Straight chain amide N,N-dihexyloctanamide (DHOA) has been found to be a promising alternative extractant to tri-n-butyl phosphate (TBP) for the reprocessing of irradiated uranium- and thorium-based fuels. Unlike TBP, DHOA displays preferential extraction of Pu(IV) over U(VI) at higher acidities (≥3 M HNO3) and poor extraction at lower acidities. Density functional theory (DFT) based calculations have been carried out on the structures and relative binding energies of U(VI) and Pu(IV) with the extractant molecules. These calculations suggest that the differential hardness of the two extractants is responsible for the preferential binding/complexation of TBP to uranyl, whereas the softer DHOA and the bulky nature of the extractant lead to stronger binding/complexation of DHOA to Pu(IV). In conjunction with quantum chemical calculations, small angle neutron scattering (SANS) measurements have also been performed for understanding the stoichiometry of the complex formed that leads to relatively lower extraction of Th(IV) (a model for Pu(IV)) as compared to U(VI) using DHOA and TBP as the extractants. The combined experimental and theoretical studies helped us to understand the superior complexation/extraction behavior of Pu(IV) over U(VI) with DHOA.

The effect of successive alkylation of the Cα atom adjacent to the carbonyl group in N,N-dialkyl amides (i.e., di(2-ethylhexyl)acetamide (D2EHAA), di(2-ethylhexyl)propionamide (D2EHPRA), di(2-ethylhexyl)isobutyramide (D2EHIBA), and di(2-ethylhexyl)pivalamide (D2EHPVA)) on the extraction behavior of hexavalent uranium (U(VI)) and tetravalent thorium (Th(IV)) ions has been investigated. These studies show that the extraction of Th(IV) is significantly suppressed compared to that of U(VI) with increased branching at the Cα atom adjacent to the carbonyl group. Small angle neutron scattering (SANS) studies showed an increased aggregation tendency in the presence of nitric acid and metal ions. D2EHAA showed more aggregation compared to its branched homologues, which explains its capacity for higher extraction of metal ions. These experimental observations were further supported by density function theory calculations, which provided structural evidence of differential binding affinities of these extractants for uranyl cations. The complexation process is primarily controlled by steric and electronic effects. Quantum chemical calculations showed that local hardness and polarizability can be extremely useful inputs for designing novel extractants relevant to a nuclear fuel cycle.

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Even though tri-n-butyl phosphate (TBP) has been the work horse for spent fuel reprocessing, some of its drawbacks are of major concern. Based on batch extraction studies, our group at Radiochemistry Division, BARC has proposed N,N-dihexyl octanamide (DHOA) as an alternative extractant to TBP for the reprocessing of three component Advanced Heavy Water Reactor (AHWR) spent fuels. To validate the batch data, twelve stage counter-current mixer settler runs were carried out on a simulated AHWR feed [∼2 g/L U + ∼2 g/L Pu + 100 g/L Th + 0.03 M HF + 0.1 M Al(NO3)3 at ∼3.5 M HNO3] (with a Pu tracer instead of 2 g/L; due to marginal variation in its extraction behavior) with 0.18 M TBP and 0.36 M DHOA/n-dodecane as extractants. Quantitative extraction of Pu and U was achieved in ∼5 stages (maintaining volume ratio (O/A) as 1.1) leaving the bulk of thorium in the raffinate. Co-extracted thorium (∼4.5%) from the organic phase was scrubbed using 4 M HNO3. Partitioning of Pu from a loaded organic phase was carried out employing 0.5 M hydroxyl ammonium nitrate (HAN) at 2 M HNO3 as the strippant. Finally, uranium was stripped quantitatively using 0.01 M HNO3. The results clearly established the cocurrent studies using 0.36 M DHOA/n-dodecane as the extractant. The proposed flow sheet using 0.36 M DHOA/n-dodecane appears attractive with respect to the volume of the organic waste generated as compared to that of 0.18 M TBP/n-dodecane. Radiolytic degradation and hydrodynamic parameters of 0.36 M DHOA were also evaluated vis-à-vis 0.18 M TBP in n-dodecane.