In many countries, geological disposal facility is considered as an option for the long-term management of radioactive waste. In the French, Swiss, and Belgian concepts, clays have been selected as suitable host formations. Considerable amounts of gas can be generated within geological disposal facilities – mainly hydrogen from corrosion of metallic elements of the repository or of the waste themselves. Although gas production will be slow, it is important to assess how gas will accumulate, migrate, and be released within the disposal system and what could be the potential system perturbations caused by gas production and pressurisation in order to properly evaluate the functioning of a geological disposal facility. To address this question, the EURAD-GAS project under the umbrella of the European Joint Programme EURAD (2019–2024) studied gas transport mechanisms in clays. The project aimed to enhance understanding of gas movement through low-permeability materials and assess its implications for barrier integrity and repository performance. This paper provides an overview of the state of knowledge on gas transport processes collected within this project and illustrates how this knowledge can be used to assess gas transport at the scale of a geological repository in clay formations.
The safe geological disposal of low- and intermediate-level radioactive waste relies on a passive multi-barrier system. This paper presents the provisional Swiss design for the low- and intermediate-level waste (L/ILW) emplacement caverns and their expected evolution. In the current design, containers are stacked within the L/ILW disposal caverns, and the remaining void space is backfilled with a porous mortar. Because cementitious materials are used in large quantities, they largely determine the geochemical conditions in the L/ILW near field. A key property of these cementitious materials is their ability to maintain a high-pH porewater environment. Such conditions limit the degradation of organic materials and suppress microbial activity. They also promote the passivation of metal surfaces, resulting in slow corrosion rates and consequently reduced gas generation. The chemical evolution of the near field is strongly coupled to the repository’s saturation history, as water is required for chemical reactions such as for the corrosion of metals, degradation of organic matter or pozzolanic reactions. Given the low permeability of Opalinus Clay, water availability in the caverns is limited, and partially saturated conditions are expected to persist for several hundreds of thousands of years. These conditions help maintain elevated pH levels, particularly in the unsaturated regions of the emplacement caverns. The substantial presence of cementitious materials also enhances the retention and slow release of radionuclides from the L/ILW near field into the host rock, once the waste packages breach. As a result, the L/ILW near field contributes to key safety functions, including immobilization, retention, and controlled release of radionuclides, as well as ensuring compatibility among repository components. The properties and long-term behaviour of these components are sufficiently well understood to allow a robust description of their evolution over extended timescales, supporting the demonstration of their barrier function and long-term performance.
The post-closure biosphere assessment and modelling in support of the general licence application for a deep geological repository for radioactive waste provides the basis for evaluating radiological consequences should radionuclides released from the geological repository reach the surface and near-surface environments. The assessment approach comprises the modelling of radionuclide migration in the biosphere following potential releases from the geosphere to a local aquifer over timescales of tens of thousands to more than a million years. A stylised approach is adopted, including reference and alternative biosphere conditions, consistent with both national and international regulations and guidelines. The model is implemented in Nagra’s SwiBAC code, which allows radionuclide fluxes to the biosphere to be translated into potential doses to humans. This process yields equilibrium biosphere dose conversion factors (BDCFs) for each radionuclide for use in the analysis of radiological consequences, which occurs in a separate assessment step. Reference biosphere calculations are based on present-day conditions in Northern Switzerland, represented by a valley setting with a temperate climate. These conditions support a wide range of potential radionuclide transport and exposure pathways, including agricultural water use, soil-plant transfer, and local food consumption. Three alternative biospheres are also assessed to address uncertainties in long-term climate and geomorphology, including (i) a warmer-drier climate, where aquifer and surface flows are reduced while irrigation is increased, (ii) a cold climate with permafrost, where the exchange between soil and the aquifer is reduced, and (iii) a drained farmland setting with a high water table. BDCFs vary significantly between these cases, increasing by up to about an order of magnitude in warmer-drier conditions and decreasing by up to about five orders of magnitude under cold-climate conditions. These results highlight the key role of the shallow aquifer and transfer pathways within the valley agriculture system. Deterministic and probabilistic sensitivity analyses further highlight the influence of sorption in the shallow aquifer, hydrological fluxes, and agricultural transfer pathways. Together, the reference and alternative cases constrain key uncertainties and provide a robust, conservative basis for evaluating long-term radiological safety.
