In order to explode an atomic bomb, a certain amount of fissile material, called critical mass, is necessary. The fissile material used in Hiroshima was uranium-235. Inside the bomb, uranium-235 was divided into two pieces, both of which are below critical mass. Using an explosive device, the two uranium-235 were brought together to form a critical mass that exploded instantaneously. This is known as a gun-barrel-type bomb.

The fissile material used in Nagasaki was plutonium-239. Inside the bomb, the plutonium-239 is divided into subcritical portions surrounded by gunpowder. When it is detonated, the conventional explosives squeeze the plutonium core in on itself to achieve a supercritical mass. This is known as an implosion type bomb