Alexandre Ruas

Molecular Dynamics Studies of Concentrated Binary Aqueous Solutions of Lanthanide Salts: Structures and Exchange Dynamics

Concentrated binary aqueous solutions of lanthanide (Nd(3+) and Dy(3+)) salts (ClO(4)(-), Cl(-), and NO(3)(-)) have been studied by means of classical molecular dynamics (MD) simulations with explicit polarization and UV-visible spectroscopy. Pair interaction potentials, used for the MD simulations, have been developed in order to reproduce experimental hydration properties. Nd(3+) and Dy(3+) have been chosen because of their position in the lanthanide series: Nd(3+) being a light lanthanide and Dy(3+) a heavy one. They are respectively coordinated to nine and eight water molecules, in pure water, involving changes in their salt hydration structures. Both MD simulations and UV-visible experiments highlight the stronger affinity of nitrate anions toward Ln(3+) compared to perchlorates and chlorides. Dissociation/association processes of Nd(3+)-Cl(-) and Nd(3+)-NO(3)(-) ion pairs in aqueous solution have been analyzed using potential of mean force profile calculations. Furthermore, from MD simulations, it appears that the affinity of anions (perchlorate, chloride, and nitrate) is stronger for Nd(3+) than Dy(3+).

High accuracy plutonium mass determination by controlled-potential coulometry

Abstract Controlled-potential coulometry is an assay method for determining the quantity of a given element from the measurement of a quantity of electricity. This method has the advantage of not being related to a chemical standard, having an extremely high trueness and reproducibility, and using low sample amounts. Nowadays, equipments intended to carry out coulometry with high trueness (typically uncertainties close to 0.1%) is seldom encountered. In order to arrive to such low uncertainties, the whole process, from the preparation of the sample test to thestorage of the material had to be refined. The qualification of the method was carried out thanks to the use of EQRAIN standard solutions. The coulometric proportioning of plutonium coming from pure solutions (without interfering elements) was extended to the proportioning of plutonium in the presence of uranium, which makes the technique not only a method of choice for standard solution certification but also appropriate for carrying out precise analyses assessments of different processes used in nuclear medium.