Effect of Molybdenum and Ruthenium on the Crystallization Tendency of a New Nuclear Glass Containing High Rare-Earth Con
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Effect of Molybdenum and Ruthenium on the Crystallization Tendency of a New Nuclear Glass Containing High Rare-Earth Concentration N. Chouard1,2, D. Caurant1, O. Majérus1, J-L. Dussossoy2, A. Ledieu2, S. Klimin3, D. Pytalev3 1 Laboratoire de Chimie de la Matière Condensée de Paris (UMR CNRS 7574), ENSCP ChimieParitech, 11 rue Pierre et Marie Curie 75231, Paris, France 2 CEA, DEN, DTCD, SECM, Laboratoire d’Etude et Développement des Matrices de Conditionnement, Marcoule, 30207, Bagnols-sur-cèze, France 3 Institute of Spectroscopy, Russian Academy of Sciences, 142190, Troitsk, Moscow region, Russia ABSTRACT The impact of Nd2O3, MoO3 and RuO2 addition on the competition between the crystallization of apatite Ca2Nd8(SiO4)6O2 and powellite CaMoO4 phases which both may appear in High Level Waste nuclear glass (under certain specific conditions of cooling and glass composition) has been studied on a simplified composition belonging to the system SiO2-Na2OCaO-Al2O3-B2O3. X-ray diffraction (at room temperature and high temperature) and scanning electron microscopy measurements have been performed on five glasses under two different thermal treatments. We show that RuO2 acts as a nucleating agent for apatite. Moreover, neodymium and molybdenum cations seem to be very close in the glassy network as Nd2O3 addition stops the phase separation of molybdates and inhibits the crystallization of CaMoO4. On the contrary, MoO3 seems to favor the crystallization of apatite. For several samples, the location of Nd3+ cations after crystallization was identified by optical absorption spectroscopy. INTRODUCTION Vitrification of high level liquid nuclear waste is the internationally recognized method to lower its impact on the environment. In France, a new confinement glass, aimed at immobilizing more concentrated nuclear waste solution than today, is currently under study [1,2]. In this context, a higher concentration of fission products such as rare-earths, molybdenum and platinoid elements (Ru, Rh, Pd) will be incorporated in this High Level Waste glass (HLW glass). Platinoid elements are well know for acting as nucleating agents and rare-earths (RE) and molybdenum can induce liquid-liquid phase separation and crystallization of apatite Ca2RE8(SiO4)6O2 and powellite CaMoO4 [3-5]. As a matter of fact, one of the major challenges in the optimization of the composition of this new glassy waste form is to avoid crystallization after melt casting in canisters. The aim of this work is to understand crystallization mechanisms by studying, for a simplified aluminoborosilicate glass belonging to the SiO2-Na2O-CaO-Al2O3B2O3 system, the impact of Nd2O3 (simulating all the rare earth elements of the fission products), MoO3 and RuO2 addition on the competition between the crystallization of apatite and powellite. EXPERIMENTAL DETAILS Glass synthesis
For this study, five glass compositions were obtained by a simplification of the HLW glass and by adding successively Nd2O3 and/or MoO3 and/or RuO2 (table I). Table I. Theoretical chemical
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