Once extracted, uranium ore is taken to a mill to be crushed and ground into a fine powder. This is then purified in a chemical process and reconstituted in a solid form known as "yellow cake", due to its yellow colouring. Yellow cake consists of 60-70% uranium, and is radioactive.
The basic aim of nuclear scientists is to increase the amount of U-235 atoms, a process known as enrichment. To do this, the yellow cake is dissolved in nitric acid and chemically processed before being heated to become uranium hexafluoride gas.
Uranium hexafluoride is corrosive and reactive and must be handled very carefully. Pipes and pumps at conversion plants are specially constructed from aluminium and nickel alloys. The gas is also kept away from oil and grease lubricants to avoid any inadvertent chemical reactions.