Evolution of thermal cracking, temperature distribution and deformation at mineral scale of Beishan granite during heating and cooling

Abstract

Thermo-mechanical property of rock is of great importance for the long-term safety of the high-level radioactive waste (HLW) geological disposal engineering. Thermal cracking, temperature and meso-deformation on the mineral grain size scale of Beishan granite were investigated during heating (160 °C ∼ 200 °C) and cooling process using online temperature, deformation and acoustic emission monitoring system. During the heating process, the temperature difference in different mineral grain was evident that the highest temperature was observed in feldspar while lower temperature in quartz and mica, and the temperature difference reached 20%. Meanwhile the feldspar expanded as the quartz and mica being compressed during heating. During the temperature holding and cooling process, the temperature difference between different mineral grain reduced and stabilized around 5%. The thermal induced expansion was recovered gradually but irreversible deformation was left, which also demonstrated by the nuclear magnetic resonance measurement that size and volume of large pore in granite both decreased after heated. The thermal cracking emerged from 25 °C ∼ 35 °C, and the thermal cracking activity was active once temperature changed during heating and cooling stage, which was calm during temperature holding stage. In addition, the cracking process was investigated by numerical modelling. The ‘confining pressure effect’ was observed during heating process, and the mechanism of thermal strengthening effect was discussed. Overall, the mild high temperature caused by decay heat of HLW could play positive role for the long-term safety of the repository, since the tightness and strength could be both enhanced by mild high temperature.