Study of thermodynamic properties of Np–Al alloys in molten LiCl–KCl eutectic

Pyrochemical methods are investigated worldwide within the framework of Partitioning and Transmutation concepts for spent nuclear fuel reprocessing. Electroseparation techniques in a molten LiCl–KCl are being developed in ITU to recover all actinides from a mixture with fission products. During the process, actinides are selectively electrochemically reduced on a solid aluminium cathode, forming solid actinide–aluminium alloys. This work is focused on the thermodynamic properties of Np–Al alloys in a temperature range of 400–550 °C and on the characterisation of the structure and chemical composition of deposits obtained by electrodeposition of Np on solid Al electrodes in a LiCl–KCl–NpCl3 melt. Cyclic voltammetry and open circuit chronopotentiometry have been used to examine the electrochemical behaviour of Np on inert W and reactive Al electrodes. Gibbs energies, enthalpy and entropy of formation and standard electrode potentials of Np–Al alloys were evaluated and compared with ab initio calculations. Galvanostatic electrolyses at 450 °C were carried out to recover Np onto Al plates and the solid surface deposits were characterised by XRD and SEM-EDX analyses. Stable and dense deposits consisting of NpAl3 and NpAl4 alloys were identified. In addition, the conversion of NpO2 to NpCl3 is described, using chlorination of the oxide in a molten salt media by pure chlorine gas.