Nuclear Physics Fundamentals

Nuclear fission is the process where a heavy atomic nucleus splits into two or more smaller nuclei, along with free neutrons and a tremendous amount of energy. This is the fundamental process behind the first generation of nuclear weapons.

The Fission Process

1. A neutron strikes a fissile nucleus (like Uranium-235 or Plutonium-239)
2. The nucleus becomes unstable and splits into two smaller nuclei
3. 2-3 additional neutrons are released
4. Enormous energy is released (about 200 MeV per fission)
5. Released neutrons can trigger more fissions, creating a chain reaction

Critical Mass

For a sustained chain reaction to occur, there must be enough fissile material present - this is called the critical mass. Below critical mass, too many neutrons escape without causing additional fissions. The critical mass depends on:

• The type of fissile material (U-235 vs Pu-239)
• The purity of the material
• The geometry and density
• The presence of neutron reflectors

Nuclear fusion is the process where light atomic nuclei combine to form heavier nuclei, releasing even more energy per unit mass than fission. This powers the sun and thermonuclear (hydrogen) bombs.

Fusion Reactions in Weapons

The primary fusion reactions used in thermonuclear weapons are:

• Deuterium + Tritium → Helium-4 + neutron + 17.6 MeV
• Deuterium + Deuterium → Tritium + proton + 4.0 MeV
• Deuterium + Deuterium → Helium-3 + neutron + 3.3 MeV

Conditions Required

Fusion requires extreme conditions to overcome the electrostatic repulsion between nuclei:

• Temperature: 50-100 million Kelvin
• Pressure: Millions of atmospheres
• Sufficient density and confinement time

In weapons, these conditions are achieved using a fission bomb as a "primary" to compress and heat the fusion fuel in the "secondary" stage.