 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 much 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.