Little Boy and Fat Man

In essence, the Little Boy design consisted of a gun that fired one mass of uranium 235 at another mass of uranium 235, thus creating a supercritical mass. A crucial requirement was that the pieces be brought together in a time shorter than the time between spontaneous fissions. Once the two pieces of uranium are brought together, the initiator introduces a burst of neutrons and the chain reaction begins, continuing until the energy released becomes so great that the bomb simply blows itself apart.

The initial design for the plutonium bomb was also based on using a simple gun design (known as the "Thin Man") like the uranium bomb. As the plutonium was produced in the nuclear reactors at Hanford, Washington, it was discovered that the plutonium was not as pure as the initial samples from Lawrence's Radiation Laboratory. The plutonium contained amounts of plutonium 240, an isotope with a rapid spontaneous fission rate. This necessitated that a different type of bomb be designed. A gun-type bomb would not be fast enough to work. Before the bomb could be assembled, a few stray neutrons would have been emitted from the spontaneous fissions, and these would start a premature chain reaction, leading to a great reduction in the energy released.

Seth Neddermeyer, a scientist at Los Alamos, developed the idea of using explosive charges to compress a sphere of plutonium very rapidly to a density sufficient to make it go critical and produce a nuclear explosion.


Little Boy
Little Boy

A replica of the Little Boy bomb that was dropped on Hiroshima.

Fat Man
Fat Man

A replica of the Fat Man bomb that was dropped on Nagasaki.

Little Boy
Preparing Little Boy

Final prepartions of the Little Boy bomb.

Little Boy
Little Boy on Tinian

The actual bomb ready to be loaded aboard the Enola Gay.

Fat Man
Preparing Fat Man

Final prepartions to the Fat Man bomb.

Fat Man
Fat Man on Tinian

The Fat Man bomb on its transport dolly

Little Boy and Fat Man
Little Boy and Fat Man

Replicas of the atomic bombs that were dropped on Japan.