Thermal effect on long-term behaviors of rocks: A DEM study

Radioactive waste generates prolonged heating of surrounding rock in nuclear waste repositories, potentially causing continuous growth of cracks. To ensure the safe isolation of nuclear waste, it is imperative to investigate long-term heating effects on mechanical properties and time-dependent behavior of rocks. This study presents a temperature-dependent stress corrosion (T-SC) model based on the discrete element method (DEM), which incorporates thermal effects through grain expansion and temperature-dependent subcritical crack growth. Beishan granite specimens are generated, and microparameters are calibrated through uniaxial compression and creep tests. Then specimens are subjected to long-term heating with various temperatures (100–400 °C). Results indicate that uniaxial compression strength (UCS) and Young’s modulus (E) exhibit strengthening-weakening transitions under short- and long-term heating. The strengthening is attributed to a compacted microstructure resulting from grain expansion, while the weakening is due to an increased crack number. Furthermore, UCS and E decrease over time from short- to long-term heating due to subcritical crack growth. Besides, the time-to-failure decreases by 2–3 orders of magnitude and becomes less sensitive to stress, and stress thresholds decrease significantly from 70 % to 44 % of UCS with increasing temperature. These findings underscore the significant weakening effects of prolonged heating on rocks when temperatures exceed 200 °C.