In this setting nuclear material is incredibly cheap, while all hydrocarbons need to be synthesized from scratch. Additionally people have shadow magic which allows them to both easily shield against all radiation except non-relativistic atoms, and separate different isotopes. So with radiation not being a major concern, nuclear tech can really be used to its fullest potential.
Nuclear tech and medicine are half a century ahead in this world, while other areas like material science are a few decades ahead, and areas like AI are actually well behind our world.
Breeder reactors are common and very cheap given lax regulations, cheap resources and undead/bound spirit labor. So there is incredible flexibility with regards to what radioisotopes can be used in various applications. This also means some isotope like californium will definitely be used in order to achieve a lower critical mass (with the help of beryllium oxide or another high temp neutron reflector).
People even have personal nuclear reactors: With shadow wards that in response to high enough radiation levels or another trigger can automatically spread magical darkness at relativistic speeds to absorb radiation, preventing meltdowns. Indeed in some contexts such as nukes (used very extensively for excavation/mining) or even reactors if their wards are extremely safe from damage people can get away with using mechanisms with concentrations of radioisotopes that should instantly go supercritical without magical darkness absorbing neutrons.
With people living in an Antarctic environment, waste heat is actually a plus, and people can afford to be profoundly wasteful because they live on an infinite flat plane that people aren't concerned about polluting so resources very cheap (it feels normal downwards acceleration but while it has otherwise realistic physics it has no true gravity).
This style of nuclear engine would be massively altered/simplified from something like the ramjet used in Project Pluto: After all spraying out large amounts of radiation can be greatly ameliorated with shadow magic, and people in this world don't mind a bit of fallout anyway as they all have radiation resistance on par with those radiotrophic fungi found in Chernobyl so open reactor designs are common.
I was imagining these engines being used very extensively as a propulsion source, compact generator, leaf blower to clear away snow, and for many other purposes. Without radiation being a concern it seems like one should be able to build this style of engine to be at least as small as a the W54 warhead used in the Davy Crockett nuclear rocket launcher.
For use as a generator the engine could be used at low power and or with something to bleed off enough speed/temperature from the exhaust for a turbine. With the engine exhaust of the engine going through a shadow barrier to block radiation and separate radioisotopes from the rest of the air stream first, before the hot air then goes to drive a small turbine. The desired intent of being this wasteful is in order to make a generator that would fit in a backpack and outperform comparably sized internal combustion based ones by leagues. Given the wastefulness of this culture, if the turbine needs to be replaced every day that's not a dealbreaker.
Its size could also make the engine usable for some truly unique applications, like powered wing-suits that can actually take off of the ground.
Is a tiny nuclear engine like this actually feasible however, given extremely good radiation shielding? and if so what kind of things would it be capable of?
My particular setting may have magic with a limited scope, but in principle you should be able to replace "magical darkness" with Clarktech radiation shielding and things would be the same. Also I don't know the extent to which incredible radiation shielding would actually be required to make the engine itself work.