A nanoscopic hydro-thermo-mechanical model for nuclear waste shale/clay repositories

Shales or highly compacted engineered clay layers are being used as buffers in deep surface nuclear waste repositories. Due to the complex natural structure and fabric of clay and non-clay minerals associated with high in situ stresses, high temperatures, and the practical difficulties in the replication of the field stress and temperature conditions in the lab testing facilities, swell potential from the macro and micro investigations does not provide reliable and universally applicable results. In this study, a comprehensive molecular-level simulation-based volume change constitutive model has been developed for clay minerals incorporating the effects of cation exchange capacity, density, water content, in situ stress state, temperature, exchangeable-cations type and proportion, pore fluids, and the dissolved salts. The molecular simulations were performed using molecular mechanics, molecular dynamics, and Monte Carlo simulation techniques. Comparing the model predictions with the results of the lab tests, the model has been proven to be quite precise in the prediction of the swell potential of these strata under various overburden pressures and temperatures. There is several fold increase in swelling of clay samples at 80 °C as compared to the equivalent specimen tested at 25 °C. The effect of higher temperature is lesser at lower initial water content (higher density) while at higher water content (lower density) the structure has been found to be more vulnerable at higher temperatures. About 100 times higher confining pressure results in the same swell at 80 °C as in its counterpart specimen at 25 °C, the corresponding swell increase factor in case of 50 °C specimens is about 45. A sharp increase in swelling with a drop in in situ pressure emphasizes the probability of higher swell as a result of an accidental reduction in in situ pressure such as the higher concentration of nuclear reactions. In this study, the cohesive energy density (CED) was found to be highly sensitive to various volume change variables, such as water content, density, CEC, type, and percentage of exchangeable and non-exchangeable cations. Contrary to all the previous models, CED-based model developed in this study is universal in nature and can be adopted for any case with minimal basic material input parameters. The good agreement found between the predicted and real values for the swell potential of the undisturbed samples suggests that the model presented here can effectively be used for the assessment of the swelling potential of shale/clay deposits to be used as buffers to the nuclear waste storage.