Smectite-rich natural clays, usually referred to as bentonite, are used as backfill material in disposal concepts for highly-radioactive wastes. The main component of bentonite is montmorillonite, characterised by a high cation exchange capacity resulting from isomorphous substitution and swelling due to the incorporation of water molecules between the stacked clay platelets, the interlayer. These properties render bentonite an ideally suited barrier material for cationic radionuclides. Caesium is such a radionuclide that is relevant in the context of nuclear waste disposal since it is highly soluble and can be incorporated in organisms.

Migration of caesium in MX-80 bentonite (Na-montmorillonite) was investigated for different dry bulk densities (1.3, 1.6 and 1.9 g/cm³) in long-term through-diffusion experiments running for up to 600 days. Diffusion experiments of tritiated water (HTO, non-sorbing) provided the transport accessible porosities. Batch sorption experiments at varying caesium concentrations should test the transferability between dispersed and compacted systems by means of the distribution coefficient K_d (m³/kg). The synthetic pore water compositions were calculated as a function of the dry bulk density. Caesium and HTO are expected to migrate via molecular diffusion through the compacted bentonite sample. Therefore, a one-dimensional numerical model was applied to determine the transport parameters, effective diffusion coefficient D_e (m²/s) and rock capacity factor α (-), from the temporal evolution of the diffusive flux and the accumulated activity.

In the dispersed systems, measured K_d values increase with decreasing caesium concentration and dry bulk density. The linear sorption isotherm indicates that sorption mainly occurred via cation exchange, predominantly with ions from the interlayer. Accordingly, the higher the cation concentration in the contacting pore water, less caesium is exchanged due to competing effects. At low concentrations (<10^-6 mol/L), however, the measured sorbed caesium concentrations do not match exactly with the isotherm. This can be attributed to impurities in form of illite-smectite mixed layers. With increasing caesium concentration, measured K_d decreases due to saturation of the high affinity sites at the illite surfaces. K_d values differ by a factor of up to ten between batch and compacted systems. With compaction, the amount of water in the interlayer decreases. This, in turn, affects the space and ability to hydrate sodium so that it is exchanged with the less hydrated caesium from the bulk solution. Accordingly, K_d values are not transferable due to thermodynamic changes of the exchange process with compaction. Consequently, caesium sorption depends on the ionic strength, the compaction and on the caesium concentration.

With increasing bulk density, the apparent diffusion coefficients of caesium decrease by a factor of ten. In general, diffusion of caesium was twice as high as that of HTO. This can be attributed to an additional transport pathway in the interlayer, which is not accessible for neutral species. For the transient phase, there was an offset between simulations and experiments, what might be explained by higher temperatures at the beginning of the experiments. Simulated and experimental steady-state phases are in line. With compaction, sorption increases, and thus diffusivity decreases.

How to cite: Hennig, T., Grossmann, S., and Brendler, V.: Diffusion and sorption of caesium depend on the dry bulk density of bentonite, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3991, https://doi.org/10.5194/egusphere-egu24-3991, 2024.

The release and migration of radionuclides towards the biosphere from underground waste repositories may pose a threat to the environment and humans. Therefore, the understanding of transport processes of radionuclides in the subsurface is crucial in order to predict the arrival in the near-surface environment for effectively mitigating and managing associated risk. Processes controlling radionuclide transport may be advection, dispersion, diffusion, sorption and decay, among others.

In the context and typical locations of existing waste repositories and test sites, diffusion-dominated transport occurs. Typically, local permeabilities and flow velocities are so small that advection and dispersion would only become significant at very large timescales. The processes of diffusion, sorption (possibly modulated by pressure and temperature) and decay may often be crucial for a) predicting the gradual release and movement of contaminants from the repository into the surrounding geological formations and b) in the geological barrier itself as part of safety analyses. Therefore, understanding of barrier-specific diffusion processes is essential for assessing long-term containment and designing effective concepts to ensure reliable isolation of radionuclides over a time period long enough for their decay to safe levels and to avoid potential environmental risks associated with the disposal of radioactive waste.

In the present desktop study, we calculate diffusion-dominated transport of specific radionuclides (Tritium, Technetium, Neptunium a.o.) for various host rocks (tuff a.o.). We calculate radionuclide breakthrough curves and timescales using the 1D analytical model by Lapidus & Amundson (1952), extended for decay in Bear (1972). In order to validate transport parameters for specific radionuclides in specific rock, we analytically try to reproduce physical radionuclide diffusion experiments. Subsequently, we use the validated transport parameters to predict timescales of diffusion-dominated transport.

References:

Bear, J., 1972. Dynamics of Fluids in Porous Media. American Elsevier, New York.

Lapidus, L., & Amundson, N. R. (1952). Mathematics of adsorption in beds. VI. The effect of longitudinal diffusion in ion exchange and chromatographic columns. The Journal of Physical Chemistry, 56(8), 984-988.

