Three oxidation experiments at very high temperatures in steam atmosphere were conducted with advanced, nuclear grade SiCf/SiC CMC cladding tube segments. One transient experiment was carried out until local failure of the sample at maximum temperature of approximately 1845°C. The failure was caused by complete consumption of the external CVD-SiC seal-coat resulting in steam access to the fibre-matrix composite with less corrosion resistance. Approaching these very high temperatures was accompanied by accelerated gas release mainly of H2 and CO2, formation of surface bubbles and white smoke. Two isothermal tests lasting 1 hour at 1700°C in steam with final flooding with water were run under nominally identical conditions. Both samples survived the tests without any macroscopic degradation. The mechanical performance of such quenched clad segments was not significantly affected with the maintenance of high capability to tolerate damages. Despite these harsh conditions of exposure, the SiC fibre-matrix load transfer remains efficient to provide ability to the composites to accommodate the deformation.