Electrochemical Behavior of Np in the Molten LiCl–KCl Eutectic

This work presents a study on the electrochemical behavior of neptunium, and in particular of the Np(III)/Np(0) redox system, in the molten LiCl-KCl eutectic. Neptunium solutions have been prepared by two different methods: from NpO 2 dissolution into LiCl-KCl by carbochlorination (Cl 2(g) + C (s) ) at 550°C and from metal Np oxidation from an Al-Cu-Np alloy by CuCl 2 . These neptunium solutions were investigated by cyclic voltammetry. A mixture of Np(III), Np(IV) species, and Np(V) soluble oxychloride was obtained from NpO 2 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 Np(IV) and Np(III) species. The second method used by Np oxidation from an Al-Cu-Np alloy gave a stable Np(III) solution. The study of the reduction reaction Np(III)/Np(0) has been performed by different electrochemical techniques. Apparent standard potentials of the Np(III)/Np(0) system at different temperatures were determined by chronopotentiometry at zero current technique [E' o Np(III)/Np(0) = -2.68 vs Cl 2(g) (1 atm)/Cl - at 450°C]. Standard free energy, enthalpy, and entropy of formation, as well as the activity coefficient of Np(III) in LiCl-KCl, were also obtained.