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The only model of nuclear reactor for which ground has been broken in the United States suffered a set back as government regulators found problems with the design of a crucial component in the shield building, the chairman of the Nuclear Regulatory Commission (NRC) said.

Computations submitted by Westinghouse, the manufacturer of the new AP1000 reactor, about the building’s design appear to be wrong and “had led to more questions,” said NRC Chairman, Gregory B. Jaczko. He said the company had not used a range of possible temperatures for calculating potential seismic stresses on the shield building in the event of an earthquake, for example.

Westinghouse needs to fix its calculations but also to explain why it submitted flawed information in the first place, Jaczko said. Earlier this year the commission staff said it needed additional calculations from Westinghouse to confirm the strength of the AP1000’s shield building. The analysis of the building’s strength and safety is all computer based.

This news comes as the commission and the American nuclear industry are facing increased scrutiny as a result of the disaster after the earthquake and tsunami damaged the Fukushima Daiichi nuclear plant in Japan in March, leading to releases of radioactive material. Various critics have asked the commission to suspend licensing of new plants, the relicensing of old ones and various other activities until the implications of the Fukushima accident are clear.

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While the commission has said it will evaluate the Japanese accident methodically, it had previously said it did not anticipate that this would cause a delay in approving the AP1000. That approval now appears to face more delays.

Westinghouse countered in a statement the “confirmatory items” the commission was asking for were not “safety significant.” It noted, and the commission agreed, the company had been the first to identify some of the problems itself.

The Southern Company has already dug the foundations and done other preliminary work for two of the AP1000 reactors adjacent to its existing reactors at Plant Vogtle near Augusta, Ga. The Energy Department promised loan guarantees for the project provided the NRC approves the design.

South Carolina Electric and Gas has broken ground for another two, 20 miles northwest of Columbia.

The commission had previously said it expected to approve the AP1000 design this summer. But a spokesman for the commission, Scott Burnell, said the decision would suffer a delay until Westinghouse submitted a third round of revised calculations.

In addition to the plants in Georgia and South Carolina, ground has also been broken on four AP1000 reactors in China, two at Sanmen and two at Haiyang. Westinghouse, owned by Toshiba, is making parts for the Chinese units.

The AP1000 was in principle designed so it would be faster to build and safer to run than previous models. The letters stand for “advanced passive,” with safety features depending on natural forces like gravity and convective cooling rather than pumps and valves, which electrical failures or floods could knock out as they were at Fukushima.

But the design involves a radical departure from those of existing nuclear plants. One change is to put a massive tank of emergency water on the roof so no pumping is required to deliver it to an overheated reactor. Another is to build a free-standing steel shell around the reactor so heat can travel through the metal and pass it off to surrounding air.

Existing Westinghouse reactors have a thick concrete dome with a steel liner that forms the reactor’s containment shell. In the new design, there is an air gap between the shell and the outer shield building, which is made of steel-reinforced concrete.

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