Nuclear rockets are a bit funny: they have a separate fuel (something that produces heat, like U-235, Thorium, etc) and propellant (something that gets heated

Nuclear rockets are a bit funny: they have a separate fuel (something that produces heat, like U-235, Thorium, etc) and propellant (something that gets heated and pushed out the back, like Hydrogen, Nitrogen, etc) . I think the idea is to carry a lifetime supply of nuclear fuel. Nuclear rockets aren't necessarily super particular about what propellant they use in conjunction with that fuel, so you can probably easily either directly scoop or otherwise easily refine/distill usable propellants anywhere you can find gas or something that is liquefiable.