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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.