You can seed expanding liquid hydrogen in the outer cavity with tungsten nanoparticles which absorb the UV radiation to heat the hydrogen for use as a propellant. You can pump the outer cavity with a UV transparent coolant and line the walls with parabolic photovoltaics (that convert UV instead of visible spectrum) for direct conversion of the black body radiation to electricity. You can theoretically even create a magnetohydrodynamic "turbine" in the outer cavity that is coupled to the spinning nuclear fuel (which can be charged plasma).
You may be thinking of an earlier that was mistakenly called nuclear lightbulb or another design that was lumped into the concept. The variation I have in mind is just as general purpose as any other nuclear reactor, it just uses a lot less fuel and has to actively maintain temperature and pressure to keep the neutron cross section energy high enough to sustain power positive fision.
There are several other reactor designs that have been researched over the last half century that can operate in a closed loop with much smaller amounts of nuclear fuel and more manageable radiation emissions. Designs like the nuclear lightbulb, which was researched by United Aircraft Corporation and NASA for almost a decade before the plug on the Mars mission was pulled in the 70s, are much better suited and are what proponents of active nuclear propulsion most often have in mind. Once there is some political will, we have decades old research to start from to build a flight capable nuclear reactor.