Group Hexavalent Actinide Separations: A New Approach to Used Nuclear Fuel Recycling

Hexavalent Np, Pu, and Am individually, and as a group, have all been cocrystallized with UO₂(NO₃)₂·6H₂O, constituting the first demonstration of an An(VI) group cocrystallization. The hexavalent dioxo cations of Np, Pu, and Am cocrystallize with UO₂(NO₃)₂·6H₂O in near proportion with a simple reduction in temperature, while the lower valence states, An(III) and An(IV), are only slightly removed from solution. A separation of An(VI) species from An(III) ions by crystallization has been demonstrated, with an observed separation factor of 14. Separation of An(VI) species from key fission products, ⁹⁵Zr, ⁹⁵Nb, ¹³⁷Cs, and ¹⁴⁴Ce, has also been demonstrated by crystallization, with separation factors ranging from 6.5 to 71 in the absence of Am(VI), while in the presence of Am(VI), the separation factors were reduced to 0.99-7.7. One interesting observation is that Am(VI) shows increased stability in the cocrystallized form, with no reduction observed after 13 days, as opposed to in solution, in which >50% is reduced after only 10 days. The ability to cocrystallize and stabilize hexavalent actinides from solution, especially Am(VI), introduces a new separations approach that can be applied to closing the nuclear fuel cycle.