The Tennessee Valley Authority’s board of directors voted unanimously Thursday to complete construction of the mothballed 1,200-MW Bellefonte nuclear unit 1 over the objections of environmentalists who said that it is unsafe to complete the unit, which has been dormant since 1988.

“You’re forcing questionable technology into an old, aging facility,” said John Noel, of the Southern Alliance for Clean Energy at the meeting on Thursday.  But board members said their concerns had been alleviated after TVA staff and executives assured them the unit would undergo rigorous inspections and have to meet the latest standards and regulations.

TVA estimates it will cost $4.9 billion to complete the reactor, in Hollywood, Ala., which was begun in 1974 and stopped in 1988 because of rising construction costs and declining need. TVA already has invested about $1.9 billion on the unit.

The board agreed not to restart construction on the unit until fuel loading is complete at Watts Bar 2 nuclear unit in Spring City, Tenn. Fuel loading will signal the end of reconstruction of that plant, which is being completed by San Francisco-based Bechtel. Construction on Bellefonte could begin as early as 2013 and is expected to be omplete by 2018.

TVA has not hired a contractor to finish Bellefonte 1. Last year, TVA awarded France-based Areva to complete a $248 million contract to complete preliminary engineering, begin licensing and procure long-lead materials.