How to cite: Peche, A., Tran, T. V., and Altfelder, S.: Timescales of diffusion-dominated radionuclide migration – an evaluation of selected existing nuclear waste repository sites and test sites , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15090, https://doi.org/10.5194/egusphere-egu24-15090, 2024.

The secure disposal of nuclear waste is of high societal concern, necessitating the development of deep geological repositories as a reliable solution. A key aspect of repository safety lies in understanding the far field, particularly the host rock, to predict the long-term behavior and migration of radionuclides within the geological environment from the deposit up to the ecosphere. This study addresses the specific challenges associated with crystalline host rocks.

Crystalline host rocks could be on the one hand of granitic composition and texture, but the term is also used for host rocks of metamorphic origin. While inside a large granitic intrusion there is little petrological variation expected, metamorphic rocks or the intrusion rim can exhibit complex structures in terms of structural geology as well as mineral composition, especially along potential fluid migration pathways. Consequently, this leads to a multitude of possible rock-composition/fluid-composition interactions and thus significantly affects the retention potential of radionuclides as opposed to the simplified model of an isotropic, uniform granite. The results of the study will allow to determine which components of the host rock are important to be included in geostatistical models which in turn serve as basis to estimate uncertainties of reactive transport through crystalline rocks.

Our study involves the development of Python code to feed chemical modelling software like PHREEQC or Geochemist’s Workbench^© with varying mineral compositions and chemical conditions of the aqueous phase, following a specific Quasi-Monte-Carlo sampling scheme. The application of compositional data analysis principles is essential to guarantee a meaningful sampling of constraint concentration data, such as mineralogical rock compositions or element concentrations in aqueous phases. Given that compositional data sum to a fixed total, each mineral content becomes a dependent variable in relation to the other contents. Recognizing and accounting for these interdependencies is crucial to ensuring the integrity of the sampling. The chemical modelling software relies on Surface Complexation Models (SCM) for each mineral phase to calculate the distribution coefficient (K_d-value) for the radionuclide (here: uranium) in the respective setting. Furthermore, a global sensitivity analysis is employed to investigate the complex interactions between mineralogical variations and radionuclide behavior. In this study, two techniques are employed namely, High-Dimensional Model Representation (HDMR) and Cumulative Sum of Univariate Nonlinear Regression (CUSUNORO) plots. The application of HDMR allows for a detailed investigation of high-dimensional parameter spaces, while CUSUNORO plots provide a visual representation of cumulative sensitivity effects.

This study presents a complete workflow of modelling how petrological variations in crystalline host rocks, including both granitic and metamorphic compositions, affects radionuclide retention. This approach advances the understanding of nuclear waste disposal and provides valuable tools for assessing the retention potential of radionuclides in diverse geological settings.

How to cite: Pospiech, S., Bok, F., Abdelhafiz, M., Duckstein, A., Plischke, E., and Brendler, V.: Understanding Geological Key Factors for Radionuclide Retention: Insights from Sensitivity Analysis on Varied Crystalline Host Rock Compositions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3471, https://doi.org/10.5194/egusphere-egu24-3471, 2024.

Geochemical background values are commonly used for the authorization of remediation measures. However, in the case of the former Königstein uranium ore mine, located in Saxony (Germany), natural uranium concentrations in groundwater cannot be directly assessed, due to active mining activities. The Königstein uranium ore depositis located within the 4th aquifer and consists of the Oberhäslich Formation and 'Wurm'-Sandstone.

This study aimed at deriving natural uranium content using batch shake test, assuming chemical equilibrium with regard to the speciation of uranium in solution and the binding to the rock matrix. For that purpose, representative subsamples from core material were taken for batch experiments and geochemical analyses. As uranium solubility strongly dependents on the redox state, the pH value, and the hydrochemistry of the target fluid, experiments were performed under various conditions.

The rock samples were analyzed with respect to geochemical and mineralogical compositions, while uranium concentration in fluid samples was measured using inductively coupled plasma - mass spectrometry. Concentrations of major cations were analyzed using cation chromatography techniques. The PHREEQC software was used to analyses the species distribution of uranium under the hydrochemical conditions of the the unaffected inflowing water from the inflow area.

Results show, that the sandstones in the middle of the 4th aquifer consist mainly of SiO₂ with more than 98 wt.-%. Two-layer clay minerals and iron oxides were identified in another sample with fractions more than 50 wt.-% kaolinite. Uranium was found in the anaerobic zone in one rock sample at 1820 ppm and once at only 25.6 ppm.

Equilibrium modeling revealed, that at a pH of 5.5 to 6 and under oxidizing conditions, uranium mainly occurs as UO₂²⁺, UO₂OH⁺, and UO₂CO₃. It can therefore be expected that the uranium species present as cations will likely be adsorbed by the solid matrix. Initial shaking tests showed that combining reduced rock materials with oxidizing water led to excessive uranium fractionation into the fluid phase. Batch tests using deionised water showed uranium concentrations between 2 and 2.5 mg/l, contrasting expectation of natural uranium concentrations in the lower µg/l range.

