The low atmospheric pressure doesn't really affect the cooling because it could be compressed arbitrarily. Shielding on Mars could just be made from surface ro

The low atmospheric pressure doesn't really affect the cooling because it could be compressed arbitrarily. Shielding on Mars could just be made from surface rock. or you bury the reactor in the first place. Radioactive waste (like the contaminated shielding) is probably a non-issue on Mars because there already is too much uninhabitable space anyway. Anyway, such things are decades or centuries in the future. In the next decades, photovoltaic or other solar energy generation will rule on Mars.