Analytical approach to USiC measurement by electron probe microanalysis
Abstract
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 French reference concept is the Sodium-cooled Fast Reactor (SFR); however, an alternative is the Gas-cooled Fast Reactor (GFR) using helium as heat-transfer fluid and operating at high temperatures (1000°C–1100°C). The fuel is constituted by a 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 both fuel and cladding under nominal and accidental 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 heat treated within a range of 1630°C to 1920°C. Characterization of the formed phases 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 complex. We have identified some analytical key points to improve the methodology taking 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 carbides measurement standards, before applying it USiC samples. The chemical compositions measured by means of a electron microprobe on the ternary phases formed at equilibrium seemed to be in good agreement with the phases measured by XRD, and were used to determine the USiC ternary diagram at 1800°C.