Engineered and natural geological barriers are used in underground nuclear waste repositories to minimize the risk of contaminant transport. Cutoff seals are one part of the engineered barriers used at strategic locations in drifts, access tunnels, and shafts to seal the repository. The optimum size and shape of these cutoffs depend on their capability to seal the excavation-induced damage around the underground excavations. The extent of the excavation damage zone around these structures depends on the in-situ stress magnitude and orientation, the stress concentration at the excavation boundary, the strength and stiffness of the rock mass, and other factors that cause stress fluctuations with time, such as the thermal pulse, bentonite swelling pressures, or earthquakes. In this paper, a parametric study for different cutoff aspect (thickness-to-depth) ratios and stress-to-strength ratios is carried out to optimize the shape and dimension of the cutoff. The results of the study show that a trapezoidal cutoff has better performance than a rectangular or a triangular cutoff. Furthermore, a time-dependent 3-dimensional finite difference model was used to determine the thermo-mechanical influence on the optimum cutoff. Incremental volumetric strain and plastic yielding induced by the thermo-mechanical loading were used to determine the change in the extent of the inner and outer excavation damage zones, respectively. The maximum increase in inner and outer excavation damage zones was 45 cm and 64 cm, respectively, when maximum rock mass temperature at cutoff tip reached 65oC. Heat from the decomposition of the nuclear waste has a minor effect on the change in the extent of damage zones.
The Nuclear Waste Management Organization recently reported that if the nuclear reactors in Canada continue to work for their planned life cycles, they will generate around 5.2 million bundles of used nuclear fuel for disposal (Noronha 2016). According to the International Atomic Energy Commission, deep storage in a stable geological formation is a sustainable way to safely manage spent fuel and high-level waste (HLW) from nuclear power reactors (Dayal 2004).