In addition to radionuclides, the waste contains non-radioactive but chemotoxic substances, which must also be safely confined in the deep geological repository. These non-radioactive, chemotoxic substances originate either from the waste inventory itself or from technical conditioning materials. Their long-term impact on humans, animals, ecosystems, and essential resources such as groundwater and soil must therefore be considered in environmental assessments (UVPV 1988). The legal framework governing these substances in Switzerland is provided by the Environmental Protection Act (USG 1983).
Within the context of a Swiss deep geological repository, such substances include a range of organic and inorganic compounds as well as metals such as cadmium, copper, and lead (Häner et al. 2014). It has to be ensured that these substances will not migrate into the CRZ and reach the groundwater in significant amounts. Generally, the behaviour of potentially chemotoxic metals does not differ from that of their radioactive isotopes with respect to transport through engineered and/or geological barriers. Therefore, the outcome of the radiological consequence analysis can also be used, by analogy, to indicate the consequences of non-radioactive substances (see Chapter 8).
In Stage 2 of SGT (BFE 2008), a risk assessment evaluated the potential release of chemotoxic substances from the waste inventory into surrounding groundwater systems (Häner et al. 2014). This assessment employed simplified, conservative scenarios that assumed transport by diffusion and no retention by either engineered or geological barriers. Even under these very conservative assumptions, the relevant toxicological thresholds for drinking water safety were largely met. It could thus be clearly demonstrated that chemotoxic elements would be of no safety concern.
Barrier properties and geological conditions
As described earlier in Chapter 3 and Section 5.2, the deep geological repository in Switzerland is planned to be constructed at a depth between 800 and 900 m below ground, within an approximately 105 m thick layer of Opalinus Clay (Section 4.2.6 of NTB 24-17, Nagra 2024i). This host rock, along with adjacent clay-rich formations, provides excellent containment for both radioactive and non-radioactive substances (Chapter 8 of NTB 24-18, Nagra 2024p). T). These formations isolate the repository from the nearest regional aquifers in the Malm or in the Keuper.
Consistent with earlier findings, the latest geoscientific investigations conducted within the framework of the general licence application confirmed that the clay-rich host rock and surrounding rocks possess large mineral surface areas (30 m2/g for the Opalinus Clay), resulting in excellent retention properties, an excess of negatively charged surfaces (illite-smectite minerals) that slow down diffusion of anionic species, low hydraulic permeability (K = 5 × 10⁻¹³ m/s), highly reducing conditions and excellent self-sealing properties. The small pore openings (distribution of diameters peaking at 2 – 3 nm) of the clay-rich rocks also reduce or prevent the transport of larger molecules in the Opalinus Clay (the relevant properties of the Opalinus Clay are described in detail in Chapter 5 of NTB 24-17, Nagra 2024i).
Following contact with water, a large proportion of the waste emplaced in the deep geological repository exhibits low solubility due to its material composition or conditioning. The material properties, along with the robust multi-barrier system, limit the release of substances into the porewater (Hummel et al. 2023, Tits & Wieland 2023, Hummel et al. 2022). As a result, the concentrations of substances that can be transported diffusively into and through the multi-barrier system are already limited. Additionally, many substances are strongly retained by sorption both in the backfilled repository tunnels and in the clay rocks. Calculations from the safety analysis (Figures 6-9 and 6-61 in NTB 24-18, Nagra 2024p) illustrate that cationic radionuclides (and thus, by analogy, potentially water-polluting elements such as Pb, Cd, or Cu) are retained within the first 10 – 20 metres of the Opalinus Clay.
Conclusion
With an improved understanding of the performance of the repository barriers and more detailed site-specific geological data acquired in SGT Stage 3, previous conservative release models – such as those used in earlier assessments (e.g., Häner et al. 2014) – are now considered overly cautious.
Updated modelling approaches incorporating more realistic physicochemical parameters, including site-specific porewater chemistry (e.g., pH, redox conditions; for more details see, e.g., Glaus et al. 2024a, Marques Fernandes et al. 2024a, Hummel & Thoenen 2023), and enhanced knowledge of retention and degradation mechanisms, have confirmed earlier findings that a deep geological repository in the Opalinus Clay offers a robust multi-barrier system: it safely contains not only radioactive materials but also non-radioactive, chemotoxic substance.