A study on the electrolysis of actinide and lanthanide fluorides in the LiF-NaF-KF melt

A fundamental study on the electrolysis of actinide and lanthanide fluorides in the LiF-NaF-KF (46.5-11.5-42.0 mol%; FLINAK) eutectic salt medium at 773K was performed by means of cyclic voltammetry as well as potentiostatic electrodeposition of metals. The positions of peak potential with the change of scanning rate in the cyclic voltammogram were examined, which enables us to identify the reversibility of electrochemical reduction and oxidation of the concerned ion at the electrode. It was considered that the reduction peaks at -0.8 V and -1.3 V (vs. the Ni-Q.R.E.) corresponded to the electrodeposition of actinide ions in the FLINAK melt. The peak at -2.05 V seems to be attributable to the multiple reductions of actinide and lanthanide ions in the melt. The deposits formed by the potentiostatic electrolysis of the FLINAK mixture with metal fluorides showed that the neptunium ion was selectively reduced up to the potential -1.2 V, whereas the mixture of actinides was deposited at a lower potential than -1.4 V. The analysis results by ICP (Inductively Coupled Plasma) and standard gamma spectroscopy of the samples revealed that salt adhesion and the relative amount of lanthanides in the cathode deposits were higher than that estimated in the range of -0.8 - -2.0 V, indicating that lanthanides are always accompanied by the reduction of actinides in the FLINAK melt in spite of their higher values of Gibbs free energy of formation than those of actinide fluorides. (author)