I've been watching "The Expanse" and considered, if the spaceships are some sort of fusion powered drive, why aren't they covered in radiator panels to reject all the heat from the reactor/drive system? How many square meters of radiator panels would a 20MW (thermal) fission reactor require? An idea for the overall design of a nuclear powered spacecraft is to have the habitable section in the nose, a looooong truss section (100s of metres) back to the reactor and rocket. This distance reduces the radiation the crew receives by some cubic function of the distance? Is is better to put the shielding right behind the habitable section or right in front of the reactor, or doesn't it matter?