It seems that all nuclear propulsion concepts extract fission energy as heat, and somehow convert a portion of that heat to energy in the desired form, such as electrical (e.g. to power an ion thruster). It always amounts to creating heat in one part of the system and dumping waste heat somewhere else. For example: using a working fluid that is heated, perhaps discharged through a turbine, cooled, and sent back to the heat source. As the turbine (for example) extracts mechanical energy from the working fluid, the equivalent heat energy is removed. Nevertheless, there is still a lot of waste heat to be dissipated somewhere.
My question is: in the higher power concepts, how is all that waste heat removed from the spacecraft? In the vacuum of space, it can only leave by radiation. So wouldn't that require a very large array of radiators? Wouldn't that add a significant amount of mass to the spacecraft, affecting the overall mass efficiency?
As a secondary question: would the radiation pressure from "waste" heat for a nuclear powered electric engine be enough to make a worthwhile contribution to spacecraft thrust?