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Compaction of porous metal oxide microspheres a multi-scale approach

Abstract : The future management of nuclear ultimate waste requires pellet fabrication of uranium-americium mixed oxide as Minor Actinide Bearing Blankets (MABB) for the transmutation of americium in sodium fast reactor [1]. In this context, we are investigating here the pelletization of innovative porous and spherical oxide precursors (lanthanides and/or uranium).Both experimental data and numerical simulations are used to optimize the pelletization step. The ultimate aim is to obtain, after sintering, homogeneous, dense and undistorted ceramic pellets. Oxide microsphere precursors are synthetized by the Weak Acid Resin process [2], which consists in loading beads of ion exchange resin with lanthanides and/or uranyle cations and mineralizing the metal loaded resin beads into oxide microsphere. Mechanical properties of a single microsphere were characterized experimentally by recording a series of crushing tests using a micro press incorporated into a Scanning Electron Microscope (SEM) to measure the tensile strength and follow in-situ the deformation and the evolution of local damage and cracks.These highly porous microspheres are composed of micronic porous aggregates, which are themselves made of individual particles. The Discrete Element Method (DEM) [3] was used to model these different length scales. Because the full simulation of a microsphere at the length scale of grains would involve prohibitive CPU time, the behaviour of two idealized spherical aggregates where grains are modelled as bonded spheres were first simulated. Building on these simulations, a full microsphere was then modelled as a porous assembly of spherical aggregates bonded together by solid bonds. The stiffness and strength of these individual bonds are fitted to obtain a reasonable match with the macroscopic crushing behaviour of a microsphere.The last step consists in simulating the uniaxial compaction of a number of oxide microspheres, for which rearrangement and breakage play an important role. Simulation results allow obtaining a direct relationship between applied pressures and compacted microstructures. In particular, in conjunction with experimental compaction data, simulations enable a better understanding of the effect of the applied pressure on the microstructure. This knowledge will help in determining the minimum pressure leading to a dense and homogeneous green pellet [4].[1]Warin, D. J. Nucl. Sci. Technol. 2007, 44, 410.[2]Picart, S.; Mokhtari, H.; Jobelin, I., Patent, WO 2010/034716, 2010.[3]Martin, C. L.; Bouvard, D.; Shima, S. J. Mech. Phys. Solids 2003, 51, 667.[4]Pizette, P.; Martin, C. L.; Delette, G. et al. J. Eur. Ceram. Soc. 2013, 33, 975.
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P. Parant, S. Picart, J.-P. Bayle, E. Remy, E. Brackx, et al.. Compaction of porous metal oxide microspheres a multi-scale approach. ATALANTE 2016 - Nuclear Chemistry for Sustainable Fuel Cycles, Jun 2016, Montpellier, France. ⟨cea-02509802⟩

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