In the reference scenario of the safety case for the Swiss repository the peak individual dose for people in the exfiltration area of ca. 0.2E-4 mSv/y is reached 0.8 Mio years after closure of the repository. The largest part of this dose is contributed by I-129. Iodine is non-sorbing on the charged surfaces of the clay minerals in the host rock. In contrast, any cations released from the technical barrier system are effectively retained by sorption within the geological barrier. The calculated dose maximum is 500 times lower than the regulatory limit of 0.1 mS/yr and 2500 times lower than the individual radiation exposure in Switzerland. The repository system therefore offers large safety margins.
The timing and maximum of the I-129 flux from the thick clay rock package forming the natural barrier of the repository into the overlying and underlying aquifers is derived from a 1D diffusion calculation. Confidence into the underlying assumptions and parameters of this calculation is gained by natural in situ analog data. Vertical profiles of Chloride measured in the porewater of the clay rock sequence confirm the assumptions for Iodine migration in the geological barrier. The close similarities between Chloride and Iodine transport in the geosphere allow to directly conclude from geological archives to dose prognosis.
We present data from natural tracers in the pore water of the clay rock sequence indicating that diffusion is the only relevant transport process for radionuclides. The thickness of the diffusion dominated package can be directly derived from the shape of the tracer profiles. Built-up times for the observed vertical distribution of the tracers between the bounding aquifers can be related to the Quaternery landscape evolution. We therefore conclude that the prognosis for the Iodine dominated dose maximum more than 2 orders of magnitude below the regulatory limit is well founded by the observations of Chloride tracer behavior in the geological past.
How to cite: Vietor, T., Schnellmann, M., Leupin, O., and Li, X.: Safety of the Swiss repository: evidence for clay host formation barrier efficiency from the geological past, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18747, https://doi.org/10.5194/egusphere-egu25-18747, 2025.
Analysing and modelling natural tracer profiles in porewaters is crucial for understanding paleo- transport and paleo-hydrological processes in argillaceous rocks, which are often considered potential host rocks for radioactive waste disposal. Following a recent deep-drilling campaign in northern Switzerland, a large dataset from eight boreholes has become available, which reveals detailed porewater, groundwater and rock properties across the Mesozoic sequence. This study aims to reproduce profiles of four different natural tracers (δ 18O, δ2H, Cl-, and Br-) from these boreholes, using a diffusion model accounting for temperature- and clay-content-dependent diffusion coefficients, in-situ anion accessibilities, and exchange of 18O during water-rock interactions in the Malm lithologies. The groundwater compositions, with respect to these four tracers, were changed at a given time in each of the three bounding aquifers, which, from shallow to deep, are: Malm (or Hauptrogenstein towards the west), Keuper, and Muschelkalk. All the 32 profiles were successfully reproduced using a consistent approach with identical aquifer evolution times specific for each borehole. The modelling concentrated on the relatively late evolution of the profiles (~last 10 Ma). The modelled timing of the groundwater composition suggests the evolution times for the Keuper aquifer of 0.1-0.7 Ma, more scattered evolution times for the Malm aquifers of 0.2-3 Ma (for the Hauptrogenstein aquifer of 0.5 Ma), and shorter evolution times for the Muschelkalk aquifer of typically less than 0.1 Ma. These times are broadly in line with studies on groundwater residence times from the same boreholes. Furthermore, sensitivity calculations were performed to access the influences of various modelling assumptions and simplifications, such as the initial conditions, the paleo-temperature conditions, the mechanism of activating the aquifers, and the uncertainties in the diffusion coefficients and in the aquifer locations. The detailed modelling study corroborates earlier interpretations that the Mesozoic rock sequence acts as an aquitard where solute transport is slow over large time scales. It also supports the palaeohydrogeologic history that has been inferred from other data. Finally, the sensitivity calculations demonstrate that these conclusions are relatively robust, even though the simplified modelling necessarily relies on several assumptions.
How to cite: Ma, J., Thomas, G., Paul, W., and Daniel, T.: Modelling profiles of natural tracers and evolution of aquifers through the Mesozoic sequence in northern Switzerland, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16060, https://doi.org/10.5194/egusphere-egu25-16060, 2025.
Anionic radionuclides are relevant contributors to the overall dose that may originate from an underground repository for radioactive waste. Clays are important parts of engineered and natural barriers of repositories due to their sealing properties. As clay surfaces are negatively charged, anions are depleted in the pore space near the clay surfaces. This partial exclusion of anions strongly affects their transport. To make reliable predictions of the evolution of a repository, e.g., for safety assessments, a thorough understanding of this phenomenon is required, as well as the ability to model the exclusion effect for different conditions.
Unfortunately, the degree of anion exclusion depends on many parameters, including the mineralogical composition of the rock, the porosity, and the porewater chemistry. Moreover, it depends on rock properties difficult to quantify such as texture or generally the pore space architecture. While the basic principles behind anion exclusion are understood and various models exist, it is not straightforward to apply these models to different rock types or different chemical conditions. At the same time, the direct determination of the anion accessibility (an average property defined as the fraction of the pore space fully accessible to an anion) by diffusion experiments is very time consuming. Fortunately, a recent deep drilling campaign in Switzerland provided a large data set including both, rock properties and anion accessibilities.
Here we profit from this data set and compare different methods to derive average anion accessibilities for various rock types and conditions. In a first approach, we build a chemical-structural model for a rock based on the fractions of different phases (minerals, porewater), the porewater chemistry, and assumptions regarding the distribution of the porewater. In a second approach, we apply Machine Learning (ML) on a training data set and build a model based on the most influencing parameters, including clay-mineral content, water content, and porewater chemistry.
