a process for chemical enrichment of uranium with respect to a lighter one of its isotopes . the process consists in contacting uranium of valence state iii and uranium of valence state iv , or a compound of uranium of valence state iii and a different compound of uranium of valence state iii . one of the phases which are contacted or the only phase is liquid . the system should be substantially free of elements which would cause uranium iii to oxidize to valence iv .