Development of Zirconolite-based Glass-Ceramics for the Conditioning of Actinides
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Development of Zirconolite-based Glass-Ceramics for the Conditioning of Actinides P. Loiseau1, D. Caurant1, N. Baffier1, L. Mazerolles2 and C. Fillet3 1 de Chimie Appliquée de l'Etat Solide (UMR CNRS 7574), ENSCP, 11 rue P. et M. Curie, 75231 Paris Cedex 05, France 2 Centre d'Etudes de Chimie Métallurgique (CECM, UPR CNRS 2801), 15 rue Georges Urbain, 94407 Vitry-sur-Seine Cedex, France 3 Commissariat à l'Energie Atomique (CEA), Centre d'Etudes de la Vallée du Rhône, DCC/DRRV/SCD/LEBM, BP 171, 30207 Bagnols-sur-Cèze, France ABSTRACT Zirconolite (CaZrTi2O7) based glass-ceramics designed for the specific immobilization of plutonium wastes or minor actinides (Np, Am, Cm) from high level radioactive wastes were investigated. To reach an efficient double containment, actinides must be preferentially located in the crystalline phase, which is homogeneously dispersed in a calcium aluminosilicate residual glass. Several heat treatments (between 950° and 1350°C) of a parent glass belonging to the SiO2-Al2O3-CaO system and containing TiO2 and ZrO2 were performed to prepare glass-ceramics. Trivalent minor actinides were simulated introducing Nd2O3 in the glass composition. Electron microscopy, X-ray diffraction (XRD) and thermal analysis have shown that devitrification processes in the bulk and on glass surface are different. They lead to the crystallization of zirconolite in the bulk and to a mixture of titanite (CaTiSiO5) and anorthite (CaAl2Si2O8) near the surface. For heat treatment temperatures greater than or equal to 1250°C, baddeleyite (m-ZrO2) crystals form at the expense of zirconolite in the bulk of glass-ceramics. XRD indicates that the order in zirconolite Ca/Zr planes increases with heating temperature. At the same time, extended defects density decreases. INTRODUCTION Specific matrices are developed in order to immobilize long-lived radionuclides such as Am, Cm, Np or Pu. Glass-ceramics are good candidates because of the double containment they offer especially if the actinides are preferentially found in the crystallized phase and if the chemical durability and the self irradiation stability of both crystallized phase and residual glass are high [1,2]. Zirconolite is one of the possible crystallized phases. It was widely studied in the case of SYNROC (titanate-based polyphase ceramics) for HLW (fission products and actinides) [3,4] and high-level actinide wastes [5,6] incorporation. Zirconolite is well-known for its excellent containment capacity and long-term behavior. Moreover, natural zirconolite can incorporate relatively large amounts of uranium and thorium in its structure [7] and shows a very good chemical durability (due to TiO2 and ZrO2, which are oxides known for their very low solubility in water) and a good self-radiation resistance in spite of metamictization phenomena [1]. Similarly, artificial actinides and lanthanides (some of that can be considered as simulating trivalent and tetravalent actinides) can be introduced in the calcium and zirconium sites of the zirconolite structure [3,5,8,9]. Few
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