Both approaches are performing relatively well for clay-rich units. However, they show weaknesses in other lithologies, especially for rocks with very low water contents or with (presumably) very specific texture. For such rocks, there is a lack of knowledge to develop suitable chemical-structural models. Also, the data base with regard to anion exclusion is currently still small for such rocks, which clearly limits the ML approach. Thus, extending the appropriate knowledge, e.g., by microstructural investigations, as well as the corresponding data base are considered as promising next steps.
How to cite: Gimmi, T., Jenni, A., Zwahlen, C., Prasianakis, N., and Boiger, R.: Modelling anion-accessible porosity of rocks based on different approaches, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10706, https://doi.org/10.5194/egusphere-egu25-10706, 2025.
Uncertainty quantification of solute transport by groundwater flow is a critical component of safety assessments of geological repositories for spent nuclear fuel. Sparsely fractured crystalline rock provides a favourable geological environment because of its low permeability and tectonic stability. Although fracture occurrence may be sparse, fracture clusters may form connected pathways for groundwater flow and solute transport from a subsurface repository to the biosphere. However, reactive solutes including radionuclides are not only affected by advective flow but also experience diffusion into the rock matrix with retention mechanisms delaying plume migration. Therefore, it is of great importance to quantify uncertainties in the representation of DFNs to obtain useful constraints on uncertainties for reactive solute transport.
In this contribution, a stochastic Lagrangian framework is used to calculate transport of reactive solutes obtained from advective particle trajectories in three-dimensional discrete fracture network models. Previous work has shown that accounting for internal fracture variability in DFN models enhances early advective particle arrival compared to the smooth fracture plane assumption. Here we investigate the effect of internal variability in permeability in DFNs for two classes of reactive solutes representing radionuclides, one dominated by diffusion and another by retention. The findings show that solutes which are dominated by retention are significantly affected by variable fracture permeability in DFNs, comparatively much more than those dominated by diffusion. We showcase how uncertainty in solute mass arrival can be quantified using the reactive transport framework methodology and discuss implications on transport assessments for cases where mechanical deformations cause changes to the internal variability of fractures.
How to cite: Frampton, A. and Stock, B.: Quantification of uncertainty for reactive solute transport in discrete fracture network models for crystalline rock with application to subsurface repositories for spent nuclear fuel, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12770, https://doi.org/10.5194/egusphere-egu25-12770, 2025.
Uranium is a redox-sensitive element in the environment that interacts with numerous compounds in groundwater. Understanding uranium behavior at the groundwater–solid interface is crucial for evaluating remediation efforts, both in restoring pre-mining conditions and assessing potential anthropogenic impacts. This study aims to identify key reactions that regulate uranium concentrations in groundwater, providing plausible limits for natural uranium levels. Investigation focus on the 4th aquifer (depth of app. 200 m), mainly sandstone, located at a former uranium mining site in Königstein (Germany).
Samples from drill surrounding the deposit were analyzed though shaking tests (batch tests), aqua regia digestions, and organic carbon content determination. Furthermore, minerals were determined using thin section microscopy, and element distributions were visualised using micro-X-Ray fluorescence analysis (µXRF).
A total of 25 g of air-dry, crushed sandstone was weighed into 50 ml centrifuge tubes which were then filled with sampled groundwater from the 4th aquifer and shaken for 27 days. Element concentration, phosphate content, and carbonate hardness were subsequently analyzed using ICP-OES, ICP-MS, ion chromatography, photometry and titration methods. Batch tests and acid digestion were performed in duplicates.
The uranium concentration in the aerobic oxygenated zone in the 4th aquifer was found to be between 1.1 and 31.9 µg/l. The solid concentration of uranium in the sandstones was between 0.1 and 41 ppm. Based on the experiments most important factors associated with high uranium concentrations in the aquifer were the amount of uranium in the bedrock, the redox potential, the pH, the carbonate hardness and the dissolved reactive phosphate content. Oversaturation of some ternary uranyl phosphate minerals determined using PHREEQC and the PSI thermodynamic database (2020), could explain a limitation of the uranium concentration in solution, due to mineral precipitation. However, these minerals have not yet been identified using previous analytical methods. Further, surface complexation was not yet implemented in the modeling approaches.
With regard to the pre-mining state of uranium levels in groundwater and the influence of anthropogenic changes, solution and precipitation reactions of ternary uranyl phosphates and complexation reactions with ternary uranyl carbonates should be considered.
How to cite: Schramm, S., Schiperski, F., and Scheytt, T.: Natural uranium-limiting processes in the groundwater of the former uranium mine Königstein (Germany), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11603, https://doi.org/10.5194/egusphere-egu25-11603, 2025.
Safe and effective disposal of Used Nuclear Fuel (UNF) within a Deep Geological Repository (DGR) must isolate and contain UNF from the biosphere for ~1 Ma. This time period is long enough for several glacial cycles to elapse; it is therefore important to understand how glaciation-related processes such as erosion and subsurface fluid infiltration may impact a DGR. Studying the history of uranium (U) minerals from U deposits, which have been impacted by Pleistocene glaciation, provides a natural analogue to investigate the potential impacts of glaciation on a DGR over Ka-Ma timescales.
