Secondary Phases

Deep geological repositories for high level radioactive waste are by their design complex multiphase systems. Geochemical gradients will occur - especially at the interfaces between the different components of the engineered barrier system (EBS). Depending on the host rock and ¬design of the repository, the EBS may contain components such as the waste package (waste form and container), backfill material (bentonite, crushed rock and/or mortar) and a concrete liner.

In contact with water, the geochemical conditions are to a large extent determined by the corrosion of metal components and the groundwater chemistry of the host rock. Corrosion and alteration of the waste form and the EBS lead to the formation of new phases which need to be considered to avoid unnecessary conservatisms in the evaluation of radionuclide mobility in future safety assessments for deep geological repositories. With respect to these new phases, different mechanisms of radionuclide retention need to be considered, including adsorption, structural uptake, and precipitation of a pure radionuclide phase.

We investigate the immobilization of radionuclides by a structural uptake on a mechanistic level with a focus on the formation of solid-solutions which can have a major impact on radionuclide solubility and retention.

Layered double hydroxides (LDH) are studied due to their capability of retaining anionic species of selenium or iodine. Within the framework of the projects ThermAc and VESPA, the structural uptake and thermodynamic properties of LDH was studied.