Uranium was used for Little Boy on Hiroshima.
Plutonium was used for Fat Man on Nagasaki.
Both incidents described above resulting in deaths were on a plutonium core. Critical mass was not achieve by creating more plutonium above subcritical. It was achieved by reflecting more neutrons into the plutonium core.
The first incident used tungsten carbide bricks to reflect neutrons that would have normally escaped back into the plutonium.
The second incident used beryllium half spheres, also to reflect.
In both incidents, the material dropped was the neutron reflecting material into direct contact. The amount of neutrons reflected back into the core greatly.
You could think of it as a mirror near a light, the amount of reflected light increases as the mirror approaches.
More neutrons reflected back into the core versus being cast away from the core decrease the amount of mass needed in the core to sustain fission reaction.
My IQ is 140, though I find it doesn't really help in everyday life lol, and I still find it VERY hard to understand a lot of this stuff.
They say it's not impossible for a reaction at the facility in Long Island I think to get out of control and swallow the earth. The odds are ridiculously small though. Still, fascinating. They made a movie based on it.