WASHINGTON — In a setback for the only model of nuclear reactor for which ground has been broken in the United States, government regulators have found additional problems with the design of its shield building, a crucial component, the chairman of the Nuclear Regulatory Commission said on Friday.

The chairman, Gregory B. Jaczko, said that computations submitted by Westinghouse, the manufacturer of the new AP1000 reactor, about the building’s design appeared to be wrong and “had led to more questions.” 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.

Mr. Jaczko said the commission was asking Westinghouse not only to fix its calculations but also to explain why it submitted flawed information in the first place. Earlier this year the commission staff said it needed additional calculations from Westinghouse to confirm the strength of the AP1000’s shield building. The building has not been built; the analysis of its strength and safety is all computer based.

The announcement comes as the commission and the American nuclear industry are facing increased scrutiny as a result of the calamity that began after an 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 clearer.