World War 2:The Manhattan Project
The design of Little Boy and Fat Man
Uranium-235 is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile (it can sustain a fission chain reaction).
The absorber shell was an 11.5 cm thick aluminum sphere also weighing 120 kg.
The plutonium pit contained 6.2 kg of plutonium alloy contained in a 9.0 cm shell.
The neutron initiator was a sphere consisting of a hollow beryllium shell, with a solid beryllium pellet inside