Fission & Fusion

How a chain reaction starts

• Each induced fission of uranium-235 needs one neutron going in but releases 2 or 3 neutrons coming out
• If any of those released neutrons strikes another uranium-235 nucleus, it can trigger another fission, releasing more neutrons, which trigger more fissions, and so on
• This self-sustaining sequence is a chain reaction:

a chain reaction is a process in which the neutrons released by one fission go on to cause further fissions, releasing more neutrons that cause still more fissions

• The number of fissions per second grows exponentially unless something stops it

Generations of a chain reaction

• Each "generation" of the chain is one round of fissions:

• In practice not every neutron causes a fission; some escape from the fuel, some are absorbed by impurities. The effective number of new fissions per old fission is called the multiplication factor k:
  • k > 1: supercritical, fission rate grows; bomb-like behaviour
  • k = 1: critical, fission rate is steady; this is what a working reactor maintains
  • k < 1: subcritical, fission rate dies away

Critical mass

• For a chain reaction to keep going at all, there has to be enough fissile material in one place so that a typical neutron meets another uranium-235 nucleus before escaping
• The smallest mass of fissile material that can sustain a chain reaction is the critical mass. For pure uranium-235 it is around 50 kg; for plutonium-239 it is about 10 kg
• Below the critical mass, neutrons escape from the surface faster than new fissions can replace them, so the chain dies. Above the critical mass, the chain takes off
• In a power station the mass of fuel exceeds the critical mass but the chain reaction is held at k = 1 by absorbing the excess neutrons (see control rods, below)
• In a nuclear weapon a supercritical mass is assembled suddenly, k jumps well above 1, and the entire fuel undergoes fission in a fraction of a second. The uncontrolled release of energy is a nuclear explosion