[en] Gold and tungsten were investigated for use as inner, high-Z shells, and as yet unsolved problems were encountered with both materials. Pure electroplated gold lacks sufficient tensile strength to be useful by itself and in addition became porous foam when heated to 923⁰K in hydrogen. Tungsten, which can be deposited by chemical reduction of WF₆, does not show substantial morphological changes on exposure to hot hydrogen, but exhibits extremely low permeability to hydrogen in coatings thicker than 25 μm. This low permeability limits the maximum achievable fuel content to an amount that can be contained by a shell with wall thickness less than or equal to 25 μm