Uranium deposits in the Kiggavik region, Nunavut, Canada occur from surface to a depth of ~500m (comparable to depths of proposed DGRs) and have been impacted by multiple post-depositional fluid events and glacial cycles. Uranium minerals comprising uraninite, coffinite, brannerite, and U-Th-Zr silicates are hosted by illite (clay) and hematite altered metasedimentary and granitic rocks. Most U minerals (U1+U2) yield ~1.55-0.3 Ga U-Pb ages indicating they have remained in-situ since before the emergence of dinosaurs despite experiencing multiple fluid infiltration events.
A smaller subset of U minerals (U3) shows stronger evidence of remobilization. U3 minerals are concentrated along redox fronts developed between geothite-bearing oxidized and bleached (clay-dominated) host rocks. These redox boundaries occur within ~5 cm of U1/U2 minerals, and are strongly associated with open fractures and porous veins. U3 minerals have 235U/207Pb ages of >0.6-65 Ma, providing minimum ages of complete recrystallization and potential large-scale radionuclide release.
Uranium-thorium disequilibrium geochronology indicates widespread leaching of soluble decay-chain isotopes, corresponding to smaller-scale release of radionuclides. This has occurred sporadically between 34-494 Ka, with major episodes correlating with periods of rapid climate change during glaciation. Oxygen and hydrogen stable isotopic values of Illite associated with U3 indicate isotopic exchange with high-latitude meteoric fluids (i.e. snow/glacial melt).
The history of U mobility in the Kiggavik region indicates oxidized glacial-derived fluids may infiltrate ≥500m into the subsurface along open fractures and mobilize radionuclides. This mobility occurs cumulatively over multiple glacial cycles and corresponds with ages of climate-induced perturbations to overlying ice sheets. Although longer distance transport from the system cannot be ruled out, the proximity of U3 to U1/U2 mineralization suggests overall transport distances are short (several cm), and geochronology indicates transport timescales are long (10’s-100’s Ka). Interactions with minerals present in both metasedimentary and granitic host rocks such as illite clay, U-oxides, and Ti-oxides have effectively restricted radionuclide mobility to rates millions of times slower than glacial movement over timescales comparable to human evolution.
How to cite: Burron, I., Fayek, M., and Brown, J.: Glaciation-induced radionuclide mobility from the Kiggavik uranium deposits: natural analogues for geological disposal of nuclear waste, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3355, https://doi.org/10.5194/egusphere-egu25-3355, 2025.
Large volumes of glacial meltwater drain along the interface between the ice sheet and its bed, thereby influencing glacier dynamics. It is known from the geological record and modern glacial systems that channelized subglacial meltwater discharge generates high erosion rates, leading to the formation of overdeepenings and tunnel valleys, some over 500 m deep. It is therefore essential to constrain the depth of potential subglacial erosion under future ice sheets when searching for locations of high-level radioactive waste repository sites.
The aim of this project is to quantify the meltwater-driven erosion under the past ice sheets in northern Germany and evaluate the erosion potential during future glaciations. To achieve this goal, we develop a next-generation dynamic numerical model simulating subglacial meltwater erosion on soft beds. In the first step, we built subsurface reservoir models at different scales and resolutions to examine the impact of model resolution on the subsequent erosion modelling. The 3D subsurface model approximately covers the area of the Northwest German Basin (40,000 km²), has a depth of 2000 m, and comprises lithostratigraphical units from the Permian Zechstein to the Pleistocene. The basin fill has a complex structure due to salt tectonics, and the main challenge was to generalize the complex lateral and vertical lithofacies/hydrofacies relationships.
Two large-scale, low-resolution subsurface reservoir models were constructed. The first model does not include Quaternary deposits. This model was created to simulate the formation of Middle Pleistocene tunnel valleys and compare/validate the results with the Pleistocene record of the Northwest German Basin. The second large-scale subsurface model includes the Quaternary deposits and will be used to simulate subglacial erosion during future glaciations. A smaller high-resolution subsurface model, covering an area of about 2000 km², will then be used to test the effects of model size and model resolution on the simulation of subglacial erosion.
How to cite: Fälber, R., Jungdal-Olesen, G., Pedersen, V. K., Damsgaard, A., Piotrowski, J. A., Brandes, C., and Winsemann, J.: Construction of hydrostratigraphic subsurface models of the Northwest German Basin: input for numerical simulation of subglacial erosion during past and future glaciations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1788, https://doi.org/10.5194/egusphere-egu25-1788, 2025.
The Low Level Waste Repository in West Cumbria disposes of low-level radioactive waste within the UK. The site requires, as part of their environmental safety case, a geological model to support hydrogeological modelling and future development of the site. Primarily the geological model must represent the variation in the sub-surface that is applicable to the groundwater, and therefore potential contaminant transport. Of particular importance are the properties and extents of the material, grouped into stratigraphic units. Recent work has been undertaken to produce an updated geological model to support the environmental safety case submission in 2026. This work included a 3D digital geological model that utilises the benefits of the volume of data collected, both legacy and more recent additional data collection, and software developments since the previous safety case submission. Boreholes, multiple geophysical datasets (including reflection, refraction, ERT and passive methods), geomorphology and sections are all interpreted within a stratigraphic framework for geological modelling. The stratigraphy applied groups lithologies of similar hydrogeological parameters, with consideration of the depositional and deformational processes, glacial events and land systems to support prediction of extents and thickness variation. Consideration of these processes supports explicit control of the digital geological model away from available observational data. The work also addresses ongoing debate surrounding the depositional environment of the near surface Quaternary deposits, identified in this work as an Ice Dammed Lake sequence limited in extent to a couple of kilometres of the repository site. The talk will discuss the recent work in more detail, providing a brief overview of the history of the site, data collected, model development, interpretations, and key findings.
