Not really a setback, because there was no place for that technology. Lack of space reactors wasn't and isn't a roadblock. Had that development continued it would have been largely wasted, and the knowledge gained would have decayed away.

It easily functions as a self-breeding terrestrial reactor that reaches criticality with 1/500th to 1/1000th the fuel of existing nuclear reactors (~20kg). The design requires active power that shuts down the reactor on failure, making any accident a complete nonevent. It can even be kickstarted without huge amounts of enriched fuel with the addition of a neutron gun (which they used to test the design without nuclear fuel). The core infrastructure required to support such a reactor are also covered by far less red tape (mostly centrifuges).

It would have changed the face of power generation at a critical time for climate change if it was allowed to reach fruition. Instead it was locked away in UTC & NASA archives.

The requirements for a space reactor, and for a terrestrial reactor, are radically different. No space reactor will make sense for terrestrial power generation. The temperatures are different, power density is different, safety cases are different. A 20kg reactor likely needs highly enriched fuel (space reactors are typically fast reactors too, so we're talking about something that's almost a bomb there.)

An external neutron source does not greatly change the needed enrichment (since any practical subcritical reactor would still need k fairly high, so it is only a bit subcritical.)

Just read the papers. There are dozens on that archive covering the design, theory, and testing - all of those issues are covered. 20kg is actually a high number optimized for a certain plasma temperature that radiates in UV without losing criticality. All things being equal (enriched uranium hexafluoride), at higher temperatures and pressures the reactor needs far less fuel but it causes too much degradation to the reactor walls to be economical or useful except in cold boots.

The terrestrial reactor and the rocket engine versions are different in how they transmit power from the reactor core to do useful work but not radically so - the core irrotational vortex compressing a high temperature plasma in a single crystal beryllium oxide chamber is the same in both designs and has been tested. The difference is in the cooling gas - hydrogen gas seeded with tungsten nanoparticles to absorb the black body radiation of the plasma for the rocket and water going through a steam turbine for terrestrial power generation.