Mechanical strength stability of tritium breeding materials under high temperature deuterium conditions

The mechanical strength of solid tritium breeding materials is one of the key factors for D-T fusion reactors. The crushing of ceramic breeder pebbles will block both tritium recovery circuit and thermal transmission. The mechanical strength changes of Li4SiO4 and Li2TiO3 pebbles exposed to high temperature and hydrogen isotope atmosphere were investigated. The crush load of Li4SiO4 decreased to 66 % of the original values when the breeders were exposed in deuterium gas at the temperature higher than 750 oC. Nevertheless, the crush load of Li2TiO3 remained stable at all the designated temperatures. Investigations involving phase analysis, defect analysis, grain size analysis, and lithium content analysis were performed to determine the causes of mechanical strength change. No new phase formation and no obvious lithium decrease were demonstrated by X-ray diffraction (XRD) and Inductively Coupled Plasma - Atomic Emission Spectrometer (ICP-AES). Defects arising from reduction reaction didn't play an important role in mechanical strength reduction. The main reason responsible for the decrease of mechanical strength of Li4SiO4 was attributed to grain growth. The grain growth behavior was understood by grain growth kinetics. The kinetic parameters of grain boundary migration for Li4SiO4 pebbles were obtained as n of 4 and activation energy Q of 140 kJ/mol. The mechanism of grain growth was given to demonstrate the mechanical strength change of ceramic breeder pebbles.