Chain reaction

A chain reaction is a self-sustaining series of nuclear fissions in which the neutrons released by one fission cause further fissions, so that the process continues and multiplies.

Description

1. A neutron strikes the nucleus of a fissile atom such as uranium-235. The nucleus splits into two smaller nuclei, releasing a large amount of energy.
2. Each fission also emits 2 or 3 new neutrons (about 2.5 on average).
3. These new neutrons strike other uranium nuclei and cause further fissions, releasing more neutrons and energy.
4. The number of fissions grows rapidly, as shown below.

   n --> U --> 3 n --> 3 U --> 9 n --> 9 U ...
        (1 fission)  (3 fissions) (9 fissions)

Conditions

• Thermal neutrons. Neutrons are most effective at causing fission when they move slowly, so a moderator (graphite, heavy water) is used to slow them down.
• Critical mass. This is the minimum mass of fissile material needed to keep the chain reaction going. If the mass is smaller, too many neutrons escape and the reaction stops.

Types

• Uncontrolled: the number of fissions increases extremely fast and releases enormous energy at once, as in an atomic bomb.
• Controlled: control rods (such as cadmium or boron) absorb extra neutrons and keep the rate steady, as in a nuclear power reactor.

Thus a chain reaction is the basis of both nuclear weapons and nuclear energy.