Medium- and low-level radioactive waste is stored in the subsurface galleries of a granitic formation in Southern Hungary. The main lithology is monzogranite. The present study focuses on the thermal behavior and characteristics of intact rocks and thermally exposed specimens. Cylindrical specimens were heated to 250°C, and 500°C in an electric oven in laboratory conditions. Physical properties (density, ultrasonic pulse velocity) and non-destructive strength tests such as Duroskop rebound value were measured on samples kept at 22°C and on samples exposed to heat. The test procedures followed the guidelines given in EN. Tests show that the bulk density was reduced after the 250 °C treatment but slightly increased due to additional heat up to 500 °C. The ultrasonic pulse velocity rapidly decreases with temperature from 22°C to 500°C. The Duroskop rebound values also show a negative correlation with temperature. Color changes are also observed since the grey specimens became increasingly brownish with increasing temperature. The test results demonstrate that with increasing temperature, the tested monzogranite becomes less dense, and micro-cracks reduce the surface strength.
Granite is considered one of the best target rocks for radioactive waste disposal. One of the critical damages linked to the mechanical strength loss of granite is the thermal shock. The study of thermal effects on rock properties is an increasing research field with publications on the thermal behavior of several lithologies, including limestone [1, 2], sandstone [3], and granite [4,5]. Besides the mechanical changes, mineralogical changes and other physical transformations are linked to increasing temperature in granite [6,7].
In this paper, the thermal behavior of monzogranitic cylindrical specimens is presented. The samples were obtained from exploratory core drillings of a radioactive waste disposal site in Hungary. At the site, low and intermediate level radioactive waste is deposited in the National Radioactive Waste Storage Facility of Hungary. The study aims to outline the physical changes caused by temperature in the monzogranite. The density, ultrasonic pulse velocity, and strength of samples were tested at room temperature and after thermal shocks of 250°C and 500°C in order to outline the thermal behavior of the host rock of the disposal site. A non-destructive strength testing device, the Duroskop, was applied to rapidly measure the strength loss of monzogranite.