How to cite: Pavey, A., Shevelan, J., and Coleman, C.: Geological modelling of Quaternary deposits in West Cumbria, UK to support Low Level Waste Repository environmental safety case., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19438, https://doi.org/10.5194/egusphere-egu25-19438, 2025.
The Low Level Waste Repository (LLWR) is the UK’s central facility for the disposal of low-level radioactive waste. The facility is required periodically to submit a safety case to the UK Environment Agency, demonstrating the continued safety of its operations for both people and the environment. Key to this is the development of a 3D hydrogeological model of the site, together with a suitable treatment of uncertainty. A baseline model, built in ConnectFlow®, conceptualises the site as a collection of distinct lithofacies units and uses observed groundwater head data to constrain the bulk permeabilities of these units. However, to quantify the effects of potential heterogeneity on flow variability, it is necessary to develop a geostatistical model for the formation process. Near-surface deposits in the region are the result of a complex Quaternary history marked by multiple glacial cycles, and recent work has interpreted the primary constituents of the regional aquifer as outwash deposits which formed following the last glacial maximum. Such deposits form as high-energy meltwaters flow from a retreating ice margin, and modern-day analogues show braided channel structures exhibiting increased channel density and decreased flow velocity with increasing distance from the glacial front. This results in a complex stratigraphy of interbedded sands and gravels. The standard Gaussian method for treating heterogeneity, whereby permeabilities are correlated as a function of distance, provides an important model, however it is unlikely to produce fluvial features such as channels which may provide preferential pathways for flow. To simulate a fluvial geology, we use the open-source AlluvSim package with stochastic parameters selected to represent a layering of braided channels and map the outputs into ConnectFlow. We generate thousands of probabilistic simulations across multiple geostatistical models and use observed groundwater head data to condition the resulting estimates for flow variability. The talk will overview these aspects in more detail, and will outline some challenges involved in heterogeneous modelling and uncertainty quantification in this setting.
How to cite: Paxton, A., Singh, T., and Woollard, H.: Modelling heterogeneity of an outwash plain: a case study at the UK LLWR, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2482, https://doi.org/10.5194/egusphere-egu25-2482, 2025.
The safe and effective deep geological disposal of nuclear waste depends on advanced technologies to monitor the degradation of radioactive waste packages. However, long-term monitoring of such facilities presents significant challenges, including restricted access to waste packages and the need to minimize intrusive equipment such as sensors that must pass through protective containment barriers.
This study explores the feasibility and benefits of using high spatial resolution Distributed Optical Fiber Strain Sensing (DOFSS) for the remote detection of package strain evolution and cracks detection, both key indicators of radioactive waste package degradation. DOFSS, widely used in civil engineering for structural health monitoring applications, has been adapted with a novel approach: the integration of optical fiber sensing cables within the concrete walls surrounding nuclear waste packages. This is achieved using 3D additive-printed support structures, which ensure precise installation positioning of the cable and enable high-performance, direct strain measurements. Laboratory experiments simulated package degradation were carried out on reduced scale samples through applying external mechanical force, sulfate attack tests, and CO₂ injection tests. The results demonstrate that DOFSS can effectively map concrete strain and track the evolution of the package shape over time. Furthermore, it can detect concrete cracking, with data analysis providing precise information on the location and width of created fractures.
DOFSS enables the monitoring of both localized and abrupt disruptions such as cracking, and diffuse effects caused by swelling, loading or temperature changes. These findings highlight DOFSS as a promising and effective method for the remote monitoring of radioactive waste package degradation.
How to cite: Simon, N., Aubert, N., Pouya, J., Dick, P., and Khadour, A.: Advanced Distributed Optical Fiber Strain Sensing for Monitoring Radioactive Waste Package Degradation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17350, https://doi.org/10.5194/egusphere-egu25-17350, 2025.
The degradation of organic substances and metal components is expected to generate gases in radioactive waste repositories. Therefore, gas-permeable plugs and seals concepts have been developed to manage gas pressure development while ensuring the containment of radionuclides and other non-radioactive contaminants.
The Gas permeable Seal Test (GAST) is an international project aimed at testing the feasibility and functionality of a gas-permeable seal under realistic scale and boundary conditions. The seal is made of a mixture of 80/20% sand/bentonite mixture and emplaced in the Grimsel Test Site (Switzerland).
After a decade of progressive seal saturation and pressurisation with water, gas flow tests were carried out between May 2022 and August 2023. Noble gases (i.e. He, Ar, Xe) were used as tracers of gas transport through the seal section of the experiment.
Consolidated interpretation of the results indicate that the gas path developed quickly through the seal (end-to-end-flow). Furthermore, the presence of the injected gas tracers both at the extraction point and in gas samples taken from inside the emplaced sand/bentonite layers demonstrated the existence of a gas phase within the seal. Finally, the absence of measurable gas leaks within the tunnel further confirmed the overall functionality of the seal system.
Acknowledgements
Grimsel Test Site Staff for support onsite
Solexperts for hardware components, support in field and with measurements
Entracers and Hydroisotop for onsite and offsite gas analyses
How to cite: Stopelli, E., Spillmann, T., Lanyon, B., de la Vaissière, R., Talandier, J., Chen, J., Cooke, A., Norris, S., Vomvoris, S., and Gaus, I.: GAST: Gas tests at the GAs permeable Seal Test – Successes and lessons learned (Grimsel Test Site, CH), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8193, https://doi.org/10.5194/egusphere-egu25-8193, 2025.
