Quantification of uncertainty for reactive solute transport in discrete fracture network models for crystalline rock with application to subsurface repositories for spent nuclear fuel

Uncertainty quantification of solute transport by groundwater flow is a critical component of safety assessments of geological repositories for spent nuclear fuel. Sparsely fractured crystalline rock provides a favourable geological environment because of its low permeability and tectonic stability. Although fracture occurrence may be sparse, fracture clusters may form connected pathways for groundwater flow and solute transport from a subsurface repository to the biosphere. However, reactive solutes including radionuclides are not only affected by advective flow but also experience diffusion into the rock matrix with retention mechanisms delaying plume migration. Therefore, it is of great importance to quantify uncertainties in the representation of DFNs to obtain useful constraints on uncertainties for reactive solute transport.

In this contribution, a stochastic Lagrangian framework is used to calculate transport of reactive solutes obtained from advective particle trajectories in three-dimensional discrete fracture network models. Previous work has shown that accounting for internal fracture variability in DFN models enhances early advective particle arrival compared to the smooth fracture plane assumption. Here we investigate the effect of internal variability in permeability in DFNs for two classes of reactive solutes representing radionuclides, one dominated by diffusion and another by retention. The findings show that solutes which are dominated by retention are significantly affected by variable fracture permeability in DFNs, comparatively much more than those dominated by diffusion. We showcase how uncertainty in solute mass arrival can be quantified using the reactive transport framework methodology and discuss implications on transport assessments for cases where mechanical deformations cause changes to the internal variability of fractures.