Radiological Consequences of Deep Geological Repository Excavation by Erosive Processes
Thatcher, K., Walke, R., Newson, R., Bruffell, L., Ludwig, A., Fischer, U., Bykov, V. & Hunkeler, P . (2024): Radiological Consequences of Deep Geological Repository Excavation by Erosive Processes. Nagra Arbeitsbericht NAB 24-08 Rev. 1.
Abstract
Nagra, the National Cooperative for the Disposal of Radioactive Waste, is responsible for the development and safe implementation of a deep geological repository for radioactive waste in Switzerland. The concept involves the disposal of low-, intermediate-, and high-level waste in a deep geological repository within a clay-rock formation (Opalinus Clay) at a depth of over 800 m. There, the disposed waste inventory will remain isolated from the surface environment, allowing radionuclides to decay, significantly reducing their radiological hazard down to a level where it is of no significant concern. In the very long term (106–107 years), uplift and erosion by both glacial and non-glacial processes may bring the repository closer to the surface, potentially resulting in the excavation of repository material. In its guidance related to deep geological disposal in Switzerland, ENSI requires assess-ment of potential dose consequences of repository excavation scenarios in the very long term. Assessment calculations have been undertaken to explore potential dose consequences of reposi-tory excavation scenarios. These have separately considered excavation by deep glacial erosion and non-glacial erosion.
- Assessment of deep glacial erosion considers an overdeepening directly impacting the repository and transporting excavated material to the near-surface region and surface, with a range of post-glacial landscape settings.
- Assessment of non-glacial erosion considers the emergence of what remains of a reposi-tory below a valley or in a hillside setting, with a range of erosion rates and valley geom-etries and the subsequent dispersal of that material by hillslope or fluvial processes.
A compartment-modelling approach is used to represent the distribution of material excavated from the repository, subsequent radionuclide release, transport and potential exposure. The expo-sure assessment assumes that humans are present in the area local to the excavated material and make maximum use of that environment. The exposure-modelling approach is based on Nagra's established reference biosphere assessment modelling.
A range of potential excavation contexts has been studied, accompanied by sensitivity calcu-lations to help identify important processes, parameters and assumptions. The assessment ex-plores potential consequences of excavation over timescales ranging from 700,000 years to four million years after repository closure. These timescales span both early hypothetical and more plausible timescales for repository excavation.
Overall, the assessment demonstrates a degree of consistency in the potential dose consequences calculated across a wide range of different excavation settings. Both excavation by deep glacial erosion and by non-glacial erosion result in potential dose consequences that are lower than or comparable with the natural, current radiation exposure in Switzerland (approximately 1 – 13 mSv/a).