 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 . 
