TY - JOUR A1 - Hennig, Theresa A1 - Kühn, Michael T1 - Surrogate model for multi-component diffusion of Uranium through Opalinus Clay on the host rock scale JF - Applied Sciences : open access journal N2 - Multi-component (MC) diffusion simulations enable a process based and more precise approach to calculate transport and sorption compared to the commonly used single-component (SC) models following Fick's law. The MC approach takes into account the interaction of chemical species in the porewater with the diffuse double layer (DDL) adhering clay mineral surfaces. We studied the shaly, sandy and carbonate-rich facies of the Opalinus Clay. High clay contents dominate diffusion and sorption of uranium. The MC simulations show shorter diffusion lengths than the SC models due to anion exclusion from the DDL. This hampers diffusion of the predominant species CaUO2(CO3)32-. On the one side, species concentrations and ionic strengths of the porewater and on the other side surface charge of the clay minerals control the composition and behaviour of the DDL. For some instances, it amplifies the diffusion of uranium. We developed a workflow to transfer computationally intensive MC simulations to SC models via calibrated effective diffusion and distribution coefficients. Simulations for one million years depict maximum uranium diffusion lengths between 10 m and 35 m. With respect to the minimum requirement of a thickness of 100 m, the Opalinus Clay seems to be a suitable host rock for nuclear waste repositories. KW - facies KW - uranium speciation KW - sorption KW - reactive transport KW - heterogeneity KW - PHREEQC KW - Mont Terri KW - repository far-field Y1 - 2021 U6 - https://doi.org/10.3390/app11020786 SN - 2076-3417 VL - 11 IS - 2 PB - MDPI CY - Basel ER - TY - JOUR A1 - Hennig, Theresa A1 - Kühn, Michael T1 - Potential uranium migration within the geochemical gradient of the opalinus clay system at the Mont Terri JF - Minerals N2 - Transport properties of potential host rocks for nuclear waste disposal are typically determined in laboratory or in-situ experiments under geochemically controlled and constant conditions. Such a homogeneous assumption is no longer applicable on the host rock scale as can be seen from the pore water profiles of the potential host rock Opalinus Clay at Mont Terri (Switzerland). The embedding aquifers are the hydro-geological boundaries, that established gradients in the 210 m thick low permeable section through diffusive exchange over millions of years. Present-day pore water profiles were confirmed by a data-driven as well as by a conceptual scenario. Based on the modelled profiles, the influence of the geochemical gradient on uranium migration was quantified by comparing the distances after one million years with results of common homogeneous models. Considering the heterogeneous system, uranium migrated up to 24 m farther through the formation depending on the source term position within the gradient and on the partial pressure of carbon dioxide pCO2 of the system. Migration lengths were almost equal for single- and multicomponent diffusion. Differences can predominantly be attributed to changes in the sorption capacity, whereby pCO2 governs how strong uranium migration is affected by the geochemical gradient. Thus, the governing parameters for uranium migration in the Opalinus Clay can be ordered in descending priority: pCO2, geochemical gradients, mineralogical heterogeneity.

KW - PHREEQC KW - reactive transport KW - sorption KW - diffusion KW - repository far-field KW - hydro-geological system Y1 - 2021 U6 - https://doi.org/10.3390/min11101087 SN - 2075-163X VL - 11 IS - 10 PB - MDPI CY - Basel ER - TY - JOUR A1 - Hennig, Theresa A1 - Stockmann, Madlen A1 - Kühn, Michael T1 - Simulation of diffusive uranium transport and sorption processes in the Opalinus Clay JF - Applied geochemistry : journal of the International Association of Geochemistry and Cosmochemistry N2 - Diffusive transport and sorption processes of uranium in the Swiss Opalinus Clay were investigated as a function of partial pressure of carbon dioxide pCO(2), varying mineralogy in the facies and associated changes in porewater composition. Simulations were conducted in one-dimensional diffusion models on the 100 m-scale for a time of one million years using a bottom-up approach based on mechanistic surface complexation models as well as cation exchange to quantify sorption. Speciation calculations have shown, uranium is mainly present as U(VI) and must therefore be considered as mobile for in-situ conditions. Uranium migrated up to 26 m in both, the sandy and the carbonate-rich facies, whereas in the shaly facies 16 m was the maximum. The main species was the anionic complex CaUO2(CO3)(3)(2-) . Hence, anion exclusion was taken into account and further reduced the migration distances by 30 %. The concentrations of calcium and carbonates reflected by the set pCO(2) determine speciation and activity of uranium and consequently the sorption behaviour. Our simulation results allow for the first time to prioritize on the far-field scale the governing parameters for diffusion and sorption of uranium and hence outline the sensitivity of the system. Sorption processes are controlled in descending priority by the carbonate and calcium concentrations, pH, pe and the clay mineral content. Therefore, the variation in porewater composition resulting from the heterogeneity of the facies in the Opalinus Clay formation needs to be considered in the assessment of uranium migration in the far field of a potential repository. KW - reactive transport KW - facies KW - heterogeneity KW - carbonate KW - PHREEQC KW - Mont Terri KW - speciation Y1 - 2020 U6 - https://doi.org/10.1016/j.apgeochem.2020.104777 SN - 0883-2927 SN - 1872-9134 VL - 123 PB - Elsevier CY - Oxford ER - TY - JOUR A1 - Steding, Svenja A1 - Kempka, Thomas A1 - Kühn, Michael T1 - How insoluble inclusions and intersecting layers affect the leaching process within potash seams JF - Applied Sciences : open access journal N2 - Potash seams are a valuable resource containing several economically interesting, but also highly soluble minerals. In the presence of water, uncontrolled leaching can occur, endangering subsurface mining operations. In the present study, the influence of insoluble inclusions and intersecting layers on leaching zone evolution was examined by means of a reactive transport model. For that purpose, a scenario analysis was carried out, considering different rock distributions within a carnallite-bearing potash seam. The results show that reaction-dominated systems are not affected by heterogeneities at all, whereas transport-dominated systems exhibit a faster advance in homogeneous rock compositions. In return, the ratio of permeated rock in vertical direction is higher in heterogeneous systems. Literature data indicate that most natural potash systems are transport-dominated. Accordingly, insoluble inclusions and intersecting layers can usually be seen as beneficial with regard to reducing hazard potential as long as the mechanical stability of leaching zones is maintained. Thereby, the distribution of insoluble areas is of minor impact unless an inclined, intersecting layer occurs that accelerates leaching zone growth in one direction. Moreover, it is found that the saturation dependency of dissolution rates increases the growth rate in the long term, and therefore must be considered in risk assessments. KW - salt dissolution KW - reactive transport KW - heterogeneity KW - density-driven KW - convection KW - PHREEQC KW - porous media Y1 - 2021 U6 - https://doi.org/10.3390/app11199314 SN - 2076-3417 VL - 11 IS - 19 PB - MDPI CY - Basel ER - TY - JOUR A1 - Steding, Svenja A1 - Kempka, Thomas A1 - Zirkler, Axel A1 - Kühn, Michael T1 - Spatial and temporal evolution of leaching zones within potash seams reproduced by reactive transport simulations JF - Water / Molecular Diversity Preservation International (MDPI) N2 - Leaching zones within potash seams generally represent a significant risk to subsurface mining operations and the construction of technical caverns in salt rocks, but their temporal and spatial formation has been investigated only rudimentarily to date. To the knowledge of the authors, current reactive transport simulation implementations are not capable to address hydraulic-chemical interactions within potash salt. For this reason, a reactive transport model has been developed and complemented by an innovative approach to calculate the interchange of minerals and solution at the water-rock interface. Using this model, a scenario analysis was carried out based on a carnallite-bearing potash seam. The results show that the evolution of leaching zones depends on the mineral composition and dissolution rate of the original salt rock, and that the formation can be classified by the dimensionless parameters of Peclet (Pe) and Damkohler (Da). For Pe > 2 and Da > 1, a funnel-shaped leaching zone is formed, otherwise the dissolution front is planar. Additionally, Da > 1 results in the formation of a sylvinitic zone and a flow barrier. Most scenarios represent hybrid forms of these cases. The simulated shapes and mineralogies are confirmed by literature data and can be used to assess the hazard potential. KW - carnallite KW - water rock interaction KW - density-driven flow KW - PHREEQC KW - Pitzer KW - equations Y1 - 2021 U6 - https://doi.org/10.3390/w13020168 SN - 2073-4441 VL - 13 IS - 2 PB - Molecular Diversity Preservation International CY - Basel ER - TY - JOUR A1 - Tranter, Morgan Alan A1 - De Lucia, Marco A1 - Kühn, Michael T1 - Numerical investigation of barite scaling kinetics in fractures JF - Geothermics : an international journal of geothermal research and its applications N2 - Barite stands out as one of the most ubiquitous scaling agents in deep geothermal systems, responsible for irreversible efficiency loss. Due to complex parameter interplay, it is imperative to utilise numerical simulations to investigate temporal and spatial precipitation effects. A one-dimensional reactive transport model is set up with heterogeneous nucleation and crystal growth kinetics. In line with geothermal systems in the North German Basin, the following parameters are considered in a sensitivity analysis: temperature (25 to 150 degrees C), pore pressure (10 to 50 MPa), fracture aperture (10(-4) to 10(-2) m), flow velocity (10(-3) to 10(0) m s(-1)), molar volume (50.3 to 55.6 cm(3) mol(-1)), contact angle for heterogeneous nucleation (0 degrees to 180 degrees), interfacial tension (0.07 to 0.134 J m(-2)), salinity (0.1 to 1.5 mol kgw(-1) NaCl), pH (5 to 7), and supersaturation ratio (1 to 30). Nucleation and consequently crystal growth can only begin if the threshold supersaturation is exceeded, therefore contact angle and interfacial tension are the most sensitive in terms of precipitation kinetics. If nucleation has occurred, crystal growth becomes the dominant process, which is mainly controlled by fracture aperture. Results show that fracture sealing takes place within months (median 33 days) and the affected range can be on the order of tens of metres (median 10 m). The presented models suggest that barite scaling must be recognised as a serious threat if the supersaturation threshold is exceeded, in which case, large fracture apertures could help to minimise kinetic rates. The models further are of use for adjusting the fluid injection temperature. KW - Geothermal KW - Fracture sealing KW - Reactive transport KW - Geochemical modelling KW - Crystal nucleation KW - PHREEQC Y1 - 2021 SN - 0375-6505 SN - 1879-3576 VL - 91 PB - Elsevier CY - Amsterdam [u.a.] ER -