How to cite: Schramm, S., Schiperski, F., Hübschmann, M., Horna, F., and Scheytt, T.: Geochemical composition of sandstone samples from drill cores of the former uranium ore mine Königstein (Saxony) and the determination of hydrochemical equilibrium concentrations of uranium by means of batch tests, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19338, https://doi.org/10.5194/egusphere-egu24-19338, 2024.

The Boom Clay (BC) in Belgium and the Opalinus Clay (OPA) in Switzerland are studied as potential host rocks for radioactive waste disposal in the frame of national programmes. In the assessment of the long-term natural barrier evolution, redox capacity is an important physical-chemical parameter to consider as it may affect the speciation and migration behavior of the released radionuclides from the waste. In this respect, iron plays an important role in the electron transfer and thus may influence the speciation and transport of many redox sensitive radionuclides. The clay minerals, commonly present in these rocks, contain iron in the octahedral and tetrahedral sheets of their structure (e.g. illite, smectite, mixed layer illite-smectite (I-S) and chlorite). Despite their relatively high abundance in the sedimentary rocks, the electrochemical activity of iron within clay minerals may vary from as high as 100% in the case of pure smectite, through 41% in the mixed 70/30 layer I-S, 10% in illite and only 2% in chlorite. The electrochemical activities of non-clay iron-bearing minerals, is low in the case of pyrite (from 2 to 12%) and zero in the case of siderite.

In order to estimate the redox capacities of BC and OPA, Fe distribution, redox state and electrochemical activity were determined by a combination of quantitative XRD analysis (QXRD), XRF, phenanthroline, ⁵⁷Fe Mössbauer spectroscopy and mediated electrochemical oxidation (MEO) and mediated electrochemical reduction (MER) measurements. In the bulk OPA clay samples, the Fe²⁺ states dominate over Fe³⁺ states, whereas Fe³⁺ states dominate over Fe²⁺ states in the studied bulk BC samples. In both cases, illite, I-S and smectite are the main Fe³⁺ carriers, while chlorite, pyrite and siderite are the main Fe²⁺ carriers. Fe distribution and the valence state are therefore controlled by the quantitative mineralogical composition in both rocks. The results of the MER and MEO measurements indicate that total electron transfer capacity (ETC) varies in the range between 45 and 64 µmol e^-/g bulk OPA and between 160 and 181 µmol e^-/g bulk BC. In most of the studied samples, the electron accepting capacity (EAC) is higher than the electron donating capacity (EDC). The measured EACs positively correlate with increasing Fe³⁺ contents, smectite content and cation exchange capacities. The correlation between measured EDCs and Fe²⁺ contents is less satisfactory, most likely due to overall low Fe²⁺ electrochemical activities reported for pure pyrite, chlorite and siderite. The quantitative mineralogical composition and available electrochemical data of the pure minerals were used to calculate the theoretical ETCs, EDCs and EACs of the studied OPA and BC samples. Despite relatively large uncertainties in the electrochemical activities in the EDC field, the correlation between theoretical and experimental EACs and ETCs is relatively good in both studied cases (R² = 0.83 and 0.76, respectively for EAC and ETC in the case of OPA and R²=0.74 and 0.84 in the case of BC). The proposed mineral electrochemical model should be tested on larger data sets to verify its general applicability to other clay formations.

How to cite: Honty, M., Hoving, A., Frederickx, L., Greneche, J.-M., and Traber, D.: Towards a comprehensive model to estimate redox capacity of clay rocks studied as natural barriers for radioactive waste disposal, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8225, https://doi.org/10.5194/egusphere-egu24-8225, 2024.

Opalinus Clay is the chosen host rock for the deep geological disposal of nuclear waste in Switzerland and is also being considered for this purpose in Germany. For the long-term integrity of the disposal site, temporally and spatially stable geochemical conditions are essential. Adjacent aquifers can induce changes into the system. It is therefore essential to investigate if and how the geochemistry and mineralogy of the sediments is influenced by the hydrogeology. Changes in the Opalinus Clay and the surrounding formations provide information about geochemical processes in the past and thus enable an assessment for the future.

In this context, a 58 m long borehole was drilled at the Swiss Rock Laboratory in Mont Terri. Drilling was conducted from the Opalinus Clay (Toarcian) through the entire Staffelegg Formation (Toarcian-Sinemurian), which contains two water-bearing sections. The groundwater, the members and their transitions were characterised with a variety of analytical methods.

Groundwater was found locally in the Beggingen (Gryphaea Limestone) and Rietheim (Posidonia Shale) members, depending on the presence of pathways in open fractures and with differences in their chemical composition. The groundwater in the Rietheim member is not directly connected to the surface, but seems to be continuously recharged by an adjacent aquifer. At the transition from the Gross Wolf to the Rietheim Member, pyrite increases and many trace elements are enriched. The Fe/Al paleoredox proxy and the enrichment of trace-metals like uranium reveal prevailing anoxic conditions in the sediment and groundwater. This is more indicative for a depositional than a mobilization feature. The influence of the groundwater can therefore be classified as limited due to its reducing conditions at this transition.

