One class of nuclear weapon , a '' fission bomb '' ( not to be confused with the '' fusion bomb '' ) , otherwise known as an '' atomic bomb '' or '' atom bomb '' , is a fission reactor designed to liberate as much energy as possible as rapidly as possible , before the released energy causes the reactor to explode ( and the chain reaction to stop ) . Development of nuclear weapons was the motivation behind early research into nuclear fission which the Manhattan Project during World War II ( September 1 , 1939 – September 2 , 1945 ) carried out most of the early scientific work on fission chain reactions , culminating in the three events involving fission bombs that occurred during the war . The first fission bomb , codenamed " The Gadget " , was detonated during the Trinity Test in the desert of New Mexico on July 16 , 1945 . Two other fission bombs , codenamed " Little Boy " and " Fat Man " , were used in combat against the Japanese cities of Hiroshima and Nagasaki in on August 6 and 9 , 1945 respectively
Trinity Test
Trinity (nuclear test)
Nuclear fission