Opalinus Clay is chosen as host rock for the deep geological disposal of nuclear waste in Switzerland and is also being considered in Germany. The underlying Staffelegg Formation comprises potentially water-bearing layers. To investigate the hydrogeological setting of these two formations at the Swiss Rock Laboratory in Mont Terri two bore holes have been drilled in the framework of the HS-Experiment (Hydrogeological Survey). The 58 m long BHS-1 starts in the Lower Shaly facies of the Opalinus Clay, crossects the entire Staffelegg formation and ends in the Triassic Klettgau formation. The shorter BHS-2 provides additional data from the carbonate-rich sandy facies and Lower Shaly facies of the Opalinus Clay. The presented data publication provides geochemical, mineralogical and petrophysical parameters of rocks, pore water and (dissolved) gases from these two drillings [1].
[1] Bonitz, Marie et al. (2024): Hydrogeological characterisation of a Lower Jurassic rock unit at the Mont Terri - I: Data of the Opalinus Clay and the Staffelegg Formation. GFZ Data Services. https://doi.org/10.5880/GFZ.3.4.2024.001
How to cite: Bonitz, M., Blessing, M., Eichinger, F., Fernández, A. M., Flehoc, C., Harrington, J., Hostettler, B., Jaeggi, D., Kampman, N., Kemp, S., Kühn, M., Lahfid, A., León, F. J., Lerouge, C., Maubec, N., Niedermann, S., Nieto, P., Ostertag-Henning, C., Regard, V., and Schleicher, A. M. and the HS-A Team: Hydrogeological characterisation of a Lower Jurassic rock unit at the Mont Terri – a new petrophysical, mineralogical and geochemical data set of the Opalinus Clay and the Staffelegg Formation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9163, https://doi.org/10.5194/egusphere-egu25-9163, 2025.
In this contribution the influence of spatial variability on uranium diffusion in clay is investigated by means of 2D reactive transport simulations at the host rock scale. The Opalinus Clay at the Mont Terri underground laboratory displays three main facies: a sandy and carbonate-rich facies, with porosity ranging from 6% to 14%, and a shaly facies with porosity from 10% to 25%. Geostatistical unconditional simulations of porosity were generated with different variogram parameters (correlation length, anisotropy of the variogram, anisotropy ratio) matching the available ranges for each facies, assuming a spherical variogram. The generated gaussian porosity was than employed to compute the tortuosity and hence a spatially variable effective diffusion coefficient. Reactive transport simulations up to 106 years were performed with the POET code using PHREEQC as geochemical engine, considering cation exchange and surface complexation as retention mechanisms, assuming otherwise chemically homogeneous medium. The considered simulation grid is a square with side of 50 m discretized in 100x100 elements and imposing constant boundary condition for uranium concentration at 10-6 molal along one whole side.
Ten independent geostatistical simulations of porosity were generated for a spherical isotropic semivariogram with correlation lengths of 5, 10, and 20 meters. We also considered anisotropic cases, with main axis of anisotropy parallel and orthogonal to the diffusion direction respectively and anisotropy ratios (ratio between the maximum and minimum correlation lengths) of 4 and 10, with a fixed range of 20 m.
The average migration length of around 22.3 meters after 1 million years is very similar to the one obtained for the spatially homogeneous, reference case. In the isotropic case, larger correlation lengths cause a more relevant spreading of the uranium profiles after 1 million years, achieving a maximum migration length of 23.66 meters with a correlation length of 20 m for the sandy facies, which is around 6 % longer than the homogeneous case. This can be taken as a rough estimate of the uncertainty of the maximum migration length due to spatial variability. Anisotropy of the variogram does not result in a significant difference in terms of covered distance.
These preliminary results highlight moderate effects of spatial variability, which is however largely unknown und needs to be estimated for each possible site. In future work more realistic geometries and facies alternance as well as spatial variability of the mineral fractions and hence the chemical retention potential of the formation will be considered.
How to cite: Fabbri, M., Hennig, T., Kühn, M., and De Lucia, M.: Effect of Spatial Variability on uranium diffusion in the three facies of the Opalinus Clay at Mont Terri, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20364, https://doi.org/10.5194/egusphere-egu25-20364, 2025.
Representative preliminary safety analyses in the site selection procedure for high-level radioactive waste repositories or the subsequent safety assessments require the analysis of the transport regime of radionuclides in e.g. the geological and geotechnical barrier. This can be done (i) using sophisticated, but time consuming numerical models on unstructured grids representing complex geological systems or (ii) using more efficient simple models such as analytical, process- or grid-simplified numerical models that are e.g. calculating in a lower-dimensional space, or simple equations based on e.g. dimensionless numbers.
The present study employs an analytical model of the solute transport equation with linear sorption and decay and a numerical simulator in order to reproduce physical through-diffusion experiments. The same set of transport parameters are used to predict temporal and spatial scales of radionuclide diffusion in low-permeability porous media. The results are then used to develop surrogate models for the estimation of timescales and maximum breakthrough distances of radionuclides. We demonstrate that an expression based on the 2nd Damköhler number can be used for the calculation of the maximum breakthrough distance for the non-sorbing radionuclides. This expression is calculated using the effective diffusion coefficient, the diffusion-effective porosity, the physical half-life and a dimensionless number. Further, we show that the timescale for reaching the maximum breakthrough distance can be roughly estimated and is two orders of magnitude larger than the physical half-life.
