General
Uranium enrichment - Principles

Natural uranium is a mixture of isotopes with different nuclear properties. It contains 0.72% atoms of the isotope ²³⁵U, 99.274 5% atoms of ²³⁸U and, in radioactive equilibrium with the latter, 0.005 5% of ²³⁴U. The half-lives of ²³⁵U and ²³⁸U are 7.1 × 10⁸ and 4.5 × 10⁹ years respectively, long enough to explain their presence in the earth's crust.

It is the ²³⁵U isotope that sustains the chain reaction in thermal neutron reactors, where it is fissile. It is also fissile, like ²³⁸U, by fast neutrons and can also be used in breeder reactors, associated with plutonium.

While the first man-made reactor diverged in 1942 (Enrico Fermi, Chicago), the first terrestrial...