Re: Little reactors?
"There could be quite a good market for those."
Assuming they work. Tiny reactors would be more difficult to control, possibly using fast fission rather than thermal. Additionally, they'd be highly inefficient because you wouldn't have enough mass to properly utilize the thermal energy. The main reason that nuclear jet engines were abandoned is that they're just TOO HEAVY to be practical.
A tiny reactor must be created with extermely enriched fuel, 'weapons grade' or better, and it requires a pretty significant mass of external things to transfer the heat. To be controllable, you need even bigger mass/geometry and it very rapidly becomes impossible to put it into a missile. A "Big Fornicating Rocket" might be able to manage a nuclear engine (and that's been proposed) but they still need some kind of fuel/propellant to eject out the tail end and so the mass of the engine must be weight against the need for separate fuel/oxidizer and the limitations of chemical reactions.
Anyway, putting a nuclear engine on a missile that's capable of running for "unlimited" time is extremely impractical. The physics and thermodynamics just don't work very well, ya know?
[not saying IMPOSSIBLE just IMPRACTICAL]