Water tanks are the most likely source of radiation shielding: propellant tanks get used up and go empty, while for any lengthy mission, water is either going t

Water tanks are the most likely source of radiation shielding: propellant tanks get used up and go empty, while for any lengthy mission, water is either going to be recycled back into the tanks or you will have to take blue water tanks and over time turn them into grey water tanks, either way you will have those tanks much more filled than the propellant.