Experimental Determination of Mechanical Property Evolution in Granites Subjected to Temperature Fluctuations: Implications for Safe Subsurface Disposal of Radioactive Waste

Granitic terrains are increasingly utilized for underground disposal of radioactive waste from nuclear power plants, owing to their abundance and superior mechanical properties. However, the heat generated by radioactive waste can elevate burial site temperatures by up to 100°C, potentially compromising structural integrity through thermal expansion and fracture generation. The response of granites to elevated temperature depends on factors such as mineral composition, volatile mineral content, grain size, and pre-existing in situ stress conditions.

This study utilizes experimental techniques namely nanoindentation, micro-CT, SEM imaging, petrography, ultrasonic wave velocity measurements, XRD, and Thermogravimetric analysis to identify and evaluate the temperature dependence of the mechanical properties of two compositionally different granite samples. The initial composition and mechanical properties of the two granite samples were determined using the mentioned techniques. The samples were then subjected to a step-by-step heating protocol ranging from room temperature to 900°C. The properties of the samples were measured at regular intervals along the heating range and were analysed to find out the correlation with temperature.

Results revealed similar but distinct thermal responses between the two samples, with the most pronounced changes occurring between 500-600°C, coinciding with the α-β transition of quartz. Petrographic analysis, micro-CT, and SEM imaging demonstrated significant microcrack development at 600°C. Ultrasonic wave velocities showed progressive reduction with increasing temperature, indicating diminishing mechanical strength. Nanoindentation studies revealed that while the reduced modulus of all minerals decreased with heating, the rate of reduction varied among mineral phases. This comprehensive analysis demonstrates that elevated temperatures substantially reduce granite's strength and structural integrity, with the rate of degradation showing some dependence on compositional variations. These findings have important implications for the selection and engineering of underground radioactive waste disposal sites. Understanding the temperature-dependent behaviour of granite can help prevent potential leakage or environmental contamination, thereby addressing key safety concerns that currently limit broader adoption of nuclear technology.