Thermo-hydro-mechanical simulation of a generic geological disposal facility for radioactive waste

Geological disposal is required for the safe and long-term disposal of legacy radioactive waste. High level waste and spent fuel generate significant heat that will cause thermo-hydro-mechanical coupled processes in the rock mass. The thermal expansion of the fluid will be greater than the grains causing a decrease in mean effective stress with the low permeability restricting Darcy flow and excess pore pressure equilibration. A decrease in mean effective stress can reduce material strength in granular materials, which may be significant near excavations where differential stress is increased. Microseismic monitoring provides cost effective, non-intrusive and three-dimensional data that can be calibrated with the stress and strain behaviour of a rock mass. However, there is no precedent for the microseismic monitoring of heat-producing radioactive waste. Generic concepts, analogue materials and data from in situ experiments are used to demonstrate the potential for the microseismic monitoring of heat-producing radioactive waste in lower strength sedimentary rocks. A mechanism for early post-closure microseismicity is demonstrated, whereby excess pore pressure decreases the mean effective stress towards yielding in shear. The rock and fluid property uncertainties are ranked according to their contribution to the excess pore pressure. Permeability is found to be important as expected, however, Biot's coefficient is demonstrably more important and yet often overlooked. Furthermore, the microseismic event locations, timings and pseudo scalar seismic moments are shown to have statistically significant relationships with the engineered backfill swelling pressure. Therefore, early post-emplacement microseismic monitoring could provide constraints for the engineered backfill swelling pressure and rock property uncertainties whilst the facility is still operational. Insights could prove timely for adapting the engineering designs, if they are not behaving as expected, in further high level waste and spent fuel tunnels.