France aims at closing its nuclear fuel cycle by recycling the valuable materials contained in the spent fuel. The most developed next generation (or Generation IV) reactor design is the Sodium Fast Reactor, like the Phénix and Superphénix reactors [1]. For their operation, these reactors use MOX fuel pellets (mixed oxides of uranium and plutonium). These small fuel cylinders, with typical diameter and height around 10 mm, are manufactured by a powder metallurgy route, using the COCA process (CObroyage CAdarache). The implementation of this process is based on three main unitary operations: preparation of the granular medium integrating a direct mechanical grinding phase (1), uniaxial pressing of the granular medium to shape the pellets (2) and sintering of the pellets at high temperature under a specific atmosphere allowing a good densification (at least 95%) with the target oxygen/metal ratio (3) [2].

During sintering various phenomena occur simultaneously: formation of a solid solution, densification, development of the microstructure and grain growth. This work is dedicated to investigate these phenomena and more precisely to study in detail the grain growth and densification mechanisms of a homogeneous MOX composition, at the scale of the spatial distribution of the U and Pu chemical elements, for different Pu compositions (from 0 to 26%mol). The objective of this work is to build sintering and grain growth maps (relationship between grain size and relative density), to obtain values for the activation energies of densification and grain growth as well as to determine the diffusion mechanisms controlling these two phenomena. The results obtained will allow to enrich the existing data [3] and to feed future numerical simulation codes.

The purpose of our communication is to present the results obtained for the UO2 reference samples (no Pu content). The powder, synthesised through a wet route, was characterised before and after the grinding step by dry powder laser diffraction (particle size distribution), scanning electron microscopy, X-ray diffraction and thermogravimetric analysis. Specific surface area, bulk and tapped densities were also measured. Compacts were then formed by uniaxial compaction and sintered using an instrumented dilatometer. During these tests, parameters such as heating rate, target temperature, isothermal duration and oxygen partial pressure were adjusted to observe the evolution of the microstructure.

[1] Tome 3 : « Les réacteurs à neutrons rapides de 4ème génération à caloporteur sodium » (cea.fr), 2012
[2] Kato M., Maeda S., Abe T., Asakura K., Uranium Oxide and MOX Production, Comprehensive Nuclear Materials, volume 2, pages 1-34, 2020 https://doi.org/10.1016/B978-0-12-803581-8.11701-1
[3] Simeon J., Frittage et développement de la microstructure du combustible nucléaire MOX RNR, Thesis, Université Grenoble Alpes, 2022