How to cite: Peche, A., Tran, T. V., Hennig, T., Kumar, V., Kringel, R., and Altfelder, S.: Surrogate models for the estimation of spatial and temporal scales of the maximum breakthrough of radionuclides in low-permeability porous media, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9265, https://doi.org/10.5194/egusphere-egu25-9265, 2025.
Reactive transport models are used to simulate the migration behaviour of radionuclides at potential disposal sites for highly radioactive waste. Previous studies for uranium in the Opalinus Clay at Mont Terri (Switzerland) show that hydrogeochemical differences, for example in ionic strength, between the host rock and adjacent aquifers lead to gradients in pore water geochemistry profiles across the entire system. This in turn decreases uranium sorption and increases migration distances [1]. Safety assessments must therefore evaluate potential variations in the boundary conditions on a site-specific basis. We simulated scenarios using the geochemical code PHREEQC applying hydrogeochemical extremes to the surrounding aquifers to assess uranium migration sensitivity. This study demonstrates how variations in boundary conditions affect uranium transport through the Opalinus Clay over a period of one million years.
We quantified the effects of potential brine and seawater intrusion, freshwater enrichment, and acidification, for example, by significantly altering the ionic strength (from 0 to 5 mol/L) and pH (from 3 to 11) at the model boundaries. The results are compared to a reference case based on current conditions at Mont Terri. Simulated uranium migration distances in the tested scenarios only differ by a few metres, depending on the concentration of aqueous ternary uranium complexes formed. This is determined by the alkalinity and the availability of Ca and Mg for complex formation, whereby less or more uranium can be sorbed [2]. In conclusion, even in the extreme cases investigated, the hydrogeochemical disturbances are buffered by the minerals in the system so that uranium migration is not significantly affected.
[1] Hennig, T. and Kühn, M. (2021): Potential uranium migration within the geochemical gradient of the Opalinus Clay system at the Mont Terri. Minerals 11 (10), 1087. DOI: 10.3390/min11101087
[2] Hennig, T., Stockmann, M. and Kühn, M. (2020): Simulation of diffusive uranium transport and sorption processes in the Opalinus Clay. Applied Geochemistry 123, 104777. DOI: 10.1016/j.apgeochem.2020.104777
How to cite: Schöne, T. and Hennig, T.: Hydrogeochemical impacts on uranium migration in the Opalinus Clay at Mont Terri, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16608, https://doi.org/10.5194/egusphere-egu25-16608, 2025.
In the Netherlands, Paleogene clay layers are eligible for the final disposal of radioactive waste. Geoscientific information from these deposits is essential to assess their suitability. An important aspect is the prediction of the geochemical behavior of radionuclides in these clay layers. Such behavior can be studied by laboratory experiments. However, these experiments are relatively short-term compared to the expected migration period of radionuclides if released from the engineered containment radioactive waste and it is very challenging to imitate disposal representative conditions in an above ground laboratory. An additional, alternative approach to predict radionuclide behavior is by studying their natural analogues that are present in the sediment of the host formation.
In this study, Paleogene deposits of two drillings were investigated. Se, U, Th, Cs, and Eu, being natural analogues for radionuclides in radioactive waste, are addressed in more detail. A wide range of analyses, such as XRD, XRF, TGA, SEM, sequential extractions and LA-ICP-MS, were performed to assess the mineralogy and associated trace elements.
The Eocene-Oligocene interval in the first core, located in the province of Zeeland, was glauconite-rich and showed an alternation of clayey and sandy layers. The top 10 m contained carbonates which were absent in the lower part of this interval. Th, Cs, Eu, Se and U correlated with clay minerals. Se and U also correlated with pyrite. The second core (Miocene-Eocene interval), located in South-Holland, also had alternating clay, silt and sand layers. In this core, Th, Cs and Eu correlated with clay minerals while Se and U only correlated to S and P.
How to cite: Hoving, A., Neeft, E., Dieudonné, A.-C., Vardon, P., and Griffioen, J.: Natural analogue study of radionuclides in Paleogene sediments in the Netherlands, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18627, https://doi.org/10.5194/egusphere-egu25-18627, 2025.
A climatic and geological long-term prognosis is relevant in order to predict the future development of a potential site for radioactive waste disposal independently of the repository. This long-term prognosis for a salt diapir in the Subercynian Basin in northern Germany first analyzes the climatic development, geological processes and then, in particular, the interactions between climate and geology. Geological processes are divided into climatically influenced and non-climatically influenced. Following the Doctrine of Uniformity and the uncertainties involved in predicting future climate developments, one can assume that there will be another ice age at the site in the next 100,000 years with the corresponding processes, i.e. permafrost formation, inland glaciation, formation of subglacial channels and stress changes due to ice loading. We consider these processes in the cold period scenario as well as the retreat of the ice sheet after 110,000 years and associated processes. Isostatic rebound of the lithosphere, which can lead to earthquakes and the reactivation of faults in the cap rock and overburden as well as in the subsalinar. The anthropogenic influence is playing an increasingly important role in the development of the climate (Talento & Ganopolski 2021). Therefore we consider a warm period scenario where the actually expected cold period will weaken considerably or even will not taking place in the next 100,000 years. The consideration of a cold and a warm period scenario thus fully covers two opposing developments. Depending on the intensity of arid or wet conditions in the warm period scenario, groundwater recharge is affected. Changed precipitation affects the processes of sedimentation and erosion.
