Geochemical behaviour and transport of radionuclides in saline systems with organic matter relevant to a final repository site
Because of their high radio- and chemotoxicity in conjunction with very long half-lives, actinides are in the focus of long-term safety analyses in the selection process for a repository site. Clay rock, one of the potential host formations, is characterised by a very high adsorption capacity towards cationic species. However, their interaction with negatively charged ligands (organic, inorganic) and charge screening effects at high ionic strengths may subvert this barrier function.
|Transport of humic matter in a clay drill core. For imaging by Positron Emission Tomography, the humic substance was radiolabelled with 124I.|
This project, realised in collaboration with the division “Surface Processes”, focuses on the effects of high salt concentrations, which are typical of ground and pore waters in clay rock. Owing to the difficult experimental handling of highly saline systems, there is a general lack of data and process understanding. In the work packages performed by our group, we take advantage of the outstanding analytical potential of radiotracer methods.
As a central part, spatially resolved investigations of transport processes are conducted by means of Positron Emission Tomography, using drill cores as well as samples prepared by compaction of powdered materials. Primarily, the influence of salinity is studied, also in systems with salinity gradients. Effects of elevated temperature and compaction pressure are investigated as well. Besides diffusion, advective transport in disturbed zones with preferential pathways is considered. Here, “self-healing” processes are a topic of interest. Model simulations are performed by finite element calculations on the basis of transport equations and statistical methods.
Advective transport of contaminant metals can be considerably facilitated by complexation with mobile carriers such as humic substances or humic-like clay organics, but little is known on their influence at high ionic strengths. Colloidal stability and metal-humic interaction are therefore investigated in the presence of NaCl, MgCl2 and CaCl2 up to high concentrations, and the effect on metal adsorption onto clay materials is studied. The additivity of interaction parameters in such ternary systems is critically examined. Surface complexation models are applied to describe competition effects on the basis of chemical equilibria. Limitations due to slow kinetic processes, leading to an increase in complex inertness, are explored in tracer exchange studies.
The project is integrated in an established research network of institutions with leading expertise in actinide geochemistry. The joint project is funded by the German Federal Ministry of Economics and Technology (BMWi).
Dr. Katja Schmeide (project coordination, division "Surface Processes")