The sensitivity of mechanical properties and pore structures of Beishan granite to large variation of temperature in nuclear waste storage sites.

Granite is the host rock of the Beishan Underground Research Laboratory (URL) for geological disposal of high-level radioactive waste in China. The mechanical behavior of Beishan granite is the key in determining whether the repository can serve safely for a long time. The surrounding rock of the repository will be exposed to thermal environment induced by radionuclide decay, resulting in significant changes in the physical and mechanical properties of the Beishan granite. This study investigated the pore structure and mechanical properties of Beishan granite after thermal treatment. The T2 spectrum distribution, pore size distribution, porosity, and magnetic resonance imaging (MRI) were obtained through nuclear magnetic resonance (NMR); uniaxial compressive strength (UCS) and acoustic emission (AE) signal characteristic of granite were investigated through uniaxial compression tests. The results showed that high temperature significantly affected the T2 spectrum distribution, pore size distribution, porosity, compressive strength, and elastic modulus of granite, and porosity gradually increases, whereas the strength and elastic modulus gradually decline with increasing temperature. The porosity of granite has a linear relationship with UCS and elastic modulus, indicating that the essential mechanism for the deterioration of macroscopic mechanical properties lies in changes of microstructure. In addition, the thermal damage mechanism of granite was revealed, and a damage variable was defined based on porosity and uniaxial compressive strength.