 The accelerated effort on an implosion design, codenamed Fat Man, began in August 1944 when Oppenheimer implemented a sweeping reorganization of the Los Alamos laboratory to focus on implosion.Two new groups were created at Los Alamos to develop the implosion weapon, X (for explosives) Division headed by Kistiakowsky and G (for gadget) Division under Robert Bacher.Although Teller was head of the T-1 (Implosion and Super) Group, Bethe considered that Teller was spending too much time on the Super, which had been given a low priority by Bethe and Oppenheimer.