Reaction Mechanisms of La3+ and Zr4+ in LiCl-KCl Eutectic Molten Salt – Cyclic and Square Wave
Voltammetry Data Simulations
C. P. Fabian1, P. Chamelot2, L. Massot2, C. Caravaca3, V.
Luca4, and G. R. Lumpkin1
1Institute of Materials Engineering, Australian Nuclear Science and Technology Organisation, Locked Bag 2001 Kirrawee DC NSW 2232, New Illawarra Road, Lucas Heights, NSW 2234 Australia.
2Procédés Electrochimiques, Université Paul Sabatier, 118, route de Narbonne 31062 Toulouse, Cedex 4.
3Departamento de Fisión Nuclear, Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, Avda. Complutense, 22. 28040 Madrid, Spain
4Programa Nacional de Gestión de Residuos
Radioactivos, CNEA, Av. General Paz 1499, Buenos Aires, Argentina.
ABSTRACT
Nuclear energy increasingly represents an important option for generating clean CO2-free electricity. This has generated a nuclear renaissance with increased interest in both innovative nuclear reactors and fuel cycle
technologies. The innovative reactor concepts proposed have mainly involved closed fuel cycles with implied recycling of irradiated fuel. Pyroelectrochemical processes based on a molten alkali chloride/fluoride electrolyte system show considerable promise, and as such have received considerable attention for fuel reprocessing. Lanthanum and zirconium are two fission products that are expected to be present in irradiated fuels.
The present investigation addresses the electrochemical reduction of La3+ and Zr4+ in LiCl - KCl eutectic molten salt using transient techniques such as cyclic voltammetry (CV), square wave voltammetry (SWV) and chrono- potentiometry (CP) from 425 to 550 oC, at two concentration levels. Experimental CV data from 50 to 500 mV/sec appears to indicate that the lanthanum reaction mechanism involve a single step reduction process, i.e., La3+ + 3e- La. Hence a theoretical simulation of this experimental data should be
straightforward as verification of the reaction mechanism.
However, zirconium experimental CV data also from 50 to 500 mV/sec appears to indicate that its reduction proceeds through an ab initio sluggish charge-transfer step coupled with an adsorption/underpotential
deposition: Zr4+ + 2e- Zr2+ followed by a second and faster step where two electrons further are transferred:
Zr2+ + 2e- Zr. A theoretical simulation of the
experimental voltammograms has been also conducted to verify this reaction mechanism. We show the parameter in their potential separation, Epp and scan rate plot to indicate the reversibility of their electrochemical process, and evaluate their standard electrochemical rate constant.
Deconvolution of the SWV data for lanthanum and zirconium confirms that the reduction of La3+ proceeds through three-electron one-step reaction mechanism while that of Zr4+ proceeds through two two-electron step reactions independent of temperature and concentrations levels.
