Understanding extractant aggregation through molecular simulation

Abstract

Solvent extraction is implemented in the nuclear fuel cycle to reprocess used nuclear fuel. One processing challenge in solvent extraction is avoiding third phase formation, which limits metal loading into the organic phase and therefore extraction efficiency. Third phase formation is known to depend strongly on extractant and solvent molecular structure for solvating organophosphorus extractants. Molecular dynamics simulations were employed to study the organic phase association of extractant molecules. Force field optimization and validation were conducted for the extractant molecules for properties relevant to organic phase association, including average molecular dipole, dimerization constant and mixing enthalpy with aliphatic solvents. Network analysis was used to quantify speciation of hydrogen bonded clusters of water and nitric acid extracted by tributyl phosphate into n-dodecane. Large cluster formation under high extracted nitric acid and water concentrations near the third phase formation phase boundary was found to be consistent with percolation theory. Simulations with uranyl nitrate/extractant clusters showed orientational dependence on the inter-cluster interactions. Structural differences between extractants resulted in changes to the uranyl-centered cluster interaction energetics. Future research may link these differences in metal-centered cluster interactions to organic phase solubility and third phase formation under high metal loading.