Redox behaviors of zirconium in molten LiCl-KCl eutectic salt

The pyrochemical process is used in recycling the transuranic elements and uranium existing in spent nuclear fuels that are decladded in molten LiCl-KCl eutectic salt. Decladding process does not perfectly eliminate Zr from spent nuclear fuels since an intermetallic (U, Zr)O_x phase exist on fuel outer surface. During the pyrochemical process, uranium and Zr can co-dissolve and co-deposit because their standard electrode potentials are close. For the optimization of such process, the research focuses on the Zirconium (Zr) oxidation and reduction behaviors while in molten LiCl-KCl salt. Through the analytical methods such as cyclic voltammetry (CV), researchers have studied the behavior of Zr in molten LiCl-KCl although its electrochemical properties are still controversial owing to many redox reactions present and mechanistic proposals that are very fragile.  Han Lim Cha and Jong-Il Yun from Korea Advanced Institute of Science and Technology demonstrated and analyzed the comproportionation reaction of Zr(IV) and Zr(0) to study the behaviors of Zirconium in molten LiCl-KCl for the very first time.

Addition of Zr powder to molten LiCl-KCl-ZrCl₄ forms the comproportionation reaction leading to a molten salt color change from colorless to dark brown. The characteristic absorption band of 530 nm indicates intermediate Zr ion formation. From the comproportionation reaction of Zr(IV) and Zr(0), Zr(II) is formed which according to research is professed as a Zr product. The analysis of the intermediate Zr ion was then conducted by the X-ray photoelectron spectroscopy (XPS) to check on the oxidation state in of the solidified salt. The oxidation state was reviewed and confirmed using the square wave voltammetry (SWV) which measures the differential current through the altering parameters regarding electric potential. It is possible to calculate the number of electrons that were involved in a reaction if the redox reaction is reversible. The Zr(III), the intermediate Zirconium ion, was the resulting product identified from comproportionation reaction between Zr(0) and Zr(IV) instead of Zr(II). Several redox peaks were identified through comparing the Zr and W working electrode (WE) with the measured cyclic voltammograms in CV examination. Based on the result of cyclic voltammetry, the fact that Zr(0) to Zr(IV) oxidation reaction occurs at O4, is clarified. Also, the Zr(0) to Zr(III) oxidation reaction occurs at O2 on the working electrode, and some steps during comproportionation reaction bring about the change of oxidization mechanism of O2.

Han Lim Cha and Jong-Il Yun study is crucial as no other research had considered Zr(0) and Zr(IV) comproportionation reaction in molten LiCl-KCl eutectic salt. Moreover, the research reported positively contributes to the recycling of Zr from spent nuclear fuel and also reduces zirconium co-deposition during electro-refining. Through the experiment conducted, the behaviors of Zr ions were identified by the use of XPS and SWV techniques. They also confirmed the Zr(III) as the products of the reaction rather than the earlier perceived Zr(II) ions. A considerable difference was shown through the changing of the working electrodes in the cyclic voltammogram of the LiCl-KCl-ZrCl₄ system. The research work is now published in journal, Electrochemistry Communications.