop crisis management exercises.

On Friday, TVA officials led a Chattanooga Times Free Press reporter and 14 other journalists on a tour of Browns Ferry - a near twin of the Japanese Fukushima Dai-ichi plant.

The goal, officials say, is to ensure safety and reassurances of Browns Ferry's "robustness" and to talk about the utility's effort to be prepared with lesson learned from the tragedy across the ocean.

"We've started doing stacked events [in tabletop exercises,] and saying what if," Preston Swafford told reporters Friday. "And we'll keep doing them until we've every one that is humanly imaginable. And that includes looking at having three reactors impacted simultaneously."

Similar looks will go on at Sequoyah Nuclear Plant in Soddy-Daisy and Watts Bar Nuclear Plant in Spring City.

Already, Swafford said, TVA officials have recognized that they want more emergency generators at Browns Ferry.

"We have ... two for each reactor. But we'd like three," he said, adding that engineers also have learned they need to stage the generators and other smaller mobile battery backups throughout the plant to ensure that they can move backup power wherever needed as it's needed.

"It's a lesson for TVA and all the nuclear industry," he said. "But in the end it adds more margin."

TVA Chief Operating Officer Bill McCollum Jr. said the exercises remain science- and fact-based.

"Safety is our top value," he said. "Our focus is to continue to learn and improve."

McCollum said TVA undertook the reviews on its own, but the utility is incorporating into them the calls for similar reviews from nuclear industry groups and the Nuclear Regulatory Commission.

Broadening the scenarios of what problems may face America's aging nuclear reactors is something Dave Lochbaum, director of the nuclear program for the Union of Concerned Scientists, long has sought.

Lochbaum is a nuclear engineer who has worked at both TVA's Brown Ferry plant and as a Nuclear Regulatory Commission training officer.

A major concern for Lochbaum is the three spent fuel pools that sit atop Browns Ferry reactors in the same location where hydrogen explosions ripped away the tops of two Japanese reactors.

"When I worked at Browns Ferry, we had high winds that tore the metal sheeting away. The pools in that plant design are not in containment," Lochbaum told a Sierra Club group in Chattanooga on Thursday evening.

Lochbaum has suggested that TVA and other nuclear plants decrease the amount of spent fuel in the pools by transferring it to cask storage. Moving the waste would provide less danger if something goes wrong.

Swafford said Friday that the concrete beneath the pool storage at Browns Ferry is strong and likely would not be impacted in an F-5 tornado, but he acknowledged that the pools could be "a mess."

He also said TVA has positioned hoses up the stairwell to provide a ready access to water should normal cooling systems be compromised.

"We're going to look at everything," he said.