Deep geological repositories for radioactive waste rely on geological barriers to isolate waste and impede or limit radionuclide migration over long geological timescales or until radioactivity has decreased to natural radiation levels. This study summarises the main geological evidence and arguments for the containment-providing rock zone (CRZ), including the host rock Opalinus Clay, as effective geological barriers for Switzerland’s deep geological repository. The information and data are part of Nagra’s comprehensive safety case within the framework of the general licence application. The geological assessment basis contains an integrated multi-scale site characterisation, covering data from 3D-seismic investigations and nine deep boreholes from the latest exploration campaign. Highlights of the geological data include: 1) high-resolution 3D-seismic datasets that image the subsurface structure, including undisturbed structural domains, key stratigraphic horizons, and the geometry of faults; 2) a comprehensive field and laboratory dataset that demonstrates that the hydraulic conductivities of the Opalinus Clay and remaining CRZ units are extremely low, that solute transport is diffusion-dominated, and that key properties have high vertical and lateral continuity; 3) independent natural-tracer profiles that developed over millions of years and confirm slow, diffusion-dominated transport across the CRZ; 4) a laboratory-derived mechanistic understanding and field evidence of self-sealing processes in the Opalinus Clay that show that fracture transmissivities decrease relatively rapidly after mechanical perturbation; 5) a systematic abstraction approach that groups geological units with similar properties and provides a modelling framework for evaluating safety- and performance-relevant processes; and 6) broad-ranging evidence that demonstrates the long-term geological stability of the Opalinus Clay with respect to the expected tectonic, geomorphological and hydrological changes during glacial-interglacial cycles over the next one million years. The synthesis presented here demonstrates how the geological data and its abstraction provide a robust and internally consistent basis for the safety assessment supporting the Swiss general licence application.
For long-term safety assessments of deep geological disposal, evaluating effective diffusion coefficients (De) and distribution coefficients (Kd) under in-situ conditions remains a critical yet challenging task due to practical limitations. This study aims to establish a methodology for evaluating these parameters under realistic geological conditions through an integrated analysis of a long-term in-situ diffusion (LTD-II) experiment and complementary laboratory through-diffusion experiments in Grimsel granodiorite. The proposed approach combines (i) post analysis of cored samples to characterize spatial variations in pore connectivity and anisotropic transport, (ii) numerical modeling that accounts for the borehole disturbed zone (BDZ), and (iii) laboratory diffusion experiments to verify the consistency and reliability of in-situ parameter estimation. The modeling reproduced both the depletion curves and the tracer concentration profiles observed in the LTD-II experiment, particularly the sharp gradients within a few millimeters from the injection hole, which were attributed to BDZ. The derived De and Kd values for sorbing tracers (Na+, Cs+, and Ba2+) were consistent between laboratory and in-situ conditions for transport distances up to several centimeters, confirming the reliability of the parameter derivation approach for sorbing species over short distances. In contrast, non-sorbing tracers (HTO and 36Cl-) exhibited depth-dependent concentration variations extending ∼50 cm from the injection hole. The De values of non-sorbing tracers were two to three times higher than those obtained in laboratory tests, yet remained within the same order of magnitude. These variations of non-sorbing tracers could be qualitatively explained by anisotropic transport along foliation and minor advection over several tens of centimeters, highlighting the importance of accounting for local structural and hydraulic variations when interpreting in-situ diffusion data. These results demonstrate that integrating in-situ diffusion data with laboratory verification and spatial profiling provides a methodology for deriving diffusion and sorption parameters representative of in-situ conditions, and clarifies the practical limitations and applicable range of transport modeling in crystalline rocks.
