Transport of dissolved radionuclides in the closure system

Once the closure system is sufficiently saturated, radionuclides can be transported in dissolved form through it and further along the shafts. Transport rates, however, remain slow and diffusion-dominated due to the favourable and to some extent redundant properties of the geological and engineered barriers. In particular, (i), the low hydraulic conductivity of the closure system, and especially of its various seals, as well as the limited capacity of the surrounding low-permeability rock to supply water, limit the rate of water flow and advective transport of dissolved radionuclides through the closure system, and (ii), the high sorption capacity of the clay materials within the seals and backfill further retards the transport of many radionuclides (Section 5.2.2 in NAB 23-21, Martin et al. 2023 and Chapter 4 in NAB 21-12 Rev. 1, Nagra 2024b).

Transport of volatile species in the closure system

As noted above, saturation with water occurs within a few hundred years in the HLW near field. The V3 shaft seals also saturate relatively rapidly. In contrast, the L/ILW near field, the sealing elements and much of the closure system of this repository section are expected to remain only partially saturated for a much longer period, facilitating the transport of repository-generated gases including volatile radionuclide species, especially ¹⁴C in the form of methane. As detailed in Section 4.3 in NAB 24-07 Rev. 1 (Nagra 2024w) these gases are, however, largely contained by the V3 seals, which have a low gas permeability once saturated. The gas within the closure system gradually dissolves in the porewater within the closure system itself and within the surrounding rock.