Sensitivity analysis of the natural-barrier-based Swiss repository system: The safety is in the rock

To protect humans and the environment from radiological risks, deep geological repositories for high-level waste rely on a system of staggered passive natural and technical barriers that collectively ensure post-closure safety. In Switzerland, the protection criteria were defined by the regulator as a dose rate. Given that the next 1 Million years have to be considered for post-closure safety, the significance of the natural barrier i.e. the clay rock sequence plays a particularly relevant role in limiting the release of radionuclides.

At the designated location close to Zurich a 250 m thick claystone sequence forms the geological barrier of the repository. This natural barrier exhibits very low hydraulic conductivities and the hydraulic gradient between the overlying and underlying aquifers is small. Thus - under present conditions - advective flow through the repository zone is practically absent. Future fault-related flow is unlikely due to the tectonic stability of location and the self-sealing processes in clay rich rocks, for which evidence is available over ranges of spatial and temporal scales.

In the absence of relevant water flux, waste particles released from the technical barrier system can only leave the repository by diffusion through the pore space of the rock. The corresponding transport parameters can be derived from lab studies and -at the meter-scale- from rock lab experiments. Transport parameters are systematically related to the rock’s clay content, as it controls the pore space geometry. These insights lead to confidence in the parametrization of nuclide species retention properties of the geological barrier.

The dose calculations for the current concept indicate a large robustness of the system due to the extremely slow transport through the natural barrier. Even with canister lifetimes at the regulatory minimum of 1000 years the clay formation limits the maximum individual dose contribution from the geological repository to a small fraction of the regulatory threshold of 0.1 mS/yr. Most of the nuclides are immobilized in the first few meters of the clay formation and decay in the barrier system. The robustness of the system is supported by the results of further alternative cases with reduced sorption in the backfill material. The deviations from the reference case in any of these cases are very small.

In comparison to technical-barrier-based systems (e.g. in crystalline rocks) canister lifetime and nearfield properties are less relevant for the radiological risk of a clay-based repository for high-level radioactive waste or a combined repository. In the Swiss clay-based repository concept the geological barrier alone is sufficient to ensure a dose far below the regulatory limit. In a stable, high clay content geological situation the technical barriers can therefore be designed with a specific focus on operational and early post closure aspects.