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Description
When irradiated nuclear fuel is dissolved in nitric acid and contacted with tri-n-butyl phosphate (TBP) during PUREX processing, many reactions can occur. There is the potential to form a range of insoluble species, especially when solutions are concentrated by evaporation, as occurs prior to high activity liquor storage or vitrification. By replacing TBP with CHON alternatives, many of the possible drawbacks may be mitigated. N,N-dihexyloctanamide has previously been identified as a potential TBP replacement, although this can be hampered by low uranium distributions symptomatic of low complex solubility in the organic phase. Through the combination of synergic extractants, uranium recovery may be improved to the point that a CHON reprocessing flowsheet is feasible. In this report, the interactions between UO22+ and several small amide-based ligand mixtures in pseudo-aqueous media are investigated to determine whether potentially exploitable uranyl-multi-ligand complexes are generated. N,N-dimethylacetamide (DMAA), malonamide, N1,N1,N3,N3,-tetraethylmalonamide (TEMA), N1,N1,N3,N3,-tetraethyldiglycolamide (TEDGA) and N2,N2,N6,N6-tetraethylpyridine-2,6-dicarboxamide (TEPDCA) were used for this study. Small ligands were used to allow easier solubility in pseudo-aqueous media as well as reduce the currently unnecessary hindrances of sterics. Preliminary UV-visible spectroscopy screening suggests UO2-DMAA-TEMA and UO2-DMAA-TEDGA systems produce ternary complexes, whereas UO2-DMAA-TEPDCA systems do not. Comparison of malonamide and TEMA spectra suggest that ligand chelation plays a significant role in interaction strength. Spectrophotometric titrations were conducted to determine likely stoichiometry and conditional stability constants of binary complexes of UO2 with DMAA, TEMA and TEDGA. Currently, spectra suggest that UO2-TEMA complexes appear to have two dominant stoichiometries dependent on ligand concentration. Jobs plot data suggests UO2 strongly prefers diamide complexes within UO2-monoamide-diamide systems. This effect has been attributed to the weak adduct formation of DMAA.
