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
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