Potential Degradation Processes of the Cementitious EBS Components, their Potential Implications on Safety Functions and Conceptual Models for Quantitative Assessment

This report presents current understanding of chemical and mechanical cement degradation processes based on a selected number of literatures. Specific attention is paid to studies carried out in the context of deep disposal applications, where possible. Preliminary implications of such degradation processes on safety functions of the repository components have also been discussed based on rather limited information. A geochemical conceptual model based on thermodynamics, and state of the art reactive transport model are proposed to help in the understanding of spatio-temporal behaviour of concrete geochemistry. Though not within the scope of this work package, an additional step is taken in carrying out preliminary geochemical calculations, including pertinent uncertainty analyses to gain a first level understanding of the chemical degradation processes of concrete. Also included are some simplified mathematical models, which are capable of exploring the chemo-mechanical behaviour, associated with decalcification and sulphate attack. Apart from the characteristics of the cement, aggregates and additives used in the makeup of the candidate concrete or mortar, the main drivers for these degradation processes are the native pore water composition, chemical nature of the waste form, presence of steel, saturated conditions, thermal field, availability of oxygen and diffusion. The literatures suggest that all the processes discussed have an impact on the performance of the cementitious components; however, many of the processes are going to be predominantly active in the long term. Short term impact can be specifically attributed to temperature variation, atmospheric carbonation and sulphate attack; the latter especially relevant during the operational phase from the point of view of retrievability.