Determination of the Structures of Uranyl–Tri‐n‐butyl‐Phosphate Aggregates by Coupling Experimental Results with Molecular Dynamic Simulations

The complex structure of a PUREX process organic phase was characterized combining experiments and molecular dynamics simulations. For the first time the molecular interactions between TBP and the extracted solutes, and the TBP aggregation after extraction of water and/or uranyl nitrate were described and analysed concomitantly. It was emphasized that coupling molecular dynamics simulations with Small and Wide Angle X-ray Scattering (SWAXS) experiments can lead to simulated organic solutions that are representative of the experimental ones, even for high extractant and solute concentrations. Furthermore, it was shown that this coupling is well adapted for the interpretation of the (SWAXS) experiments without preliminary hypothesis on the aggregates size or shape. The results link together the previous literature studies obtained for each level of depiction separately (complexation or aggregation). Without uranium, or at low metal concentration, almost no aggregation was observed. At high uranium concentration, organic phases contain small [UO₂(NO₃)₂(TBP)₂]n polymetallic aggregates (with n = 2 to 4) in which the 1:2 U:TBP stoichiometry is preserved.