[Gibbon1]

30 years ago as a student I tried calculating how big a space reactors cooling fins would need to be be. 1MW of power means 2 MW of heat you need to dissipate via radiation. I can't remember the numbers but I remember the result was dismal.

The problem becomes yes you can use a reactor or nuclear battery to power and ion rocket, which fuel efficient, very energy inefficient, and sloooow. Or you can use a nuclear rocket which avoids the need for a radiator by tossing your waste heat out the back end. Better fuel efficiency than a chemical rocket, can go faster, difficulty gamma radiation. Between those there isn't anything in between that makes sense because a high power thermal power reactor is too heavy.