Numerical study of the performance of tunnel plugs

High axial flow rates within and along tunnels, excavated for deposition of high level nuclear waste, may increase the transport capacity of potential escape routes for radionuclides that have been released to the buffer surrounding the waste canisters, or to the tunnel backfill materials, from leaking fuel canisters. High flow rates may be found if the tunnel interior has a high permeability, either initially due to the composition of the backfill material or at later times due to degradation of low permeability backfill material components. If a disturbed rock zone, a DRZ, of increased permeability was created as a result of direct damage done to the wall region during tunnel excavation, this zone may contain additional possible escape routes. Low-permeability tunnel plugs, keyed into slots cut in the rock walls, have been suggested as means of reducing axial flow rates. This paper deals with different aspects of such tunnel-plug systems. A preliminary estimate of the potential for tunnel plugs to reduce axial flow rates is made using analytical expressions. A number of numerical techniques are employed to investigate the hydraulic, mechanical and hydromechanical performance. These include μFLOW (FEM flow calculations), FLAC (finite difference mechanical calculations) and 3DEC (distinct element mechanical calculations). The mechanical calculations concern the mechanical stability in the rock surrounding the slot and permeability changes in that rock caused by stress redistribution. The results show that the effects of tunnel plugs are very significant in most cases. A discussion on the validity of the results and the applicability of the used methods is provided.