Reduction of Gd 6 UO 12 for the Synthesis of Gd 6 UO 11
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Reduction of Gd6UO12 for the Synthesis of Gd6UO11 Darío Pieck 1, Lionel Desgranges 1*, Yves Pontillon 2, Pierre Matheron 3
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CEA, DEN, DEC, SESC – Laboratoire des Lois de Comportement des Combustibles. CEA, DEN, DEC, SA3C – Laboratoire d’Etudes de la Microstructure des Combustibles Irradiés. 3 CEA, DEN, DEC, SPUA – Laboratoire Combustible Uranium. * Email address: [email protected], telephone: +33442253159, fax: +33442253285, postal address: CEA, DEN, DEC, Département d’Etudes des Combustibles, SESC/LLCC, bât. 352, F-13108 Saint Paul lez Durance, France. 2
ABSTRACT In the present work, we focus on -Gd6UO12 phase and its stability under reducing conditions. This later point is interesting regarding reducing environment that could exist in some nuclear storage sites and that could possibly degrade –compounds. A polycrystalline -Gd6UO12 sample was prepared by sintering cubic-Gd2O3 and UO2 mixed powders under an air atmosphere. The resulting pellets were then characterized and reduced by heat treatment under an Ar with H2 5% atmosphere. XRD analysis of the sample after reduction did not confirm the reduction into Gd6UO11 but a decomposition of the -compound. Preliminary characterizations of these decomposition products are presented. INTRODUCTION One possibility for safe nuclear waste storage consists in their immobilization in ceramics that can resist irradiation damage on large time scales. Compounds with structures similar to fluorite, like –compounds, have been proposed for nuclear waste immobilization [1]. Amongst the many studies that have been made concerning radiation damage on such materials, the Gd6UO12 phase which is made of urania and gadolinia mixed in a 3:1 molar ratio, was recently proved not to amorphise under irradiation, even though it undergoes an order-disorder phase transition [2,3]. This compound possesses highly ordered fluorite-related superstructures and is named -phase (delta) [2], its structure is rhombohedral and belongs to R3 space group. Its theoretical density is around 8.1 g cm-3 and it presents a yellow pale colour. Depending on the choice of a storage site, environment conditions in which immobilization matrixes are submitted can vary greatly. In some cases (deep geological storage), the environment can be very reducing compare to matrix fabrication conditions. In this work we are interested on -Gd6UO12 stability since a Gd6UO11 phase was reported in the U-Gd-O system but not characterized [4]. In order to evaluate technical feasibility of the -Gd6UO12 for nuclear waste immobilization the study of the possible reduction of the compound is mandatory. In this work, we prepared some -Gd6UO12 samples and submitted them to reducing heat treatments. Many fabrication routes have been reported by various authors [3, 5, 6, 7]. Usually, -Gd6UO12 synthesis from mixed powders of urania and gadolinia takes place in two steps: oxidation and -
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phase formation during a reactive sintering. The first oxidation step can be achieved at 400°C in air atmosphere following the reaction (Eq.1),
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