Electrochemical Behavior of Np in the Molten LiCl–KCl Eutectic
Résumé
This work presents a study on the electrochemical behavior of neptunium, and in particular of the NpIII/Np0 redox system, in the molten LiCl–KCl eutectic. Neptunium solutions have been prepared by two different methods: from NpO2 dissolution into LiCl–KCl by carbochlorination Cl2g + Cs at 550°C and from metal Np oxidation from an Al–Cu–Np alloy by CuCl2. These neptunium solutions were investigated by cyclic voltammetry. A mixture of NpIII, NpIV species, and NpV soluble oxychloride was obtained from NpO2 dissolution by carbochlorination. In order to remove any oxygenated species from the melt, this solution was contacted with metal Zr, leading in our experimental conditions to a mixture of stable NpIV and NpIII species. The second method used by Np oxidation from an Al–Cu–Np alloy gave a stable NpIII solution. The study of the reduction reaction NpIII/Np0 has been performed by different electrochemical techniques. Apparent standard potentials of the NpIII/Np0 system at different temperatures were determined by chronopotentiometry at zero current technique ENpIII/Np0 o = −2.68 vs Cl2g1 atm/Cl−at 450°C. Standard free energy, enthalpy, and entropy of formation, as well as the activity coefficient of NpIII in LiCl–KCl, were also obtained.