Meeting in Greeneville on Thursday, the board accepted a revision that adds $1.5 billion to $2 billion dollars to the estimated cost of finishing Watts Bar Unit 2, bringing the final cost of the electricity producing reactor to as much as $4.5 billion.
Completion is expected in late 2015.
TVA CEO Tom Kilgore told the board the federal utility expects the unit to be among the best operating and most economical generating facilities of the century,.
The agency blamed the increase on poor previous estimates and acknowledged that reviews to support the initial estimate were not completed.
The 2007 estimate was $2.49 billion.