Stability and Buffering Capacity of the Geosphere for Long-term Isolation of Radioactive Waste

Application to Crystalline Rock

image of Stability and Buffering Capacity of the Geosphere for Long-term Isolation of Radioactive Waste
Geological settings selected as potential host formations for the deep geological disposal of radioactive waste are chosen for, among other assets, their long-term stability and buffering capacity against destabilising events and processes. These proceedings present the outcomes of a geosphere stability workshop, held in November 2007, that focused on crystalline and other types of hard, fractured rocks. The workshop underscored the fact that many such rocks are intrinsically stable environments that evolve extremely slowly and provide good buffering against external events and processes.

The proceedings show a good understanding of the processes and events that can affect crystalline rocks and, although there is less confidence in predicting exactly when and where such events will occur and the volume of rock that will be affected, the extent of the impacts on a geological repository can be confidently addressed using bounding approaches supported by geological information from similar sites around the world.



General Framework: Crystalline Rocks as Host Formations

Nuclear Energy Agency

A number of detailed safety functions for a granitic host rock, subordinate to the main functions containment and retardation, are presented, based on SKB’s safety assessment SR-Can for the KBS-3 concept. The host rock should provide a favourable environment for the repository from the point of view of chemical, hydraulic and radionuclide transport, mechanical and thermal properties, which are further specified as a number of subordinate safety functions. These are strongly linked to the functions and properties of the canister and the clay buffer in the KBS-3 concept. Issues related to geosphere stability during the one million year assessment period are discussed and primarily relate to perturbations caused by future colder and dryer climates yielding glacial and/or permafrost conditions and the impact these perturbations may have on the safety functions. In the SR-Can assessment, it was concluded that the granitic host rocks at the analysed sites provide a sufficiently favourable and stable environment for the vast majority of the 6 000 analysed deposition holes. The residual radiological risks associated with the repository, as pessimistically calculated based on preliminary understanding of the two candidate sites, are, from the point of view of the host rock, sensitive to details in the repository layout. In particular, it is important to be able to avoid deposition holes intersected by large or highly transmissive and hydraulically connected fractures and thus to understand and being able to quantify the heterogeneous character of the fractured host rock, in particular at repository depth. A number of issues related to geosphere stability where improved knowledge could lead to more realistic assessments are also identified.


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