A self-sustaining nuclear reaction was achieved Monday at TVA's Watts Bar Unit 2 reactor in one of the most significant milestones toward activating America's first new nuclear plant in the 21st century.

Licensed reactor operators at the newest unit at Watts Bar brought the reactor to a sustained fission reaction known as initial criticality at 2:16 a.m. EDT. Unit 2 is now generating heat under its own power and within the next couple of weeks should be brought to a power level sufficient to heat steam and generate electricity for the first time.

TVA expects the Unit 2 reactor to achieve 100 percent power and be declared complete and commercially viable by this summer — nearly 43 years after construction first began at Watts Bar. The TVA plant has been stopped and started several times over the past four decades due to changing power demands in the Tennessee Valley.

Once brought into TVA's rate base this summer, the new reactor will require the federal utility to begin paying down the estimated $4.7 billion cost to complete the plant. But TVA Nuclear Chief Joe Grimes said that is still far less than the $14 billion expense of building new reactors at Plant Vogtle in Georgia or at V.C. Summer nuclear plant in South Carolina.

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"This milestone is the result of the hard work by Watts Bar employees supported by the entire TVA nuclear team," Grimes said in a statement Monday. "While this achievement is important, safety remains our top priority and we will now move forward with fully integrating the seventh unit into the fleet with that focus in mind."

The startup of the Unit 2 reactor at the Watts Bar Nuclear Power Plant near Spring City, Tenn., will add 1,411 megawatts of carbon-free electricity generation, or enough power to supply two cities the size of Chattanooga. It marks the first new nuclear plant to be added to America's electricity grid in 20 years.

The newest Watts Bar reactor will help TVA replace coal-fired generation that TVA recently has or soon will shut down at its Widows Creek, Colbert, New Johnsonville and Paradise fossil fuel plants.

TVA announced the initial criticality for the new plant on the eve of a meeting with federal regulators today in Atlanta, where TVA officials will try to outline how it has addressed Nuclear Regulatory Commission concerns that TVA is tending to suppress employee safety issues. The NRC has scheduled a hearing at 10 a.m. today for TVA to explain how it has changed its corporate culture from what regulators said was a "chilling" atmosphere for employee concerns earlier this year at the existing Unit 1 at Watts Bar, which it operates alongside the new reactor.

TVA will conduct a variety of power ascension tests at the new reactor over the next several weeks to test equipment and procedures, TVA spokesman Jim Hopson said. The unit has yet to generate any electricity, but once it reaches sufficient power and is linked with the generator on the non-nuclear portion of the plant, Watts Bar Unit 2 will begin to generate electricity.

"Our expectation would be that we would reach the point where we would start generating power in low quantities probably within the next several weeks," Hopson said.

Even anti-nuclear activists said they hope TVA is successful in its new approach.

"It's 30 years later and billions of dollars over budget, but if it is going to operate we hope it is done safely and properly by TVA," said Stephen Smith, executive director for the Southern Alliance for Clean Energy.

Contact Dave Flessner at dflessner@timesfreepress.com or at 423-757-6340.