Crystalline Host Rock Research Articles

Uptake of Eu, Th, U, and Pu by granite and biotite gneiss in Korean fresh groundwater under oxidizing and reducing conditions

Abstract The uptake of Eu, Th, U, and Pu by Korean granite and biotite gneiss was investigated in a series of batch experiments. Experiments were conducted under well-defined redox conditions, i.e., oxidizing (air), mildly reducing (Ar-atmosphere and buffered with hydroquinone, pe + pH ≈ 8–9), and strongly reducing (Ar-atmosphere and buffered with Na2S2O4, pe + pH ≈ 0.5–3). Radionuclide concentration, pH and E h were systematically monitored up to t ≤ 113 days after the addition of the radionuclide. The natural content of Eu, Th and U in pristine granite and biotite gneiss materials was quantified by means of alkaline fusion. Eu exhibited moderate sorption on biotite gneiss displaying higher distribution ratios (R d) compared to granite. This observation was possibly explained by the affinity of Ln(III)/An(III) towards biotite mineral absent in the investigated granite material. Strong sorption was observed for Th, U, and Pu in reducing systems where the predominance of the +IV oxidation state is expected. For these three systems, the strength of the uptake follows the order R d(Pu(IV)) > R d(U(IV)) > R d(Th(IV)), consistent with the hydrolysis strength of the corresponding aquo-ions. A significantly weaker sorption was observed for U and Pu under oxidizing conditions, although R d values are manifestly higher for Pu than U. Thermodynamic calculations for the oxidizing conditions predicted the predominance of U(VI) and Pu(V)/Pu(IV), explaining observed differences in retention under oxidizing conditions. These results contribute to a quantitative description and a better understanding of the retention of redox-sensitive radionuclides in crystalline host rocks. Emphasis is placed on the importance of utilizing both redox-stable probes (e.g., Eu, Th) and redox-sensitive actinides (e.g., U, Pu), as well as well-defined redox conditions for accurate predictions of radionuclide retention.

Characterization and solubility measurement of synthetic uranophane and sklodowskite under oxic groundwater conditions

Naturally occurring uranyl silicates can serve as solubility-limiting solid phases (SLSPs) of U(VI) in the oxic environment of granitic groundwater, which is likely to occur in crystalline host rock media in Korea. In this study, two synthetic uranyl silicates, uranophane (Ca[UO2SiO3OH]2·5H2O) and sklodowskite (Mg[UO2SiO3OH]2·6H2O) were prepared and used to measure the U(VI) solubility in simulated groundwater (sGW), mimicking the composition of samples collected from the underground research tunnel at the Korea Atomic Energy Research Institute (KAERI). The solid- and solution-phase species involved were examined in detail using various characterization methods. According to the powder X-ray diffraction pattern and elemental analysis results, the synthetic mineral phases, which were identified as uranophane-α and sklodowskite having layered crystal structures, were retained intact in sGW; however, in 0.1 M NaClO4, emergence of other solid phases was observed. Enhanced FTIR and Raman spectroscopic data, particularly in the regions of 790–860 and 940–1020 cm−1 for the UO22+ and SiO44− ions, respectively, enable monitoring the changes in the solid phases. Ternary Ca–U(VI)-tricarbonato complexes were identified as the dominant dissolved U(VI) species in sGW using time-resolved laser-induced luminescence spectroscopy. Furthermore, the solubility constant of uranophane was calculated (log Ks,0° = 10.3 ± 0.4) and compared with the predicted values based on our geochemical modeling analysis and previously reported ones.

