Nuclear fission and fusion

Spontaneous fission is rare. Usually, a large nucleus must first absorb a neutron before it becomes unstable enough to split.

When a nucleus undergoes fission, it splits into two smaller daughter nuclei, releases 2 or 3 neutrons, releases gamma rays, and releases a large amount of energy (mostly as the kinetic energy of the fragments).

The neutrons released by one fission event can go on to be absorbed by other nuclei, triggering further fission events — a chain reaction. If this chain reaction is left uncontrolled, the number of fissions grows exponentially, releasing energy extremely rapidly — this is what happens in a nuclear weapon. In a nuclear power station, the chain reaction is controlled so that energy is released at a steady, usable rate.

The mass of the single nucleus produced by fusion is slightly less than the total mass of the two original nuclei — this 'missing' mass is converted into the energy that is released.

The Sun, and other stars, are natural fusion reactors, fusing hydrogen nuclei into helium and releasing the energy that powers them as light and heat.