Résumé
The CEA contributes to the development of the reactors selected under the Generation IV International Forum, which will be commissioned in the middle of this century. For fast neutron reactors the reference concept is the sodium-cooled fast reactor (SFR); an alternative is the system gas-cooled fast reactor (GFR) using helium as the heat-transfer fluid and operating at high temperatures (1000°C–1100°C). The fuel could be based on uranium carbide (mixed uranium-plutonium carbide) and the cladding consists of several layers of ceramic materials (SiC) and a metal liner. Thermochemical studies are conducted by the CEA to investigate the chemical compatibility of the fuel and cladding under rated and accident conditions (2000°C).In order to compile a thermodynamic database on the formation of liquid ternary phases at high temperatures, USiC samples have been prepared with various chemical compositions then were heat treated at 1800°C. Characterization of the phases formed requires the use of a variety of analytical tools, including optical microscopy, XRD, SEM, and EPMA.The measurement of light elements — particularly C but also Si — in the presence of uranium by microanalysis is still a complex undertaking. We propose an analytical methodological approach that takes into account the phenomena that could bias the measurement, including:‒native oxidation of UC,‒spectral interference with U, C, O for carbon measurement,‒carbon contamination by the electron beam,‒sample preparation for conductivity.The method was validated on UC and UC2 measurement standards, before it was applied to USiC samples. The chemical compositions measured by means of a electron microprobe on the ternary phases formed at equilibrium are in good agreement with the phases measured by XRD, and were used to determine the USiC ternary diagram at 1800°C.