Effects of Glacial Isostatic Adjustment on Fault Reactivation and Its Consequences on Radionuclide Migration in Crystalline Host Rocks

AbstractTo assess the robustness of a safety case for a deep geological repository (DGR), it is necessary to analyze a range of scenarios covering likely, less likely, and hypothetical future developments. Crystalline rock can, under ideal conditions, provide a suitable hydrogeologic barrier due to its extremely low matrix permeability. However, this host rock is often fractured, which can compromise its hydro-mechanical (HM) barrier function. We quantify how faults that are prone to reactivation during glacial events can affect radionuclide migration around a DGR in a crystalline host rock. We extend a previously developed finite element model of coupled fluid flow and radionuclide transport to numerically solve the component transport problem before and after fault reactivation. Assuming that fault reactivation is triggered by changes in mechanical boundary conditions, we derive heterogeneous permeability distributions in the reactivated faults by evaluating the Coulomb failure stress criterion of finite element solutions of a complementary hydro-mechanical problem. Specifically, we evaluate the consequences of glacial isostatic adjustment (GIA) during a glacial cycle. We find that the increased permeability in the reactivated faults accelerates the migration of radionuclides along the fault by channeling the flow, while it is reduced in the direction perpendicular to the fault. The channeling observed is also a result of heterogeneous permeability enhancement, and the flow fields differ from those of the previous model which postulated a homogeneous permeability enhancement. Although the proposed numerical workflow has been applied to the case of GIA, it is adaptable to study hydro-mechanical processes induced by seismic events or by hydrofracking in enhanced geothermal systems.

A multi-method investigation of the permeability structure of brittle fault zones with ductile precursors in crystalline rock

Brittle faults and fault zones are among the most hydraulically active elements in predominantly impermeable crystalline host rock. They pose a significant challenge to underground infrastructure like nuclear waste repositories. Brittle fault zones frequently occur along pre-existing ductile shear zones as they introduce weakness planes in the rock.Four brittle fault zones of ductile origin were analyzed in the Rotondo Granite at the “BedrettoLab” in the Swiss Central Alps. Scanline mapping, rock sampling and permeability measurements using three different methods provide detailed insights into the heterogeneous fault zone architecture and hydrogeology. Average intact rock permeability is in the range of 10−19 to 10−17 m2. Fluid flow is channeled into single open or partially mineralized fractures of, at point-scale, up to 10−14 m2, demonstrated by selective gas probe permeameter measurements and borehole hydraulic testing. Reduced permeabilities have been measured in close proximity to these permeable features, indicating alteration of and around the fracture walls.

Evaluation of EDZ characterization technology for URL based on field investigation at BET

Detailed characterization of the Excavation Damage Zone (EDZ) is an important issue for the high-level radioactive waste (HLW) geological disposal engineering, which is closely related to the long-term safety of the HLW repository. With drill-blast experiment in Beishan Exploration Tunnel (BET, a platform for the field experiment and validation of potential technologies used for China’s HLW disposal engineering in crystalline rock), systematic field tests were carried out to evaluate the suitability of techniques in characterizing EDZ, and to study EDZ characteristic in crystalline tunnel. A series of investigation technologies were used to delineate EDZ and to examine the process of its formation, which includes: p-wave velocity measurement, ground penetrating radar (GPR), acoustic emission (AE) monitoring and borehole televiewer. P-wave velocity measurement along borehole reveal that wave velocity decreased significantly (more than 10%) in 30 cm to 50 cm from the excavation surface, and this zone is defined as EDZ. Clear images of borehole televiewer were obtained, which is helpful for EDZ delineation. GPR and borehole radar were also used for EDZ delineation. Strong reflection signals were detected in the range of 10 cm to 25 cm by GPR with high frequency antenna, indicating significant changes of physical properties of rock-mass in the zone, which is defined as Highly Damaged Zone (HDZ). Moreover, the formation process of EDZ induced by blast in crystalline rock tunnel was analyzed using AE monitoring. Damage process of surrounding rock was divided into blasting damage stage and progressive damage stage. AE events location show the induced cracking activities were clustering within 30 cm to 100 cm from excavation surface. With the experience from field investigation and result analysis, the feasibility and suitability of the techniques for EDZ characterization is evaluated. Finally a specialized EDZ characterization technique system is proposed for underground research laboratory construction in crystalline host rock in Beishan area.

