Electrochemical behavior and electrodeposition of zirconium in molten LiCl-KCl eutectic salt

Abstract

In order to develop electrometallurgical recycling processes of spent nuclear fuel and used Zr-alloy fuel cladding, it is necessary to clearly understand the electrochemical behavior and electrodeposition characteristics of Zr in molten LiCl-KCl eutectic salt. In this dissertation, a complete understanding of the redox mechanism of Zr and the development of a technique that selectively electrodeposits Zr metal on the electrode are achieved. First, the pulsed potential electrodeposition of the Zr species is demonstrated. A new reduction step of Zr species is discovered by cyclic-voltammetry analysis with a Zr electrode. Based on the cyclic voltammetry results, pulsed potential electrodepositions are performed in each reduction step. The characterization result of deposits confirmed that Zr metal could be selectively deposited on the W electrode. Subsequently, the oxidation mechanism of Zr is studied by utilizing Zr as the electrode for anodic dissolution experiments. A real-time optical monitoring system is utilized for optically scrutinizing the dissolution processes. The formation of a thin film layer on the Zr electrode is observed during passive dissolution. Also, a unique phenomenon is reported: an agglomerate is formed on the Zr electrode surface during transpassive dissolution. The chemical analysis results show that the agglomerate consists of potassium hexachlorozirconate (K2ZrCl6) and LiCl-KCl. The agglomerate formation is due to the Zr(IV) solubility limit in molten LiCl-KCl eutectic salt.