The aim is to establish a method for analytically distinguishing between features of deposition, diagenesis, alteration and mobilization. This will allow assessment of the long-term integrity of the Opalinus Clay as a host rock and the surrounding formations. The gained understanding of the hydrogeological influence on the geochemical conditions within a system is to be transferred to other potential repository sites.

How to cite: Schleicher, A., Bonitz, M., Hennig, T., Stammeier, J., Jaeggi, D., and Kühn, M.: Geochemical profiles in the hydrogeological system of the Opalinus Clay, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1317, https://doi.org/10.5194/egusphere-egu24-1317, 2024.

Understanding the movement of radionuclides over time is crucial for assessing the integrity of geological formations as barrier for a radionuclide waste repository. Long-term groundwater potential time series enable the modelling of flow and transport scenarios, which help to predict how radionuclides may migrate from the repository through the overburden into the biosphere, if the overburden as geological barrier should fail. The accuracy of numerical flow and transport models depend on the availability of reliable input data, such that long-term groundwater potential time series help to ensure that numerical flow and transport scenarios accurately represent the complex hydrogeological processes occurring over time.

However, in practice it is very common that, due to financial constraints, vandalism of measurement devices, and other logistical problems result in shorter and/or longer gaps in the ideally continuous groundwater monitoring time series. These gaps can significantly hinder the reliability and completeness of the dataset, making it challenging to perform accurate analyses.

In response to these challenges, we use machine-learning methods with monthly precipitation data from the German meteorological service (DWD), monthly groundwater recharge data generated from the hydrological model RUBINFLUX and continuous groundwater time series from state run monitoring wells as inputs to predict the missing gaps in the groundwater potential time series in the overburden of the radioactive waste repository Morsleben (ERAM).

This approach highlights the importance of continuity in the dataset for further studies, modelling, and safety assessments for radioactive waste repositories. Using machine learning techniques can help to reconstruct the missing data and provide a more comprehensive and continuous dataset for validating and calibrating numerical flow and transport models. 

References:

Bear, J., 1972. Dynamics of Fluids in Porous Media. American Elsevier, New York.

Langkutsch, U., Käbel, H., Margane, A., & Schwamm, G. (1998). Planfeststellungsverfahren zur Stillegung des Endlagers für radioaktive Abfälle Morsleben. 457. 

Peche, A., Kringel, R., Orilski, J., & Skiba, P. (2021). Hydrogeologische Modellbildung des ERA Morsleben. In Zwischenbericht Bundesanstalt für Geowissenschaften und Rohstoffe (BGR) im Auftrag der Bundesgesellschaft für Endlagerung (BGE).

Hölting, B., & Coldewey, W. G. (2013). Hydrogeologie. In Hydrogeologie. Spektrum Akademischer Verlag. https://doi.org/10.1007/978-3-8274-2354-2

Zepp, H., König, C., Kranl, J., Becker, M., Werth, B., & Rathje, M. (2017). Implizite Berechnung der Grundwasserneubildung (RUBINFLUX) im instationären Grundwasserströmungsmodell SPRING. Eine neue Methodik für regionale, räumlich hochaufgelöste Anwendungen. Grundwasser, 22(2), 113–126. 
https://doi.org/10.1007/S00767-017-0354-3

How to cite: Tran, T. V., Peche, A., Brömme, K., Kringel, R., and Altfelder, S.: Machine learning-based prediction of gaps in groundwater time series in the overburden of the Morsleben radioactive waste repository, Germany, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17411, https://doi.org/10.5194/egusphere-egu24-17411, 2024.

