They're building reactors in the state of Georgia:

https://info.westinghousenuclear.com/blog/shaping-the-future...

Other companies that manufacture nuclear components in the US include Areva, General Electric, and Framatome. But Westinghouse is the only company that has a new reactor design currently under construction in the US.

The US is still fielding new nuclear submarines and aircraft carriers. So, they ARE building new power plants, just...not commercial ones.