Redox behaviors of zirconium in molten LiCl-KCl eutectic salt

Significance 

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)Ox 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.

Redox behaviors of zirconium in molten LiCl-KCl eutectic salt based on the comproportionation reaction between Zr(0) and Zr(IV). Advances in Engineering

About the author

Han Lim Cha is a Ph.D. candidate in the Department of Nuclear and Quantum Engineering (NQe) at Korea Advanced Institute of Science and Technology (KAIST). He received his B.S. and M.S. in the NQe department of KAIST. His research interest is in the investigation of electrochemical behaviors of metal chlorides in high temperature molten salts, such as electrodeposition and in-situ spectroscopy for pyrochemical process monitoring.

About the author

Prof. Dr. Jong-Il Yun is an associate professor of Nuclear and Quantum Engineering (NQe) at Korea Advanced Institute of Science and Technology (KAIST). He received his B.S. in Nuclear Engineering from Seoul National University, his Dipl.-Ing. in Mechanical Engineering from RWTH Aachen University in 1996, and his Dr.-Ing. in Mechanical Engineering from the RWTH Aachen University in 2000.

After undertaking a research scientist positions including the 3-year postdoc at the Karlsruhe Institute of Technology for the period of 2000 – 2007, he took on the position as a faculty member in the NQe Department at KAIST in 2007.

His research focuses on aquatic geochemistry of radionuclides including actinides, pyrochemistry of used nuclear fuel in high temperature molten salts, water chemistry of reactors and laser-based spectroscopy for chemical speciation at the molecular level.

Reference

Han Lim Cha, Jong-Il Yun, Redox behaviors of zirconium in molten LiCl-KCl eutectic salt based on the comproportionation reaction between Zr(0) and Zr(IV), Electrochemistry Communications, (2017), 84:86-89.

 

Go To Electrochemistry Communications

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