A physical process that increases the proportion of the fissile uranium isotope ²³⁵U in nuclear fuel. Most nuclear reactors require ²³⁵U enrichment of 3-5 percent, with some facilities requiring even higher enrichment. Enrichment uses the mass difference between uranium isotopes ²³⁸U and fissile ²³⁵U, which is the weight of three neutrons. The uranium atoms are separated according to their weight, gradually increasing the concentration of ²³⁵U to the desired value. Enrichment options are gaseous diffusion, centrifuge enrichment or laser excitation. An older method of enrichment was the separation by magnetic fields in a device called a calutron.