The cold is a problem not for the reactor itself (it'll heat itself up quite nicely) but for the other gear involved with a settlement. Chemical plants, mining

The cold is a problem not for the reactor itself (it'll heat itself up quite nicely) but for the other gear involved with a settlement. Chemical plants, mining bulldozers, habitat seals - all of these things get seriously weird when you bring them down to ~125K. (For example: IIRC sabatier plants need to run in the high hundreds of K; if you're going to keep that going in a cryogenic environment you need either an enormous amount of insulation or to be willing to lose a lot of energy to the environment.) The obvious solution would be to use waste heat from the reactor in a combined heat and power installation, but then you run into the radiation problem. Coolant tubes or heat pipes that run a significant distance are heavy and bulky, but if you want to move your gear closer to keep that weight down you get in trouble from radiation. Note that it's not just people that are sensitive to radiation - the chemical beds required for fuel and oxygen production, electronics on any moving rovers. If you just want power, that's fine, bury it a few km away; but if you also need heat you're out of luck.