Abstract
Mixed oxide U1-yPuyO2-x (MOX) is the reference fuel for sodium-cooled fast neutron reactors (FBRs). Their plutonium content (y = Pu/(U+Pu)) and oxygen/metal ratio (O/M) must be between 0.20 < y < 0.35 and 1.94 ≤ O/M < 2.00, respectively. During irradiation, a thermal gradient occurs along the radius of the fuel pellets (~ 4 mm), reaching nearly 2500 K at their center. This gradient induces structural and microstructural modifications. In particular, plutonium-enriched (up to twice the initial content) and oxygen-depleted zones were identified in the hottest zone. These redistribution phenomena lead to local heterogeneity in the fuel's properties, including its melting temperature. However, operators need to know this temperature precisely at all points to ensure that the reactor's safety margin is respected. However, the experimental data available in the literature show variations of over 200 K, for the same composition. It is therefore essential to provide new experimental data, for a wide range of plutonium and oxygen compositions. As a preliminary step, a critical analysis of the literature concerning the U-Pu-O system was therefore carried out, highlighting the lack of data for plutonium contents of y > 0.50.The aim of this work was therefore to measure the thermo-physical and structural properties of MOX with a high plutonium content, with a comparison to the latest versions of the thermodynamic models available.Initially, pellets with plutonium fractions of y = 0.60, 0.65 and 0.70 and two initial O/M ratios (2.00 and < 2.00) were manufactured by co-milling powders, optimizing the milling and sintering steps to achieve the targeted properties. To verify that these properties were achieved, the pellets were characterized at the end of the manufacturing process using a multi-scale approach combining XRD, X-ray absorption spectroscopy (HERFD-XANES), electron microprobe and Raman spectroscopy. Sintered pellets (O/M = 1.99), with a fluorite structure, show densities in excess of 95% DT and good cationic homogeneity. Annealed pellets (O/M = 1.98) show similar overall characteristics, but with lower densities, varying between 82 and 90% DT.Secondly, melting temperature and thermal diffusivity measurements were carried out using laser heating methods. In order to study the influence of the O/M ratio, measurements were carried out under inert or reducing atmospheres. In addition, following the melting temperature measurements, quantification of the chemical composition and estimation of the oxygen stoichiometry of the samples was obtained by HERFD-XANES measurements performed on the MARS light line of the SOLEIL synchrotron. A comparison with thermodynamic modeling using the CALPHAD method was also discussed in this work. These measurements not only demonstrated good agreement between the thermodynamic model and experimental data, but also identified a minimum melting temperature, equal to 2940 ± 30 K, for a Pu content of y = 0.63.The results of this work therefore constitute unprecedented input data for the European PuMMA project, and are essential for the GERMINAL calculation code developed by CEA.