[en] We have established previously that typical DT pellets cannot be kept solid with external cooling through pure vacuum, and can only be maintained solid within DT gas-filled spaces over a limited range of cooling temperatures. This report examines whether low-density (0.01 g/cm³ or 0.05 g/cm³) polyethylene foam or helium gas have sufficient conductivity to allow inner tritium-containing shells to be kept solid. For both conductors the answer is affirmative, but some uncertainty exists concerning the conductivity of foams and the worst excursions might not provide enough conductivity. For helium much less than one millitorr pressure suffices for cooling typical pellets. For cooling the same nuclear fuel concentrated in the form of a central ball, the lightest foams are marginal but might be satisfactory at the highest excursion of our conductivity error. Required helium pressures for ball cooling are in the region of a millitorr. A simple model for the ratio of foam to solid conductivity of a substance is developed