Predicting the long term durability of concrete engineered barriers in a geological repository for radioactive waste

In order to evaluate the long term waste package integrity in a geological repository for radioactive waste, simulations of the geochemical interactions between a concrete engineered barrier and a mudrock were conducted in 1-D geometry and on time periods of up to 106 y with the reactive transport code Hytec. Scenarios involving sulfate attack are shown to potentially alter strongly a concrete engineered barrier based on pure Portland based cement. Spatial extension of chemical degradation of the host rock due to high pH fluids is restricted to a radial distance of less than 2 m of the tunnel border in 100 000 y. Results suggest that illite and quartz destabilization rates are key parameters governing the geochemical evolution of the degraded interface. Results also suggest that controls on Mg availability and speciation at the border of the altered concrete are important for a proper understanding of this system. Another key process is the progressive localized cementation of the altered mudrock. Defining a conservative and robust modelling of the effects of cementation is not an easy task, as both porosity opening and closing occurs in this reactive system. Results obtained here suggest that coupling between pH dependence of mineral stability and feedback of mineral precipitation on pH sharpen the cementation front.