<Tr> <Th> Filling weight </Th> <Td> 14 pounds (6.4 kg) </Td> </Tr> <Tr> <Th> Blast yield </Th> <Td> 21 kt (88 TJ; 1 g mass equivalent) </Td> </Tr> <P> "Fat Man" was the codename for the atomic bomb that was detonated over the Japanese city of Nagasaki by the United States on 9 August 1945 . It was the second of the only two nuclear weapons ever used in warfare, the first being Little Boy, and its detonation marked the third - ever man - made nuclear explosion in history . It was built by scientists and engineers at Los Alamos Laboratory using plutonium from the Hanford Site and dropped from the Boeing B - 29 Superfortress Bockscar . For the Fat Man mission, Bockscar was piloted by Major Charles W. Sweeney . </P> <P> The name Fat Man refers generically to the early design of the bomb, because it had a wide, round shape . It was also known as the Mark III . Fat Man was an implosion - type nuclear weapon with a solid plutonium core . The first of that type to be detonated was the Gadget, in the Trinity nuclear test, less than a month earlier on 16 July at the Alamogordo Bombing and Gunnery Range in New Mexico . </P>

Code name of the atomic bomb that was dropped on nagasaki