The aim of enrichment is to increase the proportion of fissile uranium-235 atoms within uranium.
For uranium to work in a nuclear reactor it must be enriched to contain 2-3% uranium-235. Weapons-grade uranium must contain 90% or more u-235.
A common enrichment method is a gas centrifuge, where uranium hexafluoride gas is spun in a cylindrical chamber at high speeds. This causes the slightly denser isotope u-238 to separate from the lighter u-235.
The dense u-238 is drawn towards the bottom of the chamber and extracted; the lighter u-235 clusters near the centre and is collected.
The enriched u-235 is then fed into another centrifuge. The process is repeated many times through a chain of centrifuges known as a cascade.
The remaining uranium - essentially u-238 with all the u-235 removed - is known as depleted uranium. Depleted uranium, a heavy and slightly radioactive metal, is used as a component in armour-piercing shells and other munitions.
Another method of enrichment is known as diffusion.
This works on the principle that of the two isotopes present in uranium, hexafluoride gas, u-235 will diffuse more rapidly through a porous barrier than its heavier cousin, u-238.
As with the centrifuge method, this process must be repeated many times.