High-level waste requires to be isolated safely from the human environment for around 200,000 years. After this, its activity will have to decayed to levels that no longer present a hazard.
We know from world history that social structures rarely remain stable over long time periods. The history of the earth, on the other hand, teaches us that rock formations can remain stable over many millions of years and that their properties do not change. Geological processes underground occur extremely slowly, independently of what is occurring at the surface. Rock layers that are impermeable to water can contain substances over geological timescales, far beyond the scope of human experience. The required containment period for high-level waste of around 200,000 years is short compared to these geological time spans.
It is recognised worldwide that, for high-level waste, disposal in geologically stable rock formations can ensure safety over the long time spans involved. Radiation from the waste is shielded by the waste containers, the tunnel backfill, the repository installations and the surrounding rock. The safety barriers also prevent radioactive substances from being dissolved in water and transported to the earth's surface, where they could enter the food-chains. A geological repository is therefore constructed in low permeability rock.