the core is surrounded by a thermal shield of cast iron thick weighing . masonite and steel plates enclose the thermal shield on its top and sides, forming a biological shield for radiation protection. the bottom of the thermal shield is supported by a concrete pad topped by cast-iron blocks. the graphite composition was selected to moderate the nuclear reaction fueled by of uranium slugs approximately diameter long (the approximate size of a roll of quarters), sealed in aluminum cans, and loaded into the aluminum tubes.