Long-lived radionuclides in disposal sites

In aquatic systems actinides and radionuclides can be transported in solution or as particles. The knowledge of the physical chemical interactions on the surface of the percolated minerals/rocks as well as those with the generated colloids is essential to characterize both possibilities of migration.

Aim of the research is to identify the dominating surface reactions during the sorption of actinides/radionuclides on mineral, rock and biomolecular surfaces and to qualify/describe them with suitable surface complex formation models. Decomposition into several surface reactions includes the determination of mineral specific parameters like surface binding site density and reactive surface, respectively, as well as cationic exchange parameter. The surface species are verified by spectroscopic techniques such as ATR FT-IR or TRLFS. The investigated surface complex formation constants are integrated in the mineral specific thermodynamic sorption database RES³T (Rossendorf Expert System for Surface and Sorption Thermodynamics) and allow the sorption modelling on complex rocks.

It has been shown that decomposition and mineral dissolution and precipitation can significantly influence sorption processes. Therefore the kinetics of dissolution and the formation of secondary phases should be included. This innovative approach leads to a better understanding of sorption processes on geological materials and thereby to enhanced long-term prognoses for radionuclide migration. Special attention is paid to the coupling with thermodynamic databases for aqueous chemistry and mineral formation.

In principle this approach of research can also be transferred to colloids having dimensions in nanometer ranges. On one hand colloids exhibit strong sorbat sorbens surface reactions, on the other hand they are not immobile like e.g. rock surfaces. Therefore their colloidal behavior lies between that of real dissolved actinides/radionuclides and that of macroscopic solid phases covered by radionuclides. Relevant colloids are characterized in regard to their chemical and physical properties, namely their interactions with actinides/radionuclides.

The long-term aim is to characterize the dissolved and colloidal radionuclide transport via computer programs integratively. This is required to complete the spectrum of characterization and identification methods of colloids and to enlarge the methods for investigating the species and their localization on the solid-liquid interface.

Transport process studies of dissolved solutes and colloids on the core sample scale are performed by means of GeoPET flow through experiments with complemental geophysical and geochemical characterization of the formations matrix, the fluids and the colloidal system components. These combined methods allow for the conducting of key experiments at the intersection between transport processes and chemical reactions, between experiment and modelling and at the overlap between the pore and the Darcy scales.

Intermediate Term Requirements and Aims

  • Investigation of reactions and involved species in sorption of actinides/radionuclides on mineral or rock surfaces and in soils 
  • Characterization (structural and thermodynamic) of aqueous species and radionuclides incorporated into solid phases
  • Improved understanding of redox-mediated processes, the impact of high temperatures, ionic strengths and pH values on chemical speciation
  • Development and parameterization of surface complexation models

Therefore research activities dealing with the spectroscopic investigation and the microscopic localization of the surface species will be increased. The focus lies on investigating surface complexation of uranium, extended to other actinides and fission products, in particular Np, Pu, Am, Cm, and Tc. Mineral dissolution and precipitation reactions (e.g. secondary mineralization of uranium) are also included.