The NuScale is a smaller version of the approved pressure water reactor design
which is the primary NRC design that has gotten new design variation approval over the 48-year existence of the NRC.

The NRC (from 1974-2022) has certified six other designs: the Advanced Boiling Water Reactor, System 80+, AP600, AP1000, the Economic Simplified Boiling Water Reactor and the APR1400. Designs for original pressure water reactors and the boiler water reactors were all approved under the Atomic Energy Commission (AEC) which ran from 1946 to 1974. After the AEC was replaced by the NRC, the approval of new nuclear reactor designs ground to a near halt.

It takes 7-20 years for an NRC approval and the odds of successfully getting through certification are about 20% or less. The odds seem even worse if your reactor is not submitted by Westinghouse or something Westinghouse-related. CANDU heavy water reactors (which have had versions built around the world), pebble bed reactors and high temperature reactors tried to get licenses and then applications get withdrawn after a decade or so.