Influences on Neptunium Incorporation in Uranyl Phases

Neptunium incorporation into uranium(VI) compounds has been proposed as a potential means of mitigating its release from a spent nuclear fuel repository if the waste containers are compromised. Many studies have demonstrated the feasibility of the direct substitution of Np(V) for U(VI); however, the underlying factors that affect neptunium uptake are not well understood. The objective of this research is to study the factors that influence neptunium incorporation, particularly charge-balancing mechanisms and structural considerations, in order to gain a better understanding of the fundamental chemistry of this process.

The compounds investigated in this work include Cs[(UO2)(HSeO3)(SeO3), Cu[(UO2)(PO4)]2?nH2O, Na4[(UO2)2(Si4O10)2](H2O)4, (NH4)4[(UO2)5(MoO4)7](H2O)5, and (NH4)2[(UO2)6(MoO4)7](H2O)2. Neptunium incorporation was studied in situ during the synthesis of each compound. The doped compounds were studied by single crystal x- ray diffraction, ultraviolet-visible-near infrared spectroscopy, and laser ablation-inductively coupled plasma-mass spectrometry.

In the cases of Cs[(UO2)(HSeO3)(SeO3)] and Cu[(UO2)(PO4)]2?nH2O, Np(VI) was identified in the reaction products despite starting the reactions with Np(V). Cs[(UO2)(HSeO3)(SeO3)] had the highest uptake of neptunium to date with up to 16% substitution and was also the first time a structural change occurred.

The role of different charge-balancing mechanisms was studied in Cu[(UO2)(PO4)]2?nH2O and Na4[(UO2)2(Si4O10)2](H2O)4; however, the results of these experiments were inconclusive due to the formation of Np(VI) in the former and poor LA-ICP-MS data in the latter. Incorporation in (NH4)4[(UO2)5(MoO4)7](H2O)5 and (NH4)2[(UO2)6(MoO4)7](H2O)2 was compared to study how the extended structure of the host compound affects neptunium incorporation. More Np(V) was present in the latter, which adopted a more dense framework structure.

As the understanding of the factors that affect neptunium incorporation continues to improve, the future of this research needs to evaluate how incorporation affects the properties of the uranyl host compound, which will have a strong impact on mitigating the release of neptunium from a repository.