Overall, processes associated with a glacial period have a greater impact on the site conditions than warm-period processes. The climatically influenced geological processes are of greater importance than the non-climatically influenced processes because the site is located in a tectonically quiet area. We assume that a succession of cold periods and warm periods will characterize the climate for the next 1 million years, assuming that the anthropogenic effect on the climate will no longer play a role on the long term. Based on this, the normal sequence of cold and warm periods will return and up to ten cold periods will affect the location. In particular, the associated inland glaciation, glacial erosion and subglacial channels can reshape the overburden significantly.
Talento, S. & Ganopolski, A. (2021): Reduced-complexity model for the impact of anthropogenic CO₂ emissions on future glacial cycles. Earth Syst. Dynam., 12: 1275 – 1293.
How to cite: Schütz, F. and Bebiolka, A.: Climatic and geological long-term prognosis for a salt structure in northern Germany, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6828, https://doi.org/10.5194/egusphere-egu25-6828, 2025.
A three dimensional (3D) geological model is developed for nuclear deep geological repositories (DGR) in crystalline rocks in Korea. The research includes integration of site descriptive modelling (SDM) with the 3D geological model. Thus the research will serve as a platform for various disciplines related to disposal. This is part of 9 year research project of safety case development for DGR in Korea for high-level nuclear waste.
We have adopted SKUA-GOCAD for building the 3D geological model, since the software provides excellent interpolation function for constructing geological boundaries. A testbed with the geology of crystalline rocks was selected for the 3D modelling. The geology consists of Precambrian basement, Permian to Cretaceous sedimentary cover rocks and Mesozoic granitoids. Since the geologic boundaries of the basement and the granitoids are irregular in shape, the structural and stratigraphic model in SKUA-GOCAD is difficult to apply for the formation of geologic boundaries. We have adopted point cloud methods to form the irregular geologic boundaries. The primary data for our 3D geologic model is the geologic boundaries on maps and cross sections. Then the geologic boundaries are adjusted with borehole data and geophysical data.
A relational database for SDM data with coordinate information is under development. These data share the database primary key with geologic domains and subdomains. The data table allows handling multiple spatial types (point, surface and volume) and physical quantities (scalar, vector, tensor). A visualization tool is also under development. The tool displays SDM data along with geologic elements. Drill core trajectories and the results of geophysical survey can be displayed in linear, surficial and volumetric form.
How to cite: Guk, M., Kim, S.-K., Lee, M., Kim, B., Choi, S.-Y., and Park, J.-H.: Development of three dimensional geologic model for nuclear deep geologic repositories in crystalline rocks in Korea , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7771, https://doi.org/10.5194/egusphere-egu25-7771, 2025.
Claystones are considered potential host-rocks for the long-term disposal of high-level radioactive waste (HLW). Their favorable barrier properties such as low permeability, self-sealing efficiency, potential for plastic deformation, and radionuclide sorption capacity mitigate the risk of radionuclide migration to the environment. However, these properties strongly depend on the burial history, defining effective stress and temperature conditions. This dependence complicates data transferability between sites and underscores the need to account for site-specific burial histories in assessing the formation`s barrier function.
The Maturity-project seeks to enhance our understanding on how variations in burial history systematically alter the barrier properties of claystone formations. The project focuses on a Lower Jurassic (Pliensbachian) claystone formation accessed through several shallow wells (~100 m depth) within the Hils and Sack Syncline, Lower Saxony (Germany). Previous studies from this region indicate a strong Southeast-Northwest directed burial gradient (from 1,300 m to 3,600 m) over a relatively short lateral distance (~50 km) (Littke et al., 1991; Gaus et al., 2022; Castro-Vera et al., 2024). A comprehensive research campaign aims to unravel the formations burial history and link it to alterations in its barrier properties. Moreover, a combination of in-situ and laboratory-based methods tackles open questions in the scale dependency of investigated properties.
In this contribution, we report on the general project proceedings and present initial results from various project steps. These initial results confirm a gradual increase in thermal maturity, documented by several parameters such as vitrinite reflectance and Tmax from Rock-Eval pyrolysis data. X-ray diffraction (XRD) analysis reveals a mineralogical composition dominated by clay minerals (>50%), with minimal variation across the study area. Bulk densities derived from laboratory and well logging data show an increase with thermal maturity, rising from ~2.3 g/cm³ to ~2.5 g/cm³, while porosities decrease from ~14 % to ~9 %.
References
Castro-Vera, L., Amber, S. Gaus, G., Leu, K., Littke, R. (2024). 3D basin modeling of the Hils Syncline, Germany: reconstruction of burial and thermal history and implications for petrophysical properties of potential Mesozoic shale host rocks for nuclear waste storage. International Journal of Earth Sciences, Volume 113, pages 2131-2162.
Gaus, G., Hoyer, E.M., Seemann, T., Fink, R., Amann, F., Littke, R. (2022). Laboratory investigation of permeability, pore space and unconfined compressive strength of uplifted Jurassic mudstones: The role of burial depth and thermal maturation. Zeitschrift der Deutschen Gesellschaft für Geowissenschaften, 173 (3), 469-489
Littke, R., Leythaeuser, D., Rullkötter, J., & Baker, D. R. (1991). Keys to the depositional history of the Posidonia Shale (Toarcian) in the Hils Syncline, northern Germany. Geological Society, London, Special Publications, 58(1), 311–333.
How to cite: Burchartz, R., Jalali, M., Winhausen, L., Gaus, G., Seemann, T., Littke, R., and Amann, F.: Interactions Between Barrier Properties and Burial History of a Lower Jurassic Claystone Formation – Insights, Proceedings, and Initial Results from the Maturity-Project, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10862, https://doi.org/10.5194/egusphere-egu25-10862, 2025.
