When radionuclides move away from the engineered barriers and into the Opalinus Clay, the concentrations of most radionuclides sharply decrease with distance because of slow diffusion and strong retention and remain significantly below the corresponding elemental solubility limits. The Opalinus Clay environment provides a diffusion-dominated transport environment that, combined with retardation (sorption) and anion-exclusion facilitated by the abundance of negatively charged surfaces of clay minerals, strongly limits the rate at which transport occurs.
The ability of the Opalinus Clay to immobilise radionuclides efficiently is further enhanced by the incorporation (absorption) of some radionuclides into minerals, as supported by the findings of (Bradbury et al. 2017). This process reduces their mobility and contributes to long-term immobilisation.
Transport of dissolved radionuclides
Based on evidence from studies of the Opalinus Clay and of compacted and saturated bentonite described in NTB 23‑08 (Glaus et al. 2024a), alongside observations from natural clay analogues, it is expected that the long-term transport of radionuclides in the host rock and clay-rich rocks in the over- and underlying rock units will primarily be dominated by diffusion. In above cited work, diffusion experiments have been carried out for many anionic, cationic and neutral species. In addition, studies on the diffusivity of tracers in the Opalinus Clay have made it possible to estimate the diffusion rates of radionuclides using performance assessment, see NTB 24‑22 Rev. 1 (Nagra 2024u). Diffusion studies in the laboratory and field experiments give consistent values for different spatial and temporal scales (Leupin et al. 2017). In the Opalinus Clay and in clay-rich rocks in the over- and underlying rock units, the phenomenon of anion exclusion has been experimentally observed, and corresponding diffusivities have been determined and documented in NAB 23‑26 (Van Loon et al. 2024) and NTB 23-08 (Glaus et al. 2024a).
Temperature also impacts diffusion processes. The relationship between temperature and diffusion coefficients follows an Arrhenius-type behaviour (Van Loon 2014). When extrapolating these data to the conditions encountered in the host rock (approximate T ≈ 45 – 50 °C at 900 m depth), the expected effective diffusion coefficients for neutral species are approximately 10‑11 m2 s‑1, while for anions, they are approximately 10‑12 m2 s‑1. The experimentally determined low values for this parameter indicate the presence of anion-exclusion effects. This phenomenon implies that certain anions experience restricted diffusion in the host rock compared with neutral species, further impacting the overall radionuclide transport behaviour in a positive way. The understanding of these diffusion characteristics allows for an accurate assessment and prediction of radionuclide migration to be made for the prevailing conditions.
Besides the diffusion-dominated transport process, sorption is a critical factor controlling radionuclide transport in both the near field and geosphere. Scientific understanding of radionuclide retardation by sorption has been demonstrated through numerous experimental studies in the laboratory and in field studies and sorption data have been compiled in NTB 23‑06 (Marques Fernandes et al. 2024a).
The potential influence of complexing organic molecules naturally occurring in the Opalinus Clay on the sorption of radionuclides has been investigated by Glaus et al. (2001). Their study examined whether the presence of organic matter affects the sorption behaviour of radionuclides of specific elements, namely Ni, Eu, and Th, on the Opalinus Clay. The findings of the study revealed that the complexing organic molecules in the Opalinus Clay had no significant impact on the sorption of Ni, Eu, and Th. In other words, the sorption behaviour of these radionuclides on the Opalinus Clay remained largely unaffected by the naturally present organic matter.
Advection in the host rock and within the clay-rich rock zones is expected to play a minimal role in radionuclide transport. Occurrences of preferential flow through fractures in the Opalinus Clay at depths within the repository zone are not expected. The presence of fractures in the Opalinus Clay has been observed, but at the depth relevant for the repository system, i.e., with the prevailing compressive tectonic influence and lithostatic pressure, the natural self-sealing mechanism prevents enhancement of any fluid flow. Experimental studies conducted 300 m underground at the Mont Terri rock laboratory (confirmed that artificially created pathways formed in the Opalinus Clay did not lead to irreversible changes in permeability (Guglielmi et al. 2025). Despite these findings, there are remaining uncertainties that need to be considered, and minor increases in fracture transmissivity cannot be completely ruled out. Hence, some advective transport with matrix diffusion perpendicular to the direction of flow is possible. Nevertheless, modified or enhanced transmissivities under such circumstances are expected to be limited, i.e., fall below < 10‑10 m2 s-1 m s as is described in NTB 24‑22 Rev. 1 (Nagra 2024u).
Colloids can exist in Opalinus Clay porewater, but their mobility is expected to be extremely low due to the specific properties of the host rock. The small pore sizes, typically ranging from 2 – 15 nm with limits of 2 to 100 nm (see Section 5.3.4 in NTB 24-17, Nagra 2024i), and the presence of the charged surfaces of clay platelets, contribute to the immobilisation of the colloids. In addition, the moderate ionic strength of the Opalinus Clay porewater further limits colloid movement (Mäder & Wersin 2023). Considering these factors, colloid-facilitated transport through the Opalinus Clay can be ruled out as a relevant transport mechanism for radionuclides (Section 3.3.6 in NTB 23-03, Kosakowski et al. 2023).
Transport of volatile species in the CRZ
Repository-generated gases could penetrate the CRZ as a separate gas phase but, because of the slow gas production rate, they mostly dissolve in porewater. Gas (mainly H2) is therefore transported through the host rock, predominantly via diffusion in the porewater and to some extent by water displaced due to the gas pressure build-up in the underground structures. Some dissolved gas can reach the over- and underlying confining geological units, depending on the amount of gas generated in the repository and the transport properties of the barriers. The existence of a continuous gas phase across the host rock and its confining geological units can, however, be excluded, as documented in NTB 24-23 (Nagra 2024o). Regarding the radionuclide 14C, the generally slow migration of gases together with the relatively fast decay of 14C ensures that no significant release into the biosphere occurs, as can be seen from the analysis of radiological consequences in Chapter 8, with more details in Section 7.2 of NTB 24-18 (Nagra 2024p).