A Study on Previous Cases of Seismic Reflection Surveys for Deep Geological Disposal Site Investigations in Crystalline and Sedimentary Host Rocks

A Study on Previous Cases of Seismic Reflection Surveys for Deep Geological Disposal Site Investigations in Crystalline and Sedimentary Host Rocks

Corrosion evaluation for engineered barriers in crystalline host rock – challenges for container development in the German site selection process

Abstract. Crystalline rock is being considered as a potential host rock for a deep geological disposal facility for high-level radioactive waste in the ongoing site selection process in Germany. As crystalline rock is commonly highly fractured, the designation of a containment-providing rock zone is potentially not possible. In this case, according to German law (§ 23 (1) and (4) StandAG), the safe containment of the nuclear waste for 1 million years must be guaranteed by engineered and geo-engineered barriers. Therefore, it is of great importance to evaluate and understand processes that might lead to corrosion of the canister from the outside or from the inside. The long-term integrity of the waste canister depends on various parameters such as the material, microstructure, and coating of the canister; the properties of the geo-engineered barriers; the local pore water composition; the radiolysis on the inside of the container; and the tectonic strain. In a recent project (Long-Term Integrity of Canisters in Crystalline Host Rock – LaKris), the challenges of determining the durability of waste canisters in crystalline host rock have been addressed by comparing and evaluating different international concepts for canister design and discussing their applicability to the requirements in Germany. Further focus lies on the evaluation of predominant corrosion mechanisms while also considering the geochemical environment of the crystalline host rock in Germany that the canisters might be subjected to. This contribution will highlight the activities within the project with regard to evaluating the corrosion properties of different potential canister materials such as copper, cast iron, and stainless steel. The aim is to deepen the understanding of individual processes and their combined effects as well as their classification into favourable or unfavourable processes and conditions for the derivation or justification of requirements. It also includes a review of the existing requirements on canister integrity according to the regulatory framework as well as a discussion about important hydrogeochemical parameters, which could influence the corrosion of the canister, including the limited availability and uncertainty of such data.

BARIK: an extended Hoek–Brown-based anisotropic constitutive model for fractured crystalline rock

Abstract. The disposal of heat-generating radioactive waste in deep geologic formations is a global concern. Numerical methods play a key role in understanding and assessing the disposal scenarios of radioactive waste in deep geological repositories. However, the complexities of the thermal, hydrological, mechanical, chemical, and biological processes associated with the disposal of radioactive waste in porous and fractured materials constitute significant challenges. One of the most challenging issues in this field is the complex material behavior of fractured crystalline rock. The presence of fractures makes the rock anisotropic, nonlinear, and dependent on loading paths. Additionally, the Biot coefficient cannot be considered constant throughout the critical and subcritical fracture development regions. These factors make the development of an accurate constitutive model for fractured crystalline hard rock a critical component of any deep geological disposal project. Furthermore, to demonstrate the integrity of the containment-providing rock zone in crystalline host rock, the qualitative integrity criteria must be quantified so that numerical simulation can be performed with concrete numerical values. Part of this assessment for a crystalline host rock is a dilatancy criterion, which is currently based on the Hoek–Brown constitutive model. BARIK is the German acronym for the research project on which this paper is based. This contribution provides an overview of the development and verification of the BARIK constitutive model, an extended Hoek–Brown model for fractured crystalline hard rock that takes into account up to three fracture systems. The model enables the consideration of the matrix and joint behavior of the rock separately, with each component having unique strength characteristics and failure criteria. These criteria are formulated such that suitable consideration of the strength-reducing properties of the respective fracture systems during barrier integrity verification is possible. The BARIK model has been implemented into two computer codes, FLAC3D and MFront for OpenGeoSys, allowing for the identification and evaluation of any inaccuracies that may arise from the use of different codes. The model enables isotropic–elastic, orthotropic–elastic, isotropic–elasto-plastic, and orthotropic–elasto-plastic calculations of the matrix, making it a valuable tool for the site selection process and for the construction and long-term safety of underground repositories. Furthermore, this poster presentation will show how the constitutive model was evaluated in relation to the dilatancy criterion and how the BARIK constitutive model's suitability for conducting an integrity assessment was validated. In conclusion, the development of BARIK is a significant step forward in the understanding and modeling of the complex material behavior of fractured crystalline hard rock. This contribution will provide insights into the development and verification of this model for the safe disposal of radioactive waste.

