Solvent extraction is utilized by the nuclear energy industry as a separations technique for reprocessing used nuclear fuel. In the industry standard PUREX (Plutonium URanium EXtraction) process, an amphiphilic extractant molecule like tributyl phosphate (TBP) selectively transfers metal ions of interest, such as uranium or plutonium or thorium, from a dissolved aqueous solution into a dodecane organic phase. It has been shown that the aggregation chemistry of similar extractant molecules varies drastically despite similar molecular structure. This difference in behavior is evident in third phase formation, an undesirable phenomenon that occurs at high extracted metal ion concentrations.  TBP is more prone to this phenomenon than the similar extractant diamyl amyl phosphonate (DAAP).  This study examines the structure and interaction of post-extraction TBP-metal ion complexes in their organic diluent using molecular dynamics (MD) simulations.  Initial studies involve the development of an MD model of TBP more accurate than those commonly used in the literature. To ensure a realistic and robust model, comparison to relevant experimental properties was performed. TBP density, average molecular dipole and dimerization constant in dodecane were calculated and successfully validated against experimental values. Neat TBP simulation density and dipole were calculated as 980 ± 2 and 3.28 ± 0.02, compared to experimental values of 972 and 3.32-3.35. The calculated dimerization constant was measured as 0.18 ± 0.04 compared to an extrapolated experimental value of 0.20 ± 0.01. Once the methodology for successfully modeling TBP was determined, alternative extractants like DAAP were also modeled. These extractant molecules will be implemented in molecular dynamics simulations of third phase conditions to identify the role that the molecular-level structural differences between TBP and DAAP play in inter-complex interaction.

Use of molecular dynamics to evaluate tributyl phosphate and diamylamyl phosphonate containing systems

249th National Meeting (2015)

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