Abstract
AbstractDBE TECHNOLOGY, BGR and GRS are developing a methodology to demonstrate the safety of a repository for high-level waste and spent fuel (HLW/SF) in clays according to the requirements of the German regulating body. In particular, these requirements prescribe that the barrier effect of host rocks must not be compromised by a thermal impact resulting from HLW/SF emplacement. To substantiate and quantify this requirement, we carried out a literature survey of research on thermally-induced changes on clay properties. Effects thus compiled can be divided into thermo-hydro-mechanical and chemical-biological-mineralogical effects and were analysed with regard to their relevance to the integrity of clay host rocks. This analysis identified one effect of major influence within each group: thermal expansion and compaction as well as results of microbial activities. Importantly, it further revealed that a moderate temperature increase above 100°C cannot be expected to compromise the integrity of the geological barrier according to the current knowledge state. Evidence is presented in this paper that temperature increases up to 150°C can actually contribute to an improved performance of a radioactive waste repository by increasing the consolidation of the clay and sterilizing the repository's near-field to depress the deteriorative microbial effects. A quantitative temperature criterion for thermal impact of HLW/SF on clay host rocks is accordingly proposed.
Highlights
AN integrated methodology to demonstrate the safety of a high-level waste and spent fuel (HLW/SF)-repository in clays in Germany is being developed by DBE TECHNOLOGY, BGR (Bundesanstalt für Geowissenschaften und Rohstoffe) and GRS
One key element is the demonstration of the integrity of the geological barrier
The regulating body requires a demonstration of the integrity of the containment-providing rock zone within host rocks for a period of 1 million years by meeting different integrity criteria (BMU, 2010)
Summary
AN integrated methodology to demonstrate the safety of a HLW/SF-repository in clays in Germany is being developed by DBE TECHNOLOGY, BGR (Bundesanstalt für Geowissenschaften und Rohstoffe) and GRS. The analysis of the thermo-hydro-mechanical effects reveals no deterioration but does reveal a positive impact of a temperature increase beyond the ECT (50‒80°C) up to 150°C on the barrier properties of host rocks. The period during which the thermal input of a HLW/ SF repository heats the host rocks to temperatures relevant for abiotic chemical and mineralogical processes only within a limited period of time and within a distance limited to several metres around the emplacement boreholes or tunnels During this time no appreciable abiotic illitization (and no change in swelling properties) thermochemical sulfate reduction or kerogen transformation to raw oil and natural gas is to be expected. Evaluation of the thermally induced effects in clay with respect to the impact on its barrier properties (ECT = Expansion-Contraction-Threshold) based on available studies for temperatures from 20°C to 150°C
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