WASHINGTON  The Nuclear Regulatory Commission said Thursday that it had rejected a design by Westinghouse for a new reactor because a key component might not withstand events like earthquakes and tornadoes.

The rejection raises the possibility of delays in building 14 planned reactors in the United States, including two twin-reactor projects in Georgia and South Carolina that are leading the pack. Westinghouse, which is owned by Toshiba, promised to conduct tests as quickly as possible to try to satisfy the agency staff that the design was sound.

The new reactor, called the AP1000, is intended to be faster to build and safer to run than previous models. The letters stand for “advanced passive” and the number is the estimated electrical output in megawatts.

Resolving design issues before construction is viewed as a crucial part of the nuclear industry’s plan for a revival without the delays and cost overruns that bedeviled the industry in the 1970s and ’80s.