Identification of knowledge gaps regarding iodine, neptunium and technetium sorption in the context of deep geological nuclear waste disposal

Sorption on mineral surfaces present in (geo-)technical barriers and in host rocks (especially in clay rock and partly in crystalline rock) is a key process constraining the transport of radionuclides from a deep geological nuclear waste repository into the biosphere. The sorption behavior of radionuclides highly depends on the environmental conditions within the deep geological repository system, which may vary spatially and over time. Therefore, a great variety of system parameters involving different sorbing minerals and environmental conditions (e.g. redox condition, ionic strength, pH, presence of complexing ions or microorganisms) need to be considered to assess the mobility of radionuclides.

This study provides a high-level overview of which systems (i.e. combinations of radionuclides, minerals and environmental conditions) have already been investigated extensively and which systems have been addressed in only few studies or not at all. The developed systematic evaluation of the state of knowledge concerning the sorption of iodine, neptunium and technetium (as representatives for safety-relevant elements) in different oxidation states includes a literature survey and a categorization of the references in a literature database with regard to the studied systems. The overarching goal of this evaluation is to identify persisting knowledge gaps and to assess the relevance for the ongoing site selection procedure in Germany and the long-term safety assessment of deep geological nuclear waste repositories in general.

Preliminary results show that iodine, neptunium and technetium sorption has already been extensively studied at neutral and slightly alkaline conditions and at low to moderate ionic strengths. Also, the influence of carbonate and divalent cations (mainly Ca) has received significant attention. However, some environmental conditions constitute knowledge gaps in sorption studies for all three examined elements and all considered solids. These are for example: high ionic strength (> 1 M), high temperatures (> 25°C) and the influence of organic ligands and microorganisms. The outcome of the project will be a sorption literature database with the possibility to search and filter references for the assigned categories.