Optimization of new extractant molecules for U(VI)/Pu(IV) separation

Abstract : The standard industrial solvent extraction process PUREX involves tri-n-butyl phosphate (TBP) as extractant to separate uranium(VI) and plutonium(IV) from fission products. TBP extractant has been used for decades but has some limitations such as its non-incinerable nature and the formation of some troublesome degradation products by radiolysis. The partition between uranium and plutonium requires the reduction of Pu(IV) to Pu(III) by introduction of reducing and stabilizing agents such as uranium(IV) and hydrazinium nitrate. New monoamide extractants showing higher selectivity are studied in order to separate U(VI) from Pu(IV) in one single cycle without reducing plutonium. This new extraction system would allow the reprocessing of MOX fuels with a higher Pu content. Amongst potential candidates, unsymmetrical N-methyl-N-alkyl monoamides are so far the most promising molecules to achieve U(VI) and Pu(IV) separation without redox chemistry. The effect of structural modifications (alkyl chains length and ramifications) on extraction performances (U and Pu distribution ratios, U/Pu selectivity) and physico-chemical properties (viscosity and loading capacity) was evaluated. In particular, the length of alkyl chains (total number of carbon atoms on the molecule) was adjusted to give enough lipophilicity preventing from third phase formations, but in the same time maintain a reasonable viscosity (compatible with a good hydrodynamic behavior).