A study of plutonium(IV) in solution combining theoretical and experimental approaches

The actinide speciation in solution is central to the understanding and improvement of separation processes for the nuclear fuel cycle as well as environmental speciation problems. However, the radioactivity of these elements combined with their chemical complexity make the study of their speciation a real experimental challenge. That is why, bringing into play the molecular modeling may be a key help in getting new insights on this issue. In this work, coupling quantum chemical calculations to experimental data (thermodynamics, EXAFS, UV-visible spectroscopy) for studying plutonium(IV) speciation in an organic phase containing N,N-dialkyl-amide ligands enabled us to identify the formation of several complexes having different coordination modes of the ligand to the plutonium. In parallel, as no force field has been reported for describing plutonium(IV) by classical molecular dynamics, we decided to tackle the development of such a potential for this element. This development has been initiated with the parameterization of actinide - water interactions considering the series from thorium to berkelium at their +IV oxidation state. The chosen ab initio parameterization approach as well as the final force field are presented. Then, the first molecular dynamics results are compared to available experimental data. Finally, the limits and future developments concerning both the parameterization approach and the analytical potential are discussed in detail. (author)