Joint project: Geochemical retention of radionuclides on cement alteration phases (GRaZ); Subproject B
Karlsruher Institut für Technologie, Institut für Nukleare Entsorgung
Universität des Saarlandes, Anorganische Festkörperchemie
Technische Universität München, Fachgebiet Theoretische Chemie
Universität Potsdam, Institut für Chemie / Physikalische Chemie
Technische Universität Dresden, SG Strahlenschutz, Professur Radiochemie
Universität Heidelberg, Physikalisch-Chemisches Institut
Fig. 1: Nuclear waste disposal in deep geological
formations (Sachs et al., 2009).
The long-term disposal of high-level nuclear waste in deep geological formations behind multiple barriers is recognized worldwide as preferred strategy for nuclear waste management. In the site model NORD, developed within the project “AnSichT” (Lommerzheim and Jobmann, 2014), claystone is considered as a potential host rock. Concrete and bentonite will be utilized as backfill and buffer material as well as for borehole sealing in the geotechnical barrier surrounding the nuclear waste containing canisters. Pore waters of the North German clay deposits are characterized by high ionic strengths up to 4 M (Brewitz, 1982). The contact of such highly saline formation waters with concrete will result in an enhanced corrosion of concrete which will lead to formation of secondary phases and to the evolution of highly alkaline cement pore waters (10 < pH < 13). The hyperalkaline solutions, in turn, can react with the bentonite buffer as well as with the surrounding clay host rock and can change their retention potential towards radionuclides.
So far, the retention of radionuclides on cement phases was mainly studied at low ionic strengths whereas the radionuclide retention on clay minerals was almost exclusively studied in the pH range up to pH 10. To improve the understanding of radionuclide retention processes on cementitious materials and bentonite at conditions expected in the near field of a repository which is influenced by highly saline formation waters, radionuclide retention studies have to be performed at highly saline as well as alkaline to hyperalkaline conditions.
In this project, the long-term stability of U(VI) doped calcium silicate hydrate (CSH) phases, as main component of hardened cement paste, in saline solutions with ion compositions typical of North German clay deposits is studied. Furthermore, the retention behavior of Ca-bentonite (Calcigel®) towards U(VI) in diluted Gipshut solution at alkaline to hyperalkaline conditions is studied. The batch sorption studies are supported by spectroscopic measurements such as time-resolved laser-induced fluorescence spectroscopy (TRLFS), Fourier-transform infrared (FT-IR) spectroscopy and X-ray absorption spectroscopy (XAS) to identify adsorbed or incorporated U(VI) species on a molecular level. These studies will be extended by studying the retention of further radionuclides on these materials under comparable conditions.
Furthermore, the effect of polymeric cement additives, so-called superplasticisers, on radionuclide retention on CSH phases and bentonite at high pH values is studied. Applying polycarboxylate ethers as new generation of superplasticisers, complexation and sorption experiments in dependence on pH and electrolyte content are performed to identify conditions for radionuclide mobilization.
The project will contribute to an improved process understanding and to an enhancement of the thermodynamic database with regard to the influence of alkaline brines on the retention behavior of cement phases as well as clay minerals influenced by a cementitious system towards radionuclides.
This research project is performed in collaboration with the R&D projects of the cooperation partners listed above.
- Sachs, S., Schmeide, K., Křepelová, A.: Humic substances and their influence on the mobility of actinides in clay formations. In: Nuclear Safety Research. Biennial Scienitific Report 2007-2008, Vol. 3, Wissenschaftlich-Technische Berichte, Forschungszentrum Dresden-Rossendorf, FZD-509, Dresden, 2009 p. 32).
- Brewitz, W.: Eignungsprüfung der Schachtanlage Konrad für die Endlagerung radioaktiver Abfälle. GSF-T 136 (1982).
- Lommerzheim, M., Jobmann, A.: Endlagerkonzept sowie Verfüll- und Verschlusskonzept für das Standortmodell NORD. TEC-08-2014-Z (2014).
- Fritsch, K., Schmeide, K., Steudtner, R., Müller, K., Brendler, V., Stumpf, T.: "Uranium(VI) sorption on montmorillonite at high ionic strengths: Surface complexation modelling in the NaCl and CaCl2 systems." Geochim. Cosmochim. Acta (2017, under review).
- Philipp, T., Schmeide, K., Stumpf, T.: Retention of uranium(VI) by Ca-bentonite at high ionic strength and high pH studied by batch sorption tests and TRLFS. 7th International Conference on Clays in Natural and Engineered Barriers for Radioactive Waste Confinement, 24.-27.09.2017, Davos, Schweiz.
- Wolter, J.-M., Schmeide, K., Stumpf, T.: Interactions between U(VI) doped CSH phases and high saline brines. Migration 2017 - 16th International Conference on the Chemistry and Migration Behaviour of Actinides and Fission Products in the Geosphere, 10.-15.09.2017, Barcelona, Spain.
- Philipp, T., Schmeide, K., Stumpf, T.: U(VI) sorption by Ca-bentonite under alkaline and saline conditions as a function of pH and carbonate content. Migration 2017 - 16th International Conference on the Chemistry and Migration Behaviour of Actinides and Fission Products in the Geosphere, 10.-15.09.2017, Barcelona, Spain.
- Wolter, J.-M., Schmeide, K., Stumpf, T.: Interactions between U(VI) doped CSH phases and high saline brines. ABC Salt V Actinide and Brine Chemistry in a Salt Repository Workshop (V), 26.-28.03.2017, Ruidoso, USA.
- Becker, M., Lippold, H.: Complexation and adsorption of [152Eu]Eu(III) to superplasticizers and bentonite at variable salt concentrations. 2nd Petrus-OPERA PhD and Early-Stage Researchers Conference on Radioactive Waste Management and Disposal, 27.06.-01.07.2016, Delft, Niederlande.