Oxidation of SiCf-SiC CMC cladding tubes for GFR application in impure helium atmosphere and materials interactions with tantalum liner at high temperatures up to 1600°C

Abstract

The oxidation kinetics of SiCf-SiC cladding tube segments and sandwich tubes with an inner tantalum layer in impure helium atmosphere prototypical for GFRs was investigated at ambient pressure in the temperature range between 900 °C and 1600 °C using a thermogravimetric device. The transition from passive oxidation (formation of a protective silica scale) to active oxidation (volatilization of silica due to the formation of SiO and other volatile species) occurred between 1200 °C and 1300 °C. Hence, during target operational temperatures of 900–1000 °C, the CMC cladding material should withstand long-term operation with respect to oxidation and corrosion. At higher temperatures beyond 1300 °C, i.e. under accident conditions, volatilization of composite constituents would lead to serious degradation of the SiC-based cladding tubes. Severe interactions between the tantalum layer and silicon carbide with the formation of Ta silicides and carbides were found to become significant at temperatures higher than 1300 °C.