Geothermal resources have been emerged as a practical and cost-effective clean energy alternative to fossil fuels in many countries. The study of geothermal water can provide valuable insights for sustainable long-term utilization of geothermal energy and for understanding deep geologic environments for diverse emerging geo-energy technologies. For this study, chemical and isotopic tracers (δ²H, δ¹⁸O, δ¹³C, ¹⁴C) of thermal groundwaters over South Korean peninsula were investigated to evaluate their geochemical characteristics and reservoir parameters (i.e., temperature, recharge and circulation depth of groundwater, and circulation dynamics). Based on geochemical characteristics, geologic setting, and geographical location, South Korean thermal groundwaters were categorized into five groups (Kim et al., 2020): saline, CO₂-rich, high-pH alkaline, sulfate-rich, and diluted freshwater. These distinct hydrochemical characteristics are determined by complex geochemical processes which include calcite dissolution, plagioclase feldspar hydrolysis, CO₂ gas dissolution, cation exchange, precipitation of secondary minerals (clay, calcite, fluorite), gypsum dissolution, and seawater mixing. However, each group tends to show no systematic change among outflowing temperature, hydrochemistry, and stable isotopes over time (i.e., radiocarbon age), which may indicate that thermal waters have reached an equilibrium through deep circulation over very long periods of time (millennial scales). All groups of thermal water originated from deeply circulated meteoric water, and their stable water isotope data (δ²H, δ¹⁸O) show a systematic fractionation pattern depending on the recharge altitude (Choi et al., 2023). Temperatures of geothermal reservoirs at depths were estimated by using chemical geothermometers considering the ambient geology and measured outflow temperatures. The results obtained from K-Mg, Li-Mg, Na-K-Ca, Na-K, and Si geothermometers ranged widely from a minimum of 28°C to a maximum of 207°C, with an average of 61 to 148°C. The estimated depths of groundwater circulation to form South Korean thermal waters fall between about 1.0 and 3.3 km, if calculate from the calculated average reservoir temperature and geothermal gradient data in each location. The estimated circulation depths tend to increase with decreasing stable water isotope data (δ²H, δ¹⁸O) and increasing radiocarbon (¹⁴C) ages. This study, conducted across South Korea, provides important information about the origin and evolution of deep thermal water, which may be helpful to the efficient and sustainable management of geothermal resources and to the selection of suitable sites for geologic CO₂ storage or high-level radioactive waste disposal. Acknowledgements: This study was supported by the Institute for Korea Spent Nuclear Fuel (iKSNF) and the BK Plus project in Korea.

References

Choi, J., Yu, S., Park, S., Yun, S.T., Lee, J., Park, J. (2023). Hydrochemical and Isotopic Assessment of Deep Groundwater: Residence Time, Circulation and Inter-Aquifer Mixing in South Korea. Goldschmidt 2023 Abstracts. https://doi.org/10.7185/gold2023.20028

Kim, K.H., Yun, S.T., Yu, S., Choi, B.Y., Kim, M.J., Lee, K.J. (2020). Geochemical pattern recognitions of deep thermal groundwater in South Korea using self-organizing map: identified pathways of geochemical reaction and mixing. Journal of Hydrology 589, 125202. https://doi.org/10.1016/j.jhydrol.2020.125202

How to cite: Choi, J., Lee, J.-H., Park, S., Seo, H., and Yun, S.-T.: Investigating Geochemical Characteristics of South Korean Geothermal Waters and Their Reservoir Condition Using Combined Hydrogeochemical, Isotopic and Geothermometric Approaches, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15040, https://doi.org/10.5194/egusphere-egu24-15040, 2024.

Characterizing the host rocks for high-level radioactive waste disposal requires considering various rock parameters such as petrophysical (e.g., porosity, permeability and storativity) as well as mechanical (e.g., Young’s modulus) properties. The burial history and thermal maturity of rock formations, particularly shale, significantly affect these properties. Understanding the relationship between the thermal maturity and the referred properties is essential for transferring data among various test sites.

Pliensbachian shales (Lower Saxony Basin, Hils area) are characterized by a notably homogenous mineral composition. Previous studies on this formation suggest a highly variable burial history over a lateral distance of about 50 km, reaching maximum burial depths between 1300 m in the southeast to 3600 m in the northwest of this region (Gaus et al., 2022; Littke et al., 1987, 1991). In the context of the MATURITY project, eight boreholes were drilled at five different locations with varying thermal maturity along this axis. Geophysical well logging was carried out in all boreholes to investigate mineralogical (clay mineral content), petrophysical (porosity), and rock mechanical properties (e.g., dynamic Poisson's ratio). First results revealed a homogeneous clay mineralogy in all boreholes. However, significant differences were observed in both the dynamic-elastic behavior and porosity of the investigated shale formation at different locations. The obtained data can be used to establish correlations between burial history and various shale rock properties, contributing to a site-independent characterization of shale formations for long-term disposal of high-level nuclear waste.

 References

• 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., Baker, D. R., & Leythaeuser, D. (1988). Microscopic and sedimentologic evidence for the generation and migration of hydrocarbons in Toarcian source rocks of different maturities. Advances in Organic Geochemistry, Vol.13(Nos 1-3), 549–559.
• 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., Grohmann, S., Winhausen, L., Gaus, G., Seemann, T., Erbacher, J., Littke, R., and Amann, F.: The Influence of Burial History on the In-situ Petrophysical and Mechanical Characteristics of Pliensbachian Shale, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16083, https://doi.org/10.5194/egusphere-egu24-16083, 2024.

Climate evolution over millennial timescales significantly influences various environmental aspects, including thermal conditions, regional sea levels, geomorphological evolution, weathering processes, and hydrogeological evolution. This intricate relationship has direct implications for nuclear waste management facilities, as climate change alters the frequency and intensity of extreme events, posing risks during construction, operation, and post-closure phases. Understanding these impacts is crucial for ensuring safety and to building civil society's confidence in radioactive waste management programes. This contribution centers on Belgian scenarios.

