Fission Weapon - Implosion-Type Design

The Manhattan Project, a secret U.S. program to develop a nuclear bomb during World War II, developed the concept of an implosion triggered device. In this method, subcritical masses are compressed together in a sphere by implosion. Scientists faced the problem of controlling and directing the shock wave uniformly across the sphere.

An implosion device consists of:

1. A tamper of uranium-235
2. Explosive in between the tamper and the core
3. A plutonium-239 core

The detonation sequence in modern implosion devices is as follows:

1. The explosives fire, creating a shock wave
2. The shock wave propels plutonium pieces towards the neutron initiator in the center
3. Neutrons are released and the fission reaction takes place
4. The bomb explodes

Fat Man, the second nuclear bomb ever to be deployed in war, was an implosion-type nuclear bomb. It had an efficiency of approximately 17%. Implosion type devices are more efficient than gun-triggered devices because the density of the masses increase in addition to being combined. Fat Man took 560 nanoseconds to detonate. The explosion was equivalent to roughly twelve thousand tons of TNT.

Almost all nuclear powers today employ the implosion method in all of their nuclear stockpile.