Uranium exists in several forms, or isotopes, only one of which can sustain the energy-releasing chain reaction that makes the element so powerful. The isotope of interest to nuclear engineers is known as U-235. This makes up less than 1 per cent of naturally occurring uranium. The heavier, more stable - and thus less useful - isotope U-238 makes up almost all of the rest.
To create a chain reaction, engineers have to increase the proportion of U-235 from its natural concentration. This is the "enrichment" process. The enrichment begins with a chemical treatment of the mined uranium ore to create a concentrate called yellowcake. This is then combined with fluorine atoms to create uranium hexafluoride gas. At the Iranian enrichment facility, the gas is fed into a cylindrical centrifuge, which spins to set the gas rotating. Just as passengers in a car are thrown sideways when taking a bend, the rotation of the centrifuge throws the heavier gas molecules, the ones carrying the U-238 atoms, to the outside walls of the centrifuge.
The result is a higher proportion of the desired U-235 atoms in the gas in the middle of the centrifuge. This "enriched" gas is drawn off and fed into another centrifuge, where the process starts again. Repeating this procedure in a "cascade" of centrifuges will eventually yield a gas with the desired concentration of U-235 atoms. Once this has been achieved, the fluorine atoms are chemically stripped from the gas to give a "packet" of enriched uranium.
The higher the concentration of U-235 atoms, the smaller the lump of uranium needed to create a chain reaction. A nuclear power plant typically needs concentrations of 3 to 5 per cent. A missile warhead would need to be composed of more than 90 per cent U-235, but a conventional nuclear bomb could be built at concentrations as low as 50 per cent. Although the 20 per cent enrichment achieved by Iran's scientists is not enough to create the material for a bomb, it is a significant step forward. That's because the more enriched the uranium, the more efficient each processing step becomes; the Iranians have, in effect, done the hard part.