The most popular model for the next generation of nuclear power plants in the United States will have to be redesigned or tested again to ensure greater strength in its shield building, federal regulators said Thursday.

The Nuclear Regulatory Commission announced that it has rejected part of the design of the Westinghouse AP-1000 reactor, which the Tennessee Valley Authority and a half dozen other utilities want to build across the Southeast.

The NRC's concerns about the structural integrity of the plant's shield structure could delay the required regulatory approval for the new plants, officials said. In a meeting with regulators this summer, Westinghouse's vice president for regulatory affairs, Ed Cummins, said that having to test the proposed shield building design "could have a significant impact on schedule," according to an NRC summary of the July 14 meeting.

But in a prepared statement Thursday, Westinghouse sought to downplay the chances of any delay in the scheduled design safety certification.

"We have fully committed the resources necessary to both quickly and definitively address the NRC's concerns, and we are confident that we will meet all applicable requirements," the company said.

Westinghouse expects to gain complete regulatory approval for the design of its new reactor in 2011 to allow the first of the new nuclear plants to be built by 2016, probably at Plant Vogtle near Waynesboro, Ga.

Georgia Power Co.'s Vogtle plant is the first of 14 AP-1000 reactors planned by Southern utilities. TVA is studying plans to build two AP-1000 reactors at its Bellefonte nuclear site in Hollywood, Ala.

TVA spokesman Terry Johnson said TVA's current schedule envisions getting licensing approval for a new Bellefonte reactor by 2013 and then building the unit in time for a 2018 startup.

In 2006, the NRC endorsed the design of the AP-1000. But problems developed when Westinghouse proposed an amended design to prefabricate parts of the shield building around the reactor containment. The changes were made, in part, to respond to NRC's aircraft safety standards adopted this year to limit chances of an terrorist attack on a new nuclear plant from an airplane crash.

"This is situation where fundamental engineering standards will have to be met before we can begin determining whether the shield building meets the agency's requirements," said Michael Johnson, director of the NRC's Office of New Reactors.

NRC officials said the building could be vulnerable because it has to hold up an elevated tank with more than 6 million pounds of water.

Ed Lymans, a nuclear scientist for the Union of Concerned Scientists, said he was encouraged that the NRC staff was trying to verify the safety of key reactor systems "in the face of tremendous political pressure to speed up approval" of the new reactors.

Westinghouse's AP-1000 design uses passive safety systems, rather than relying upon so many pumps, valves and mechanical equipment employed in existing plants to shut down the reactor in the event of an accident. The simplified design uses only half as many safety-related valves and 80 percent less safety-related piping, allowing the plant to be built in 36 months.

Westinghouse has located a nuclear service facility in Chattanooga to help prepare for what the company expects to be a revival in nuclear power plant construction with the new design.