Nuclear generation didn't always require a steam turbine. Radioisotope thermoelectric generator is an old tech that doesn't need any kind of turbines or liquids to function. It's still being used on the two Voyager spacecrafts. Of course it doesn't meet DARPA's requirement of bypassing the thermal middleman but it can be scaled to be reasonably small and can generate a few hundred watts.
The issue with RTGs is that they don’t scale up, not that they don’t scale down. I’m not sure what the biggest RTG is, but they are usually measured in hundreds of watts.
Then there is the issue of the radioactive waste.
It’s useful for situations where refueling or maintenance are not options and access to solar light is poor. Pretty niche requirements, and the radiation issue limits applications to military levels of security.
The thermocouples those use are wildly inefficient and only make sense in cases where no moving parts that might break is the utmost priority (spacecraft, extremely remote lighthouses or radio relays, etc.)
Betavoltaics are another option, which actually skip the thermal step, but those are only good for very small amounts of power.
