Hydrogen is really the only propellant that makes sense for a nuclear thermal rocket. A nuclear reactor can't get substantially hotter or higher pressure than a chemical rocket engine, the reason it offers high specific impulse (basically efficiency) is because since you don't need chemical energy from the propellant to heat the propellant, 100% of your thrust can come from low molecular weight propellants (ie hydrogen). Helium will also give you better performance than chemical rockets, but substantially worse than hydrogen, and it's even more deeply cryogenic. Anything heavier than helium is going to provide little to no advantage over a chemical rocket, certainly nothing to justify the atrocious thrust to weight ratio and the extreme engineering challenges.