Influence of Thermal Treatment on the Thermal Conductivity of Beishan Granite

Granitic rocks are potential rock types for hosting high-level radioactive waste (HLW) repositories at depth. A better understanding of rock thermal conductivity is essential to develop HLW repositories successfully. In this work, experimental investigations on the thermal conductivity of thermally treated Beishan granite were conducted. Disk specimens preconditioned at 105 °C were heated to different temperatures (200, 300, 400, 550, 650, and 800 °C) and then cooled to room temperature for testing. Conventional physical properties such as bulk density, porosity, and P-wave velocity were measured under the effect of thermal treatment. Scanning electron microscope was used to characterize thermally induced microcracks in the rock. Thermal conductivities of the treated specimens under dry and water-saturated conditions were determined using the transient plane source method, and the effect of water saturation on the thermal conductivity was investigated. The influences of temperature and axial compression stress on the thermal conductivity were also studied. Results indicate that the thermal conductivity of the specimens depends strongly on the thermal treatment temperature. The thermal conductivity decreases nonlinearly with applied temperature, because of growth and propagation of microcracks in the specimens. On the other hand, water saturation plays an important role in increasing the thermal conductivity. In addition, significant differences exist in the thermal conductivity behaviors of the specimens when subjected to different ambient temperatures and compression stresses. Based on the experimental data, models considering the effect of porosity were established for describing the effects of water saturation, ambient temperature, and compression stress on the thermal conductivity of thermally treated rock.