NuScale got far enough to get approval to build a test reactor at the Idaho Reactor Testing Station, which is in Outer Nowhere for good reasons. But they never got enough funding to build it.
The trouble with most of these small modular reactor schemes is that their big pitch is mostly "we don't need a big, strong, containment vessel because ... reasons."
There's no inherent problem in building a small nuclear reactor. Here's one from 1957, near Oakland, CA.[1] It's safety if something goes badly wrong that's a problem.
History:
- Chernobyl - meltdown and fire, no containment vessel, major disaster.
- Fukushima - meltdown, too-small containment vessel, large disaster.
- Three Mile Island - meltdown, big strong containment vessel, plant lost but no disaster.
Alternative reactor history:
- Fort St. Vrain - high temperature gas-cooled, subject to helium plumbing leaks in radioactive zone, shut down and plant converted to natural gas.
- AVR reactor, Germany - pebble bed reactor, had pebble jam, had to be shut down, extremely difficult to decommission.
- Sodium reactors - prone to fires.[3]
- Molten salt reactors [4] - require an attached chemical plant that reprocesses radioactive molten salt.
Most of the problems of nuclear reactors in practice involve plumbing. Everything in the radioactive zone has to last half a century or so without maintenance. That's possible with distilled water as the working fluid, but everything else tried has not worked well.
[1] https://www.youtube.com/watch?v=A1O8xAB_FDI
[2] https://en.wikipedia.org/wiki/AVR_reactor
[3] https://www.nuclearsafety.gc.ca/eng/resources/research/techn...