Electrochemical Behavior of Plutonium in Molten Fluoride Medium

Abstract

A pyrochemical process for slag recycling containing trapped actinide particles into a CaF2-CaO matrix is currently in progress. Since the process requires to melt the slag (melting point above 1300°C), an additional amount of CaCl2 must be used to decrease the operating temperature below 1000°C. Then, a reducing agent (calcium) is added into the molten medium to remove the actinides in a stable metallic form after decantation. The clean salt product meets the radiological requirements to be discarded to the dedicated way (ANDRA). However, the extraction process is operated in a molten chloro-fluoride salt which is complex and quite unknown. Pu investigation in simplified media, chloride and fluoride, must lead to understand the mechanism of Pu reduction in chloro-fluoride media. Pu behavior has already been investigated in molten chloride media via electrochemical techniques by J. Serp et al. [1], O. Shirai et al. [2] and Lambertin et al. [3]. Nevertheless, data regarding this actinide behavior in molten fluoride media are incomplete [4]. Consequently, a detailed electrochemical study was carried out on plutonium in fluoride salt. The selected solvent is a mixture of two well-known salts, LiF and CaF2, melted in a crucible made of vitreous carbon in an oxygen free atmosphere (argon). Plutonium ions were introduced into the melt in the form of PuF4 powder. The setup consisted of a three electrodes assembly: tungsten as working electrode, vitreous carbon as auxiliary electrode and the potentials were referred to a platinum electrode. Cyclic voltammetry, square wave voltammetry and chronopotentiometry were coupled with thermodynamic calculations (potential-oxoacidity diagram) to investigate the electrochemical system. The acquired data will be used for a better understanding of Pu extraction from the melt. [1] J. Serp, R.J.M. Konings, R. Malmbeck, J. Rebizant, C. Scheppler, J.-P. Glatz, Electrochemical behaviour of plutonium ion in LiCl-KCl eutectic melts, J. Electroanal. Chem., 561 (2004) 143. [2] O. Shirai, T. Iwai, Y. Suzuki, Y. Sakamura, H. Tanaka, Electrochemical behavior of actinide ions in LiCl-KCl eutectic melts, J. Alloys Compd., 271-273 (1998) 685. [3] D. Lambertin, S. Ched’homme, G. Bourgès, S. Sanchez, G. Picard, Plutonium chemical properties in NaCl-KCl and CaCl2 at 1073 K, J. Nucl. Mater., 341 (2005) 124. [4] C. Hamel, Séparation actinides-lanthanides (neodyme) par extraction électrolytique en milieux fluorures fondus, vol. 3, 2005. Thesis, Toulouse.