Reactive-transport model of fracture-matrix alteration by hyperalkaline fluid interactions: influence of boundary conditions on sealing behavior

Abstract Fractured rocks in the Excavation Damage Zone (EDZ) and intact host rocks of a geologic repository for radioactive waste could be altered by reactions with hyperalkaline solutions leached from cementitious materials in the repository. Alteration could cause individual fractures to open or seal under the control of coupled fluid-flow, solute-transport, and water-rock interaction processes. Resultant changes in the bulk hydraulic conductivity and transport properties of the altered host rock may affect the groundwater flow field around the repository, thereby complicating efforts to reliably assess the long-term safety of the repository in limiting radionuclide releases to the biosphere. A discrete-fracture, reactive-transport model was used in the present study to evaluate fracture opening/sealing behavior caused by reactions involving hyperalkaline solutions with minerals in a representative repository host rock. Differences in assumptions underpinning the model were found to strongly affect system behavior. Model results indicated the rock matrix adjacent to a fracture would seal relatively quickly, for example, if a constant-concentration condition was assumed along the fracture-matrix boundary. Conversely, the model predicted that the matrix would not seal completely near the fracture surface if a constant-concentration boundary was assumed at the inlet rather than along the fracture-matrix interface. Fracture flow would slow by precipitation of secondary minerals in this latter case, however, causing the fracture porosity and permeability to decrease. Sensitivity analyses based on a critical evaluation of initial and boundary conditions in discrete-fracture models can provide important insights concerning phenomena controlling fracture opening/sealing behavior and related impacts on fluid flow and radionuclide transport.