In-situ High Temperature X-ray Diffraction Study of the Am-O System
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In-situ High Temperature X-ray Diffraction Study of the Am-O System E. Epifano1, R. C. Belin1, J-C Richaud1, R. Vauchy1, M. Strach1, F. Lebreton1, T. Delahaye2, C. Guéneau3, P. M. Martin1 1 2 3
CEA, DEN/DTEC/SECA/LCC, Cadarache, 13108 Saint-Paul-Lez-Durance, France CEA, DEN/DRCP/SERA/LCAR, Marcoule, 30207 Bagnols-sur-Cèze Cedex, France CEA, DEN/DPC/SCCME/LM2T, Saclay, 91191 Gif sur Yvette, France
ABSTRACT In the frame of minor actinide recycling, (U,Am)O2 are promising transmutation targets. To assess the thermodynamic properties of the U-Am-O system, it is essential to have a thorough knowledge of the binary phase diagrams, which is difficult due to the lack of thermodynamic data on the Am-O system. Nevertheless, an Am-O phase diagram modelling has been recently proposed by Gotcu. Here, we show a recent investigation of the Am-O system using in-situ High Temperature X-ray Diffraction under controlled atmosphere. By coupling our experimental results with the thermodynamic calculations based on the Gotcu model, we propose for the first time a relation between the lattice parameter and the departure from stoichiometry. INTRODUCTION Minor actinides (MA) like Am, Np and Cm significantly contribute to the long-term radiotoxicity of the spent nuclear fuel. One of the options envisaged for reducing the nuclear waste inventory is the transmutation of these elements in fast neutron reactors. The possibility of using advanced fuels containing MA has led to the necessity of investigate the structural and thermodynamic properties of these elements and their compounds. In this frame, the attention is mainly focused on U-Am mixed oxides [1], [2]. The assessment of the thermodynamic behavior of the U-Am-O ternary system is not possible without a thorough knowledge of the simpler binary systems. Unfortunately, the Am-O system is far from being well known: its phase diagram is still not defined, even if various representations have been proposed [3] [4]–[7]. In the O/Am ratio ranging from 1.5 to 2.0, one dioxide, two sesquioxides and one intermediate cubic phase (C-AmO1.62) have been identified [8]–[10]. The FCC fluorite phase AmO2-x has a large hypo-stoichiometric existence domain above 1300 K; for this phase, DTA measurements pointed out the existence of a miscibility gap [4]. The sesquioxide exists in two crystal structures: the hexagonal A-type P-3m1 Am2O3 (s.g. (space group) 164, prototype La2O3) and the BCC C-type Am2O3 (Ia-3 structure, s.g. 206, prototype Mn2O3) [9], [10]. Moreover, another C-type intermediate phase with an O/Am around 1.61 has been reported for temperature higher than 600 K [10]. However, the existence domains of these phases are not well defined. An attempt to give a coherent version of the Am-O phase diagram was made by Thiriet and Konings [6]. Recently, a thermodynamic model of the Am-O system with the Calphad method has been proposed by Gotcu et al., [11]. However, because of the lack of thermodynamic data, some simplifications were necessary for the modelling: the intermediate C-type phase (AmO1.62) w
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