Study of the Electrochemical Reduction of Zr[sup 4+] Ions in Molten Alkali Fluorides

Abstract

The electrochemical behavior of Zr 4+ in potassium-free fused alkali fluorides has been studied in the temperature range of 773-1123 K. With an inert tungsten or molybdenum electrode, a well-defined oxydo-reduction wave was observed during cyclic voltammetry experiments. The reduction of Zr 4+ is a single four-electron reversible step, and the process is diffusion controlled. Chronopotentiometry measurements give rise to the diffusion coefficient values of Zr 4+ ions. For instance, a value 2.92 ϫ 10 −5 cm 2 s −1 was found at 1010 K. The reduction of Zr 4+ ions was also studied using a nickel electrode for which Ni-Zr alloys formation was pointed out as revealed by cyclic voltammetry. The analysis of these deposits by scanning electron microscopy ͑SEM͒ and energy dispersive X-ray showed the presence of a multilayered Ni-Zr alloy with different Zr contents. On graphite electrodes, zirconium carbides have been evidenced on cyclic voltammograms and the formation of Zr metal was observed by SEM.The pyroelectrochemical route is considered as one of the most promising options in an innovative nuclear fuel cycle. 1 Indeed, this process allows the separation of actinides from lanthanides by electrorefining in fused electrolytes. 2-7 For instance, Lantelme et al. 5 have performed the extraction of lanthanum in fused alkali chlorides via the formation of La-Ni alloys using Ni and Ni-Ti alloyed electrodes. Recently, Massot et al. have reported the formation of Sm-Ni alloys in molten LiF-CaF 2 . 6 Several studies have also been devoted to the electrochemical behavior of Zr ions in fused LiClKCl due to the electrorefining of binary U-Zr Experimental Breeder Reactor II driver fuel. 8,9 In this refining process, Zr is commonly considered as a nuisance 10 due to its slightly higher potential than uranium. 9,11,12 Consequently, the current efficiency for the actinides recovering is significantly decreased by the Zr dissolution and, thus, the electrochemical behavior of Zr in such a medium was widely studied.The interest devoted to the preparation of Zr and Zr-based alloys is also because these materials can be used for various applications, such as corrosion-resistant materials or as catalysts in various processes, such as for example, for the hydrogen evolution reaction. 13 For example, when H 2 is used as a fuel in small fuel cells, it is a convenient way to store it is as a hydride because it is compact and easy to operate. A typical metal hydride uses an AB 2 -type alloy, where A is, for example, Zr or Zr-Ti alloys and B is a mixture of transition metals. 14 Another original use of stainless steel-zirconium alloys was proposed by Argonne National Laboratory 15 for the express purpose of immobilizing radioactive metal fuel components left behind following the electrometallurgical treatment of spent nuclear fuel. 16 Such materials have been proposed for the consolidation and the disposal of waste stainless steel, Zr, and noble metalfission products, such as Nb, Mo, Tc, Ru, Pd, and Ag recovered from spent nuclear fuel assemblies....

Heat-transport properties of molten fluorides: Determination from first-principles

“…Future work should focus on the prediction of these quantities for other salts like LiF-ThF 4 , for which very few experimental data is available. It is important to note that nowadays an important experimental work is aimed at the determination of structural [33][34][35][36][37][38][39] and electrochemical [40][41][42][43] properties of the molten fluorides, but only one group is involved in studies concerning their thermal properties [44][45][46](some work has also been done on the system LiF-NaF-BeF 2 in the framework of the fusion energy reactors [47]). In the future simulations will be of particular importance to complete the gaps in the necessary databases.…”

Transport in molten LiF–NaF–ZrF4 mixtures: A combined computational and experimental approach

“…It is routinely performed in roomtemperature ionic liquids [10,11], but up to now no such study appeared for molten fluorides. Estimates can be done from electrochemical measurements, and for example a value of 2:92 Â 10 À5 cm 2 s À1 was determined for the diffusion coefficient of Zr 4+ present in small concentration in a LiF-NaF mixture at 1010 K [22]. In MD simulations, the diffusion coefficient can also be computed from the mean-squared displacements through the Einstein expression:…”

Zirconium(IV) electrochemical behavior in molten LiF-NaF

“…In fluoride media, two different Zr (IV) ions reduction mecha nisms have been proposed: either a two step reduction [12] or an one step reduction [10,11]:…”