SCK CEN, with extensive experience in studying climate change impacts on nuclear waste management, shares its insights from projects such as BIOCLIM, BIOMOSA, and EURAD. The Belgian case, with its diverse climate regions, offers valuable data applicable to other countries facing similar climate challenges. For example, SCK CEN's involvement in EURAD - UMAN has already prompted recommendations for managing uncertainties in the geosphere, particularly in climatic evolution. Key areas of focus could include validating permafrost depth models, modeling hydrochemical changes resulting from permafrost development, conducting detailed palaeo-hydrological studies, and considering the influence of decompaction on host rock properties. This study serves as a critical resource for understanding and addressing climate-related challenges in nuclear waste management, fostering international collaboration and knowledge exchange.

How to cite: Montoya, V. and Beerten, K.: Climate Change Impacts on Nuclear Waste Management: Insights from Belgium and Recommendations for Future Research, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19823, https://doi.org/10.5194/egusphere-egu24-19823, 2024.

The distinctive properties of salt, such as low permeability, high thermal conductivity, and self-sealing features, make it a suitable medium for storing heat-generating radioactive waste. Understanding the thermal, hydrological, and mechanical (THM) processes, including permeability evolution and brine migration around the heat source, is crucial for safety. The Brine Availability Tests in Salt (BATS) at the Waste Isolation Pilot Plant were conducted to investigate these processes, employing an array of sensors and techniques like electrical resistivity tomography (ERT), fiber optic distributed temperature sensing (DTS), and strain sensing (DSS). These techniques monitored temperature changes, brine movement, and stress conditions in the salt.

This presentation highlights the results from ERT, DTS, and DSS in controlled heating experiments, focusing on the response differences across various heating events. The analysis, augmented by Discrete Element Models (DEM) simulations, showed that resistivity changes were sensitive to temperature and brine movement. A significant decrease in resistivity, especially beyond temperature effects, indicated brine migration or permeability changes. The DTS and DSS data captured the evolving thermal and mechanical responses of the salt to heating and cooling cycles, including salt deformation, and creeping towards the drift wall.

Joint analysis of ERT, DTS, and DSS data provided an integrated understanding of THM processes in salt during heating. Consistent heating and brine migration patterns were observed across different events. Ongoing work aims to combine all monitoring data for a deeper insight into the coupled behaviors in salt formations, offering valuable calibration and benchmarking for numerical models.

How to cite: Wu, Y., Wang, J., Chen, H., Luo, L., Otto, S., Kuhlman, K., Zheng, L., and Birkholzer, J.: Geophysical Understanding of Coupled Thermal-Hydro-Mechanical Dynamics in Rock Salt During Heating, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7277, https://doi.org/10.5194/egusphere-egu24-7277, 2024.

Long-term stability of engineered barrier system (EBS) materials in repository conditions is a primary concern for radioactive waste repository success. EBS designs generally include bentonite as barrier between the canister and host rock to provide: 1) a physical barrier to prevent natural fluid from interacting with the waste package and 2) a chemical barrier that attenuates radionuclide migration. Bentonite interaction with host rock and groundwater in water-saturated geologic repositories may result in the formation of secondary minerals that affect the sealing properties of the bentonite. Specifically, the formation of zeolites may negatively affect bentonite swelling properties but may also help attenuate radionuclide migration through sorption and incorporation into zeolite structures. Here we discuss the experimental formation of analcime-group zeolite minerals with Na-, Ca-, and Cs-endmembers (analcime, wairakite, and pollucite, respectively) through hydrothermal bentonite alteration in geologic repository conditions.

Experimental work was conducted at elevated pressure and temperatures conditions relevant to underground repositories. High-pressure Cs-bentonite experiments were completed from 150-400 °C and 500-1000 bar for 14 to 62 days. Gold capsules were loaded with a 2:1 water:rock ratio of unprocessed bentonite from Colony, Wyoming and an aqueous fluid containing 2 molal CaCl + CsCl + NaCl with an initial pH of ~5.8. At 200 °C Cs was found to be entrained in different phases including the zeolite clinoptilolite. Clinoptilolite is an accessory mineral in the initial bentonite mineral assemblage and the results suggest incorporation into existing clinoptilolite rather than any Cs-clinoptilolite precipitation. At reaction temperatures ≥ 300 °C and < 450 °C we observed newly precipitated crystals of pollucite in the bentonite matrix. After reaction at 450 °C only trace amounts of Cs were identified in mineral phases: instead, we observed the formation of analcime-wairakite minerals with little or no pollucite formation. Comparing these results to analcime-wairakite-pollucite stability at a range of pressure-temperature conditions sheds light on conditions that promote Cs immobilization through incorporation in pollucite crystal structures. These results expand the range of experimentally observed zeolite formation and define pressure and temperature fields that promote Cs entrapment during hydrothermal bentonite alteration. The potential benefits of radionuclide immobilization through zeolite formation are contextualized by thermodynamic and kinetic geochemical modelling of Na-, Ca-, Cs-zeolite formation and the anticipated effects to bentonite and EBS properties over time.

