Optimization Studies for the Recovery of Thorium from Advanced Heavy Water Reactor High Level Waste (AHWR-HLW) Solutions Using Green Solvents

Investigations on preferential Pu(IV) extraction over U(VI) by N,N-dihexyloctanamide versus tri-n-butyl phosphate: evidence through small angle neutron scattering and DFT studies

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.

Characterization of the species formed during the extraction of thorium employing tri-n-butyl phosphate and N,N-dihexyl octanamide as extractants by laser desorption/ionization time-of-flight mass spectrometry

Laser desorption/ionization-time-of-flight mass spectrometry (LDI-ToF-MS) has been applied to identify and characterize the organic phase species formed during the extraction of thorium nitrate by 1.1 M tri-n-butyl phosphate (TBP) and N,N-dihexyl octanamide (DHOA) solutions in n-dodecane. The aqueous phase thorium concentrations (at 4M HNO3) have been suitably chosen to get Loaded organic phases with/without third-phase. The extracted species have been characterized for the first time using LDI-ToF-MS. The results show feasibility of the use of this technique for understanding the extraction mechanisms and third-phase formation behavior of different extractants. The different chemical species observed using this technique are consistent with those observed by small-angle neutron scattering (SANS).