High-temperature reactor designs are essential. Radiative heat transfer scales with temperature to the fourth power.
So if you reject 100 MW of waste heat at, say, 400°C (750°F), a radiative heat transfer area of 10,000 m^2 (i.e. 100m x 100m square) would be sufficient. That's quite big and hot, but 100 MW is a lot!
Radiative heat rejection at 100°C would require 10x as much area.
Disposing heat at a higher temperature would also mean that you need to dispose of more heat (~2x in the case of 400°C vs sub-100°C) and losing energy which could've been used for useful work such as generating electricity.
It may be reasonable to do at early stages, but after the colony is able to produce metals in situ it would be better to build more radiators. Especially considering that you also need to dispose latent heat from other sources.