Thermal conductivity evaluation for bentonite buffer materials under elevated temperature conditions

In most countries, the upper limit of the buffer temperature in a geological repository for nuclear waste is set below 100 °C because of possible illitization. This smectite-to-illite transformation likely causes the swelling function of the buffer to deteriorate. However, stipulation of a higher temperature for the buffer would significantly increase the disposal density and cost effectiveness of the repository. Hence, understanding the characteristics and providing a database of information pertaining to the buffer at elevated temperatures is crucial. This study involved an evaluation of the thermal conductivity of bentonite buffer materials, as this property is the most important parameter for determining the design temperature. An experimental system suitable for operation at high temperatures was established, and was used to measure the thermal conductivity of bentonite buffer materials at different saturation values in the range 0–0.22 from room temperature to 150 °C. The dry density of the bentonite buffer materials is approximately 1.75 g/cm³, and the higher the initial saturation, the higher the measured thermal conductivity. The specimen and probe were placed in an oven and completely sealed with heat resistance tape to prevent water from evaporating from the pores of the specimen. The thermal conductivity of the bentonite buffer materials, which initially increased with temperature, decreased slightly between 100 and 150 °C. In general, the thermal conductivity values were similar between room temperature and 150 °C.