E = m c 2 (a little bit of mass can create a lot of energy)

Enrico Fermi in 1942 built a sub-critical fission reaction in a squash court at the University of Chicago.

Over a period of months, experiments were run in which Uranium 235 fuel interacted with the neutron absorbing material of the pile and control rods. As control rods were removed, more neutrons were free to strike other U-235 atoms, inching closer and closer to a chain reaction, and releasing energy in the form of heat.

Different geometries of the pile were closely studied, and agreed very well with theoretical studies.

Fat Man (below) Little Boy (above)

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Physicist Robert Oppenheimer & General Leslie Groves, the primary figures in the Manhattan Project, the super-secret, pedal-to-the-metal US effort to develop a fission weapon during the Second World War.

Little Boy, a U-235 based “rail gun” bomb. By 1945, enough U-235 had been collected to make one of these bombs. 2 half-spheres of uranium were sent towards each other, creating a “critical mass” when they touched. Manhattan Project physicists were confident this bomb would work, and it was not tested until August 8, 1945

The geography of the city helped to concentrate the blast, which killed perhaps 70,000 people in minutes, and perhaps 50,000 more over the weeks and months to follow.

The Fat Man Design used Plutonium as its fissionable material. Plutonium is synthetic : it does not occur in nature. Because of the characteristics of Pu, triggering a chain reaction required increasing the DENSITY of the Pu. This was done by surrounding it with a shell of high explosive and IMPLODING it. This was the bomb design tested at Trinity, and the bomb dropped on Nagasaki.

Because of an off target drop, the bomb was detonated about 2 miles off target, and did not cause as much chaos as the first, even though it was twice as powerful.