Coupling time-dependent sorption values of degrading concrete with a radionuclide migration model

For many near-surface disposal facilities, concrete is an important engineered barrier. Knowledge on the durability of such concrete components and its relation to radionuclide sorption is important for a defensible safety assessment. One of the most challenging issues in safety assessment concerns the long time scales involved and the time evolution. Some processes are addressed by implementing conservative assumptions in the radionuclide migration models while others, such as chemical concrete degradation, however, can be estimated for long time periods by the use of coupled geochemical transport models. Chemical degradation typically occurs as the result of decalcification, dissolution and leaching of cement components and carbonation. These reactions induce a gradual change in the solid phase composition and the concrete pore-water composition. In this study the time-dependency of the concrete mineralogy and porewater was coupled with sorption values that are characteristics for the four concrete degradation states. Sorption values were obtained from a literature review. This time-dependency of the sorption values was implemented in a one-dimensional radionuclide migration model used for release calculations from the planned near-surface disposal facility at Dessel, Belgium. Calculated releases are discussed for radionuclides typical for low- and intermediate level short-lived (LILW-SL) waste.