Systematic evaluation of the state of knowledge on radionuclide sorption in the context of deep geological nuclear waste disposal

Sorption on mineral surfaces present in the (geo-)technical barriers and in the host rock (especially in clay rock and partly in crystalline rock) is an important process constraining the transport of radionuclides from a deep geological nuclear waste repository into the biosphere in case of canister failure. For a reliable safety assessment, it is therefore important that the amount of sorbed radionuclides can be assessed adequately and that there is a sufficient understanding concerning the underlying mechanisms. The sorption behavior of the radionuclides highly depends on the environmental conditions within the deep geological repository system, which may vary spatially and over time. As a consequence, a great variety of system parameters involving different radionuclides, sorbing minerals and conditions (e.g. redox conditions, ionic strength, pH, concentration of ligands) need to be considered and investigated.

The presented study provides a high-level overview of which systems (combinations of radionuclides, minerals and environmental conditions) have already been extensively investigated and which systems have been addressed in only few studies or not at all. It is not aiming to build up a database for distribution coefficients or thermodynamic data, nor analyzing certain sorption mechanisms in detail. The systematic evaluation of the state of knowledge concerning the sorption of safety-relevant radionuclides, includes a literature survey and categorization of the references in a literature database according to the studied radionuclides, minerals and conditions. The overarching goal of this evaluation is to identify persisting knowledge gaps and to assess their relevance for the ongoing site selection procedure in Germany and the long-term safety assessment of deep geological nuclear waste repositories.

Prerequisite to identify safety relevant knowledge gaps, is to define requirements on the level of knowledge in the first place. Therefore, radionuclides, minerals and environmental conditions with great relevance for deep-geological nuclear waste disposal were identified during the initial phase of the project. Also, requirements on the form of sorption data (e. g. distribution coefficients, thermodynamic data, process understanding) were discussed. Based on this, the literature survey was started and is still ongoing. This contribution will discuss the methodology and present the current status of the evaluation with preliminary results for selected systems.