Evolution of sorption properties in large-scale concrete structures accounting for long-term physical-chemical concrete degradation

Long-term safety of radioactive waste disposal facilities relies on the longevity of natural or engineered barriers designed to minimize the migration of contaminants from the facility into the environment. Especially for near surface disposal facilities, such as planned by ONDRAF/NIRAS for the Dessel site in Belgium, long-term safety relies almost exclusively on the containment ability of the engineered barriers (EB) with concrete being the most important EB material used. Due to the long time frames typically involved in safety assessment, the chemical, physical and mechanical properties of concrete evolve in time. Alterations in concrete mineralogy result in pH evolution and in sorption behaviour for many radionuclides as a result of chemical degradation processes. Application of dynamic sorption models for concrete requires an adequate knowledge of long-term concrete degradation processes, knowledge of the effect of changing mineralogy on radionuclide sorption and knowledge of large-scale system behaviour over time. Moreover, when implemented in safety assessment models, special attention is required to ensure model robustness and transparency of the implementation. The discussion in this paper focuses on the sorption properties of concrete: selection of data, rescaling issues, and the hypotheses used to build a dynamic model for large-scale concrete structures.