KNOXVILLE, Tenn. — The Tennessee Valley Authority’s board of directors voted unanimously yesterday to begin a five-year plan to finish a second nuclear reactor at the Watt Bar Nuclear Plant on the Tennessee River.

The Spring City station, about 50 miles south of Knoxville, was the last new nuclear plant to come on line in America when it fired up one of its two planned reactors in 1996.

The second reactor was mothballed in mid-construction in 1985 when TVA shut down its entire nuclear program over safety concerns.

The plan to finish it is expected to cost about $2.5 billion, likely funded by the public utility’s revenues and adding debt. It was approved after a $20 million internal study on the feasibility of finishing the reactor determined it was already about 60% complete. TVA is the nation’s largest public utility, providing wholesale electricity through 158 distributors to about 8.7 million consumers and directly to several dozen large manufacturers in Tennessee, Kentucky, Mississippi, Alabama, Georgia, North Carolina, and Virginia.

The plan has faced opposition from anti-nuclear advocates and environmentalists, who asked for a year’s delay to study the plan further.

Bill Sansom, the chairman of the eight-member board, said future forecasts for power requirements in the TVA’s booming coverage area could require more nuclear projects, as well as increased conservation.

“This isn’t ‘either/or’ as it comes to conservation,” he said. “We need this and all the conservation you can bring on.”

TVA’s distributors and major industrial customers supported the project, saying TVA has shown nuclear power to be reliable, low cost and less polluting. The project would create about 2,200 construction jobs, the utility estimated.

“It makes good economic sense for TVA to add additional nuclear capacity to its fleet of generators,” said Jack Simmons, president and CEO of the Tennessee Valley Public Power Association, representing TVA’s distributors.

Walt Brockway, speaking on behalf of the Tennessee Valley Industrial Committee, representing industrial customers, told TVA directors that “a second unit at your Watts Bar facility would seem to make eminent good sense at this point.” Mr. Brockway is energy manager of Alcoa Inc.’s massive aluminum operation near Knoxville.

The agency is buoyed by the restart of its oldest reactor at the Browns Ferry plant in northern Alabama in May after a five-year, $1.8 billion renovation.

The Browns Ferry reactor, like Watts Bar Unit 2, had been silent since TVA shuttered the agency’s entire nuclear program in 1985 over safety concerns and regulatory fallout from the Three Mile Island accident.

But unlike Browns Ferry Unit 1, the second Watts Bar reactor has never run before. That would make Watts Bar Unit 2 the first new commercial reactor in this country — the 105th overall.

That will likely make the project a target of opposition. So will Watts Bar’s unique role as the only commercial reactor in the country that also works for the military — making tritium for nuclear weapons for the Department of Energy since 2003.

While Watts Bar Unit 2 is not expected to make tritium, security concerns remain for the site especially during construction involving thousands of workers.

“There are a lot of people that will be in this fight,” said Ann Harris, a former TVA whistleblower at Watts Bar and now an activist with the Sierra Club. “The antinuclears. The safety advocates. The people who work on conservation.”

Opposition to Watts Bar Unit 1 was fierce. Whistleblower complaints forced large amounts of cabling and piping to be replaced, delaying the reactor and driving the cost to $7 billion. Protesters blocked plant entrances and demonstrators were removed from TVA board meetings.

“This time people will have a lot more knowledge,” Ms. Harris said. “There are lots of opportunities to ask for public hearings, [to seek] injunctions and media that didn’t exist before.”