Prior to the construction of radioactive waste disposal facilities, a groundwater flow model was proposed to guide site monitoring, and it is updated based on new data collected during construction. Disposal facilities located in crystalline bedrock in coastal regions are particularly susceptible to seawater intrusion, which can lead to the formation of fracture zones and increased permeability. Therefore, the fluid movement may deviate from the predictions of the initial groundwater flow model.
Conventional site assessment methods, such as borehole-based groundwater sampling, provide high accuracy but are limited in delivering comprehensive spatial interpretations. To address these spatial limitations, a long-range ground-penetrating radar (GPR) system equipped with real-time sampling was applied. This advanced GPR system enables deeper penetration, facilitating the evaluation of seawater intrusion zones and associated hydrogeological characteristics. The GPR survey identified seawater intrusion zones that showed a strong correlation with the electrical conductivity data of groundwater samples. The GPR results indicated that the groundwater flow model had overestimated the extent of seawater intrusion, necessitating modifications to improve its accuracy. In conclusion, GPR has proven to be a valuable tool for accurately assessing seawater intrusion zones and validating groundwater flow models. Furthermore, the GPR survey highlights its suitability not only for seawater intrusion assessments but also for long-term site monitoring in disposal facility settings.
Acknowledgement
This study was supported by the Nuclear Safety and Security Commission (No. 1805020-0421-CG100).
How to cite: Yu, H., Yu, Y., and Hyun, S. G.: The applicability of long-range GPR for monitoring seawater intrusion and validating groundwater flow models near LILW disposal facility, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14635, https://doi.org/10.5194/egusphere-egu25-14635, 2025.
Understanding the geological characteristics of potential sites is essential for ensuring the long-term safety of high-level radioactive waste repositories. In Korea, studies on geological disposal have been limited compared to other countries, making it crucial to develop site-specific models to assess the long-term stability of the geosphere. This study focuses on the geological characterization and conceptual modeling of a potential test site for a deep geological repository.
The core objective of our research is to construct a site descriptive model that comprehensively characterizes the geological features influencing the stability of a repository. Our approach involves detailed analyses of lithological distributions, fault damage zones, and deformation structures. By integrating surface geological surveys, borehole data, and geophysical measurements, we aim to create a reliable framework for assessing subsurface conditions. Particular attention is given to defining buffer zones through respect distance criteria from major faults, ensuring that disposal facilities are placed in geologically stable areas. This conceptual framework serves as the foundation for further geomechanical and hydrogeological assessments.
To develop this site descriptive model, we first conducted 3D geological modeling as a key preliminary study. Our methodology included linear structure analysis, geological map interpretation, and surface geological investigations. These steps were crucial in identifying and visualizing both regional and local geological features in three dimensions. The resulting 3D geological model provides a detailed visualization of the subsurface environment, offering insights critical for repository site selection and design.
Our study emphasizes the importance of detailed geological investigations and advanced modeling techniques in the assessment of potential radioactive waste repository sites. By establishing a comprehensive framework for characterizing subsurface conditions, we contribute to Korea’s long-term radioactive waste management strategies, aligning with international best practices for ensuring the geosphere’s long-term stability.
How to cite: Park, J. Y. and Jin, K.: Preliminary Geological Characterization and 3D Modeling for a Radioactive Waste Repository in Korea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14552, https://doi.org/10.5194/egusphere-egu25-14552, 2025.
Argillaceous rocks, such as Opalinus Clay, are considered potential hosts for high-level radioactive waste repositories due to their low permeability and ability to retard radionuclide migration. In these formations, diffusion is the primary transport mechanism for radionuclides. Previous laboratory studies have indicated that effective diffusion coefficients for non- or weakly-sorbing radionuclides increase exponentially with temperature between 0 and 70°C, suggesting that higher temperatures could enhance diffusion rates. However, the impact of temperature on radionuclide diffusion under in-situ conditions remains underexplored.
To address this gap, the DR-C experiment was initiated in 2019 at the Mont Terri Underground Research Laboratory (URL) in Switzerland. This in-situ study aims to investigate radionuclide diffusion in Opalinus Clay under a controlled thermal gradient. The experimental setup includes two 5-meter-long boreholes: one equipped with a heating module maintaining an 80°C temperature at the clay interface, and a control borehole at ambient temperature. A cocktail of radioactive tracers, including HTO, 129I-, 22Na+, 137Cs+, 60Co2+, and 133Ba2+, will be injected to monitor diffusion behavior. The injection is scheduled to commence at the beginning of 2025 and will run for one year. Upon completion, overcoring and subsequent chemical analyses will determine diffusion profiles, enhancing understanding of temperature effects on radionuclide migration in clay-rich host rocks.
This research is crucial for assessing the long-term safety of geological disposal facilities for radioactive waste, particularly concerning "worst-case scenarios" involving potential canister failure during the thermal phase. Insights gained from the DR-C experiment will inform safety assessments and contribute to public confidence in geological disposal solutions.
How to cite: Montoya, V., Pochet, G., Jaeggi, D., Heberling, F., Graupner, B., Bower, W., Deissmann, G., Agnel, M., Magri, F., Vinsot, A., Borkel, C., Dietl, C., Bernier, F., Barroo, C., Surkova, M., Yang, Y., and Ratnayake, S.: In-situ radionuclides diffusion experiment in a thermal gradient in the sandy facies of Opalinus Clay , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15435, https://doi.org/10.5194/egusphere-egu25-15435, 2025.
