The Tennessee Valley Authority has completed another round of critical testing of the Watts Bar Unit 2 reactor, which the utility plans to start up later this year, pending regulatory approval.

TVA said today it successfully passed the first in the series of pressure tests for the new reactor following a series of hot functional tests of plant equipment during July. The latest structural integrity and integrated leak rate tests involved increasing pressure inside the containment vessel to check welds and other seals to make sure they function as designed. More than 200 valves, cable penetrations, airlock doors and other openings also were checked without any significant problems being uncovered, TVA spokesman Scott Brooks said.

"The tests were completed successfully on the first performance, which reinforces the quality of the construction," Brooks said. today.

TVA is continuing startup testing to verify the reliability of all safety-related systems at Watts Bar, including emergency AC power systems, prior to preparing for initial fuel load.

The U.S. Nuclear Regulatory Commission must still issue an operating license for TVA to load nuclear fuel into the Unit 2 reactor. The NRC has authorized its staff at its Atlanta regional office to issue the license once TVA verifies the reliability of plant systems and passes NRC inspections.

Unit 2 senior manager of operations Tom Wallace walks up the stairs to the Unit 2 cooling tower of TVA's Watts Bar nuclear plant Wednesday, April 29, 2015, in Spring City, Tenn. TVA plans for the nuclear plant's second reactor unit to come online by the end of the year.

"It is important that we verify the containment vessel provides a tight barrier and has been built to rigorous industry standards," Mike Skaggs, senior vice president for Watts Bar operation and construction, said in a statement today.

TVA Chief Nuclear Officer Joe Grimes said the success of the tests "speaks to the impressive quality work done by the construction craft in achieving this objective."

The Unit 2 reactor is a Westinghouse pressurized water reactor capable of generating about 1,150 megawatts, or enough power to supply the electricity needs of about 650,000 homes.

TVA started construction of Watts Bar in 1973 but has stopped and started work on the nuclear plant several times over the past 42 years in response to changing power demand and plant regulations. The Unit 2 reactor is scheduled to become the first new American reactor to come on line in the 21st century and will be the first new U.S. nuclear unit added to America's electric grid since TVA completed the Unit 1 reactor at Watts Bar in 1996.

TVA has invested more than $6 billion in the Unit 2 reactor. Recovering that investment will cost TVA an extra $450 million next year in depreciation and interest expenses, according to TVA Chief Financial Officer John Thomas. But the new reactor will cut TVA fuel costs by $200 million a year and help TVA comply with new carbon limits on electric utilities.