How to cite: Zandanel, A., McKanna, A., Rock, M., Sauer, K., and Caporuscio, F.: Temperature and reaction time dependence of (Na,Ca,Cs)-zeolite formation., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3648, https://doi.org/10.5194/egusphere-egu24-3648, 2024.

Bentonite is a crucial part of the engineered barrier system (EBS) of the deep geological repositories (DGR) for nuclear waste. However, as a natural material, it harbors diverse indigenous microorganisms whose metabolic activity might compromise the long-term integrity of EBS. One of the critical questions for microbial activity prediction in DGR is the microbial reaction to the early hot phase of nuclear waste repository evolution. Our study investigates the impact of gamma radiation and heat on microbial survivability in bentonite (Czech bentonite BCV and reference bentonite MX-80), considering variations in compaction, temperature, and saturation levels as these factors are known to influence microbial reaction to extreme conditions. Previous results suggested average radiation tolerance and high heat tolerance of indigenous bentonite microorganisms. We aimed to confirm these findings during the medium-term experiment (18 months) at the repository simulating conditions. Four experimental setups were conducted using compacted BCV and MX-80 bentonites under anoxic conditions differing in initial saturation level (15-20 wt. %), heating temperature ( 90 or 150°C), and irradiation (0.4 Gy/h). An additional set of bentonite powder samples heated at 90°C or 150°C for 1, 3, and 6 months was further included to unravel the time effect of heat exposure on microbial survivability. Microbial community analysis, involving natural incubations in the form of bentonite suspension, enrichment cultures, microscopy, and molecular methods, was conducted for each sample to estimate microorganism survival following exposure to extreme conditions.

Contrary to expectations, no microbial growth and recovery were detected in any treated samples except for the additional set. Fresh samples, suspension incubations, and enrichment cultures were checked negative for microbial presence in almost all the cases in the four main experimental sets except for the positive controls. On the other hand, the samples from the additional set showed microbial recovery after heating at 90°C for 1, 3, and 6 months. However, exposure to 150°C for only 1 month resulted in bentonite sterilization. These medium-term data pointed out the high tolerance of indigenous bentonite microorganisms to heat but also indicated that developing a sterile bentonite layer in the proximity of hot and radiation-emitting metal canister is highly likely at the early stage of the DGR evolution. However, the particular development will depend upon the DGR design and actual levels of temperatures and irradiation. To enhance our understanding and predictability of microbial processes in the bentonite sealing layer of DGRs for nuclear waste, further microbiological experiments simulating conditions in distant bentonite layers subjected to lower temperature and studying potential microorganism penetration from non-sterile to sterilized zones are needed.

How to cite: Bartak, D., Šachlová, Š., Kašpar, V., Říha, J., Večerník, P., Hlaváčková, V., and Černá, K.: Assessing microbial activity and survivability in heated and irradiated bentonite samples under deep geological repository relevant conditions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17828, https://doi.org/10.5194/egusphere-egu24-17828, 2024.

In this contribution we have developed a reactive transport model to assess the hydro-chemical evolution of a L-ILW disposal cell in indurated clay rocks, involving the interaction of different components/materials and the expected hydraulic and/or chemical gradients over 100 000 years. The L-ILW disposal cell leverages a multi-barrier concept buried between 200 and 800 m below the surface. The multi-barrier system is comprised of the waste matrix (i.e. backfilling the waste drums), the disposal container, the mortar backfill in the emplacement tunnel (where the disposal containers are located) and the clay host rock. The dimensions and design of the emplacement tunnel (e.g. 11 × 13 m) and disposal cells represent and consider some aspects taken into account in the designs of some European countries. In addition, tunnel walls reinforced with a shotcrete liner and the Excavation Damaged Zone is considered in the concept. The model is implemented in OpenGeoSys-6, an open-source version-controlled scientific software based on Finite Element Method which is capable of handling fully coupled hydro-chemical models by coupling OpenGeoSys to iPHREEQC. First calculation results, demonstrate that the most important processes affecting the near-field chemical evolution are i) the degradation of the concrete and cementitious grouts with porewater migrating inwards from the host rock and ii) the significant quantities of reactive and non-reactive gases (i.e. hydrogen, carbon dioxide and methane) that are generated as a result of: i) the anaerobic corrosion of metals present in the waste and containers and ii) the degradation of organic compounds by microbial and chemical processes. Several simulation cases were carried out under different assumptions, specially related to the saturation of the domain, the final goal was to have a 2D model of the system representing a more realistic geometry which will potentially help in design optimization decisions.

How to cite: Montoya, V. and Garibay-Rodriguez, J.: 2D reactive transport model for assessing the chemical evolution of a low and intermediate-level waste repository in clayrock, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18849, https://doi.org/10.5194/egusphere-egu24-18849, 2024.

