Spent nuclear fuel reprocessing is an important issue for developing sustainable and economically attractive nuclear power generation. Generation of liquid radioactive wastes can be minimized by changing from the current solvent extraction technology (PUREX process) to non-aqueous pyrochemical reprocessing in molten salts. High resistance to radiation and absence of neutron moderators allow reprocessing of high burn-up fuels (including those with high plutonium content) after a short cooling time. Lowering working temperatures can be achieved by using low-melting eutectic mixtures as solvents. Developing a feasible process requires in-depth knowledge of behavior of all SNF components.

Zirconium is one of high yield fission products and also a widely used construction material. Pyrochemical reprocessing of SNF requires separation of fission products and fissile materials using, for example, electrolysis. Electrode potential of zirconium is quite close to that of uranium and developing a method for efficient separation of these two elements requires comprehensive data on their electrochemical properties. The aim of the present study was determining zirconium electrode potentials in the melts based on LiCl–KCl–CsCl low-melting ternary eutectic mixture (m.p. 263 ^oC).

Preliminary experiments performed using cyclic and square wave voltammetry, and cathodic polarization showed that zirconium ions in different oxidation sates (+4 and +2) can be present in the melts in contact with zirconium metal. Electrode potentials were determined by electromotive force measurement technique using the following galvanic cells: ZrǀMCl–ZrCl₄ǁMCl–AgClǀAg or ZrǀMCl–K₂ZrF₆ǁMCl–AgClǀAg (MCl = LiCl–KCl–CsCl eutectic). Zirconium containing electrolytes were prepared by dissolving zirconium tetrachloride or potassium fluorozirconate, K₂ZrF₆, giving purely chloride or mixed chloride-fluoride melts. The experiments were performed between 300 and 800 ^oC. An example of the experimentally measured temperature dependencies of zirconium electrode potentials in LiCl–KCl–CsCl–K₂ZrF₆ melts in shown in the Figure. The experimental potential values were then used for calculating formal standard electrode potentials of zirconium in LiCl–KCl–CsCl eutectic based melts.

Figure. Equilibrium electrode potentials of zirconium in LiCl–KCl–CsCl–K₂ZrF₆ melts. Total zirconium concentration (wt. %) is given on the plot for each set of data. Ag/AgCl (1 mol. % in LiCl–KCl–CsCl) reference electrode.