Electromagnetic isotope separation was used in combination with gaseous diffusion to build the first atomic weapons. Uranium in the form of solid uranium tetrachloride (UCl4) is put into an evacuated chamber and heated to high temperature to produce UCl4 vapour (boiling point of 792 °C at 101 kPa). The vapour is then bombarded with an electron beam to break down the molecules to generate U+ ions and other species. The charged particles are accelerated by an electrical potential to form an ion stream. The ion stream is passed through a strong magnetic field, which causes the ions to start to travel in a circular path by acting upon the particles according to the charge to mass ratio. The charge to mass ratio of the 235U+ ion is slightly higher than for the 238U+ ion so that the magnetic field will cause those ions to travel in a tighter radius, the result being that the ions of the two isotopes will separate into two beams. Careful positioning of collectors then allows the two ions to be separated. The principle of ion generation, magnetic separation and collection is essentially the same as found in mass spectrometers.

The technology provides effective separation but has a number of drawbacks, in particular:

• Not all of the UCl4 is converted to ions, so that a significant proportion of the starting material is not collected.

• Many of the ions ‘miss’ their designated collector, so that they are again not collected.

• The material is deposited within the equipment so that it quickly becomes contaminated and must be cleaned out both to allow continued operation and to recover feed material that would otherwise be wasted.

• It is a batch process with a significant time between runs.

• Many thousands of units would likely be required for a commercial scale enrichment facility.

For these reasons, the technology has never progressed to civil, commercial use.