Depleted uranium
Additionally, (n, 2n) reactions with fast neutrons also convert small amounts of 235U to 234U. This is countered by the rapid conversion of available 234U into 235U through thermal neutron capture. Spent fuel may contain as much as 0.010% 234U, or 100 parts per million, a higher fraction than in natural uranium's 55 parts per million. Depleted uranium separated during the enrichment process contains much less 234U (around 0.001%) which makes the radioactivity of depleted uranium about one-half of that of natural uranium. Natural uranium has an "equilibrium" concentration of 234U—the point at which an equal number of decays of 238U and 234U will occur.