 The impracticability of a gun @-@ type bomb using plutonium was agreed at a meeting in Los Alamos on 17 July 1944 . All gun @-@ type work in the Manhattan Project was directed at the Little Boy , enriched @-@ uranium gun design , and the Los Alamos Laboratory was reorganized , with almost all of the research oriented around the problems of implosion for the Fat Man bomb . The idea of using shaped charges as three @-@ dimensional explosive lenses came from James L. Tuck , and was developed by von Neumann . To overcome the difficulty of synchronizing multiple detonations , Luis Alvarez and Lawrence Johnston invented exploding @-@ bridgewire detonators to replace the less precise primacord detonation system . Robert Christy is credited with doing the calculations that showed how a solid subcritical sphere of plutonium could be compressed to a critical state , greatly simplifying the task , since earlier efforts had attempted the more @-@ difficult compression of a hollow spherical shell . After Christy 's report , the solid @-@ plutonium core weapon was referred to as the " Christy Gadget " . 
