Nuclear fission releases the energy for nuclear power stations and weaponry. They requires that the concentration of fissile material is quite high. They will usually use Uranium-235, and since only 0.7% of natural uranium is this isotope (it is usually Uranium-238) is necessary to use a good technique to filter out this lighter isotope. This is the enrichment and can be done by four methods:

• Mass spectrograph (balow)– The element is injected into a magnetic field. The different masses of the isotopes will cause them to follow different paths. The isotopes will then flow into their individual collectors.

• Gaseous diffusion separator – This has been a popular method of separating the isotopes. Uranium hexafluoride (²³⁵UF₆ and ²³⁸UF₆) as a gas is fed into a gaseous diffusion separator. The thin nickel alloy (stage) that acts as a filter where the ²³⁵UF₆ finds it easier to get through because of its smaller mass. Because of the very small difference in molecular weight (349 to 352) the amount of separation is small it may need to be filtered thousands of times.

• Centrifuge for uranium – A device that holds uranium and spins fast enough to force Uranium 238 to the outside due to its slightly heavier weight. It is in a vacuum. It is suspended and driven magnetically.

• Laser isotope separation – Uses laser light to selectively ionise uranium 235 atoms which can be drawn away from the uranium 238 atoms. It is under development.