The CEA contributes to the development of the reactors selected under the Generation IV. Thermochemical studies are conducted to investigate the fuel and the cladding used for GFR reactor and the materials used of neutron absorbent in SFR reactor under nominal and accidental conditions (2000°C). For the GFR reactor, the fuel studied is constituted by a mixed uranium-plutonium carbide and the cladding consists of several layers of ceramic materials (SiC) and a metal liner. 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. Other types of samples were also prepared with the same temperature treatment to study the phases formed during the interaction between sheath type steel, B4C and fuel UO2 in SFR reactor. Materials characterization requires the use of a variety of analytical tools (XRD, EBSD, SEM, and EPMA). The measurement of light elements by microanalysis is still complex and must taking into account numerous phenomena : - native oxidation of UC, - spectral interference with U, C, O for carbon measurement, - carbon contamination by the electron beam, - chemical shift for B, C, O. The analytic method was validated on standards before applying it USiC and UBCO samples. The chemical compositions measured were in good agreement with the phases measured by XRD. A study of EBSD texture between sheath type steel, B4C and UO2 was also conducted to better understand the diffusion mechanisms.