Uranium enrichment is the process, through which the isotopic proportion of U-235 is increased from 0.72 per cent to up to 94 per cent.
Uranium is considered low-enriched if its isotopic proportion of U-235 remains below 20 per cent. Most commercial reactors use low-enriched uranium (LEU) below five per cent as fuel, which is also often referred to as “reactor-grade uranium”. LEU does not deteriorate and can be safely stored for many years.
If uranium is enriched beyond 20 per cent, it is considered highly enriched. Uranium with such high isotopic proportions of U-235 is mostly used in naval propulsion reactors (for example in submarines), nuclear weapons and some research reactors.
Different methods can be used to increase the isotopic proportion of U-235. Typically, the yellow cake is converted into a gaseous form, called uranium hexafluoride. This gas is then pumped into fast spinning cylinders — centrifuges — where heavier isotopes, such as U-238, are pushed towards the walls of the cylinders, and the lighter U-235 stays in the centre of the cylinders. This enables to “filter out” and collect the gas with higher concentrations of U-235. The process can be repeated until the isotopic proportion of U-235 is sufficient. The acquired gas then goes through a process of re-conversion, which enables it to turn U-235 into the form of black power — uranium dioxide.
