Understanding transport pathways for any radionuclides released from the repository is necessary to guide performance assessment, focusing on key aspects of barrier functionality and barrier evolution over time, and to evaluate potential radiological consequences. This section provides a broad-brush description of how radionuclides are expected to migrate through repository barriers. It is based on the qualitative description of the expected phenomenological evolution of the repository as presented in Chapter 9 in NAB 24‑20 Rev. 1 (Nagra 2024m).

A monitoring period starts after the completion of waste emplacement operations. After an initial period of monitoring, the disposal area accessways will be backfilled and sealed. This is followed by a second monitoring period that is currently assumed to last several decades (as e.g., described in the Waste Management Programme (Nagra 2021c). After this monitoring period, the whole repository is closed¹³ and the post-closure period begins. Evolution in the post-closure period with respect to the fate of radionuclides can be described in terms of five periods (which overlap to some extent):

1. The radionuclides are contained in the waste packages. In the case of HLW disposal canisters, this period is expected to last at least 1,000 years. No period of complete containment can be assumed for L/ILW disposal containers. However, the L/ILW containers, backfilled with cement mortar, contribute to the retention of radionuclides.

2. A small proportion of the radionuclides in the HLW are released after canister breaching. Some releases are relatively rapid (e.g., gaseous fission products), but most radionuclides (from activated SF assemblies or vitrified glass from reprocessing) are released very slowly, congruently with the degradation of the waste matrices.

3. The radionuclides released from the waste packages start to migrate through the surrounding bentonite buffer and cementitious backfill and into the host rock and/or the closure system. This migration is inhibited, reduced, and delayed by the physical and chemical environment created by the bentonite buffer and cementitious backfill.

4. The small fraction of radionuclides (corresponding to less than 1% of the initial activity) reaching the host rock continues to migrate through the CRZ away from the repository. This migration is diffusion-driven and very slow, allowing for further decay.

5. Only a very small proportion of the initial radionuclide inventory (corresponding to less than 0.1% of the initial activity) leaves the CRZ and enters the deep aquifers that bound it. These radionuclides may eventually reach a local aquifer and the biosphere, where they are further dispersed and diluted. Nevertheless, this can lead to radiological exposure of humans and the environment, albeit at extremely low levels.

In the following sections, additional details of these processes are presented.