The Mark 5 design was the first production American nuclear weapon which was significantly smaller than the 60 inch (150 cm) diameter implosion system of the Fat Man nuclear bomb design first used in 1945, down to 39 inches (99.1 cm) diameter. The Mark 5 design used a 92-point implosion system (see Nuclear weapon design) and a composite Uranium/Plutonium fissile material core or pit. The Mark 5 and W5 were a pure fission weapon. There were at least four basic models of core design used, and sub-variants with yields of 6, 16, 55, 60, 100, and 120 kilotons have been reported.