Implosion-Triggered Fission Bomb
Early in the Manhattan Project, the secret U.S. program to develop the atomic bomb, scientists working on the project recognized that compressing the subcritical masses together into a sphere by implosion might be a good way to make a supercritical mass. There were several problems with this idea, particularly how to control and direct the shock wave uniformly across the sphere. But the Manhattan Project team solved the problems. The implosion device consisted of a sphere of uranium-235 (tamper) and a plutonium-239 core surrounded by high explosives. When the bomb was detonated, this is what happened:
  • The explosives fired, creating a shock wave.
  • The shock wave compressed the core.
  • The fission reaction began.
  • The bomb exploded.

Fat Man was this type of bomb and had a 23-kiloton yield with an efficiency of 17 percent. These bombs exploded in fractions of a second. The fission usually occurred in 560 billionths of a second.

In a later modification of the implosion-triggered design, here is what happens:

  • The explosives fire, creating a shock wave.
  • The shock wave propels the plutonium pieces together into a sphere.
  • The plutonium pieces strike a pellet of beryllium/polonium at the center.
  • The fission reaction begins.
  • The bomb explodes.