Cutoff Modelling for Excavation Sealing in Underground Nuclear Waste Repositories

When placement rooms in a deep geological repository are constructed, different excavation damage zones (EDZs) with connected fractures (inner excavation damage zone - EDZi) close to the excavation surface and disconnected fractures (outer excavation damage zone - EDZo) further away form. A cutoff is an excavation constructed perpendicular to the placement room axis that creates a cross-sectional enlargement of the excavation, which when filled with a sealing material such as bentonite, seals the EDZi to minimize radionuclides transport through this zone. This research developed a framework to help determine the dimension and shape of an optimum cutoff, examining a wide range of geological scenarios using continuum numerical modelling. In the first stage, dimensions of different EDZs induced due to the excavation of the placement room were determined. In the second stage, based on the dimensions of EDZi from the first stage, rectangular, triangular, and trapezoidal cutoffs with different dimensions under different geological settings were constructed. In the third stage, the effect of different variants such as shape and orientation of placement room, rock mass properties and anisotropy, and heat from the nuclear fuel on the extent of EDZi around the optimum cutoff was studied. The depth of EDZs was predicted based on the sign and magnitude of volumetric strain inside the plastic zone from the numerical model. The extent of EDZs increases with an increase in the ratio of maximum stress around the excavation to the strength of the rock mass. The trapezoidal shaped cutoff, with a minimum thickness to depth (aspect) ratio (0.2), was found to be the optimum in this study based on its ability to induce a minimum increase in the EDZi after the construction of the cutoff. The radial extent of EDZi for the optimum cutoff is smaller for the granite followed by limestone and mudstone. The radial extent of EDZi for the optimum cutoff is lower for the limestone and mudstone with the absence of bedding planes compared to when bedding planes are present. Furthermore, for the given stress situation in this study, the radial extent of EDZi for the optimum cutoff for the horizontal placement room and vertical room (shaft) is found approximately equal. The radial extent of EDZi for the optimum cutoff for a circular placement room is smaller than the rectangular placement room. Finally, a maximum temperature of 64 oC at the cutoff increases from the mechanical only model the radial extent of EDZi and EDZo by 72 cm and 61 cm, respectively. This study helps in understanding excavation damage zones and designing cutoff for underground excavations, especially in underground nuclear waste repositories.