The European Union’s Horizon 2020 project EURAD (European Joint Progamme on Radioactive Waste Management) aim is to implement a joint Strategic Programme of research and knowledge management activities at the European level, bringing together and complementing EU Member State programmes in order to ensure cutting-edge knowledge creation and preservation in view of delivering safe, sustainable and publicly acceptable solutions for the management of radioactive waste across Europe now and in the future. The broader scope of the work package ACED (Assessment of the chemical evolution in intermediate and high level radioactive waste disposal cells) in EURAD is the assessment of the chemical evolution at the disposal cell scale involving interacting components/materials and thermal, hydraulic and/or chemical gradients. Four generic disposal cells that are representative for the most important aspects of disposal designs in several national disposal programs throughout Europe, formed the basis for the research activities in this work package.

Conceptual, mathematical and numerical models were developed and implemented to describe the geochemical evolution in the combined engineered barrier system and the immediately surrounding host formation (clay or granite) from a more detailed scale up to a spatial scale of a few meters and time scales up to 100 000 y. ACED demonstrated the applicability of advanced coupled reactive transport models to provide an integrated view on coupled geochemical processes in the multi-barrier system of a deep geological repository. Insight in the geochemical evolution was obtained from more detailed numerical models at the scale of the waste package (neglecting influences from the host rock) up to models with the detailed geometry and the different materials in the disposal scale (depending on the system including nuclear glass, organic waste, metallic waste, steel, cementitious materials, bentonite, and host rock). Beside models that integrated as detailed as possible the available information, approaches were tested to decrease the model complexity while preserving the key findings of the more complex models; a process also known as model abstraction. Simplifications were made with respect to dimensionality, chemical complexity and numerical accuracy as well by implementing surrogate models based on machine learning tools to replace the parts of the models requiring the most computational time.

ACED made a significant step forward in process-based modelling of geochemical interactions  in the near field of a geological repository where the engineered barrier system interacts with the host rock. The models and insights obtained for the generic disposal cells can form a basis for more specific research studies in national disposal programs.

Acknowledgement: The research leading to these results was funded by the EURAD work package (European Joint Programme on Radioactive Waste Management of the European Union, EC grant agreement nº 847593)

How to cite: Jacques, D. and Montoya, V. and the EURAD - ACED Team:: EURAD ACED – Modelling the geochemical evolution of disposal cells in deep geological repositories, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21286, https://doi.org/10.5194/egusphere-egu24-21286, 2024.

​In order to place experiments in real geological conditions, a new section of the already active generic underground research facility Bukov (Bukovská et al. 2019) is recently being constructed. It is located in the central Bohemian Massif, in former uranium mine Rožná, at c. 550 m below the surface. During the excavation of new tunnels and laboratory chambers, a complex of geomechanical, structural, geological, hydrogeological and geophysical measurements and monitoring activities is continuously performed. The main objectives of this in-situ characterization project are to understand and 3D visualize the structural record and its hydrogeological systematics. Other objectives are to determine the excavation damage zones, the effect of blasting, the physical and mechanical properties of the rocks, and testing the excavation and drilling methodology.

The laboratory is situated mostly in the intensively deformed paragneiss, high-grade amphibolites and migmatites, typical for the European Variscan belt. Geological observations have revealed a complex polyphase deformational pattern of metamorphic foliations, strongly influencing the superposed fault and fracture systems, hydrogeological conditions and rock mass properties. Based on multidisciplinary research of borehole and tunnel data, possible problematical geological structures are identified and investigated. Finally, all obtained data and experience will serve to development of work-flows of documentation and rock-mass suitability classification system during building of the future real deep geological repository.

The construction will be completed in 2024 and the underground research facility will become part of the national planning and siting strategy for the deep underground repository for high-level radioactive waste. The data from this research will contribute to the understanding of the geology of the region and will be used for experiments addressing long-term safety and technical feasibility. Such results from the depth can allow correlation and validation of data collected from surface research (Soejono et al. 2021) in the potential sites for deep geological repository construction.

 Bukovská, Z., Soejono, I., Vondrovic, L., et al., 2019. Characterization and 3D visualization of underground research facility for deep geological repository experiments: A case study of underground research facility Bukov. Czech Republic. Eng. Geol. 259, 105186. https://doi.org/10.1016/j.enggeo.2019.105186

Soejono, I., Bukovská, Z., Levá, B., et al., 2021. Interdisciplinary geoscientific approach to radioactive waste repository site selection: the Březový potok site, southwestern Czech Republic. J. Maps 17, 741–749. https://doi.org/10.1080/17445647.2021.2004942

How to cite: Mareda, L., Soejono, I., Bukovská, Z., Švagera, O., Rukavičková, L., Řihošek, J., Kry, J., Zelinková, T., Chabr, T., Morávek, R., Souček, K., Vavro, M., Waclawik, P., Sosna, K., Nedvěd, J., Mareček, L., and Macáková, A.: New underground research facility for deep disposal of radioactive waste Bukov II, Czech Republic: objectives, geological constraints and current status, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21252, https://doi.org/10.5194/egusphere-egu24-21252, 2024.