Haben, Aaron

► Research

Retention of repository-relevant elements at C-S-H phases

The safe and economically as well as ecologically sustainable storage of HLW (High Level Waste) from nuclear power and armaments is a difficult and important issue of our time. An international consensus has been reached on final disposal in deep geological host rock formations using a multi-barrier system. For the Federal Republic of Germany, storage is planned in boreholes reinforced with sand, clay and cement-based materials in approx. 600 m deep clay rock strata (model NORD). Since the long-term retention of potentially escaping radionuclides is to be ensured by the multi-barrier system, valid proof of the performance of the technical components envisaged there, including the construction materials to be used, is required. One of the most important components here are cement-based materials and, due to the time period required for a safe repository, also their alteration products. The main ageing phases of hydrated cement are portlandite and C-S-H phases, whereby the proportion of portlandite decreases as the ageing process progresses. Thus, the proof of performance of radionuclide retention at C-S-H phases is essential for a valid proof of safety of a potential repository.

Within the framework of the doctoral thesis, the retention of repository-relevant elements at C-S-H phases is therefore investigated under the general conditions of the joint project GRaZ II (Geochemical Radionuclide Retention at Cement Alteration Phases) funded by the BMWK (formerly BMWi). Usually, diffusion and / or batch experiments are used for such questions. However, these are either extremely time-consuming or not close to nature due to unrealistic solid-liquid ratios. Alternatively, HPLC experiments with mini columns have recently emerged as a promising link between the conventional methods. Therefore, the focus here is on method development for performing mini-column sorption experiments and elaboration of existing methods. In doing so, results from the applied methods (batch and column) are always compared with each other and, if necessary, supported by suitable modelling. Due to the framework conditions set by the GRaZ II joint project, the influence of dissolved iron as well as organic ligands, such as cement additives, on radionuclide retention at cement corrosion phases under hyperalkaline conditions at medium to high ionic strengths is also quantified during method development.