BARIK: laboratory programme within the framework of the development of an extended Hoek–Brown-based anisotropic constitutive model for fractured crystalline rock

Abstract. The disposal of heat-generating radioactive waste in deep geologic formations is a global concern. Numerical methods play a key role in understanding and assessing the disposal scenarios of radioactive waste in deep geological repositories. However, the complexities of the thermal, hydrological, mechanical, chemical, and biological processes associated with the disposal of radioactive waste in porous and fractured materials constitute significant challenges. One of the most challenging issues in this field is the complex material behaviour of fractured crystalline rock. The presence of fractures makes the rock anisotropic, nonlinear, and dependent on loading paths. Additionally, the Biot coefficient cannot be considered constant throughout the critical and subcritical fracture development regions. These factors make the development of an accurate constitutive model for fractured crystalline hard rock a critical component of any deep geological disposal project. Furthermore, to demonstrate the integrity of the “containment effective rock area” in crystalline host rock, the qualitative integrity criteria must be quantified so that numerical simulation can be performed with concrete numerical values. Part of this assessment for a crystalline host rock is a dilatancy criterion, which is currently based on the Hoek–Brown constitutive model. This contribution provides an overview and first results of the laboratory programme, which was performed as a part of the research project. The aim was to generate a fundamental and extensive dataset for an anisotropic material used for verification and validation purposes of the newly developed constitutive model. The rock material for the test programme was “Freiberger gneiss” because of its pronounced anisotropic properties regarding deformation and strength as well as the good access to obtain a larger amount of sample material. A wide range of basic tests have been performed, e.g. determination of density and porosity, measurement of ultrasonic wave velocities to get dynamic elastic properties, Brazilian tests, and uniaxial compression tests to obtain strength data. Additionally, a large number of more complex multi-stage triaxial compression tests with examination of the post-failure region and hydromechanical coupled triaxial compression tests have been conducted or are in progress. The hydromechanical part, in particular, plays an important role in examining and quantifying the evolution of the micromechanical damage process, which changes the permeability and Biot coefficient and therefore the effective stresses inside a saturated rock material. Tests were conducted with different orientations of the structural planes in relation to the loading directions.

Geology and evolution of fissure systems in fractured basement rocks, Calabria, southern Italy: implications for sub-unconformity reservoirs and aquifers

Basement-hosted fissure-fill networks in sub-unconformity settings are increasingly recognized globally and have the potential to act as important subsurface reservoirs and/or migration pathways for hydrocarbons, geothermal fluids and groundwater. We examine well-exposed fissures from exhumed crystalline upper Carboniferous basement rocks in southern Italy (Calabria) and describe their nature, origin and evolution. The basement rocks record the emplacement and exhumation of their plutonic protoliths. The evolution of these rocks includes their initial intrusion in the late Carboniferous, followed by veining, folding and rifting events, to eventual exhumation at the surface when fissuring occurred in the mid-Miocene. The fissure network hosts fossiliferous marine sediments, wall rock collapse breccias and limited mineralization with vuggy cavities. In the basement below the main erosional unconformity, fissure-fills form up to 50% by volume of the exposed rock. The fills are notably more porous (up to 15–25% matrix porosity) than the ultra-low-porosity (<1%) crystalline host rocks. We present field observations, palaeostress analyses of fault slickenlines and fracture topology analyses demonstrating that these exceptionally well-connected fissure networks are related to rifting and penetrated to depths of at least 150 m below the main Miocene erosion surface.

Impacts of Crystalline Host Rock on Repository Barrier Materials at 250 °C: Hydrothermal Co-Alteration of Wyoming Bentonite and Steel in the Presence of Grimsel Granodiorite

Direct disposal of dual-purpose canisters (DPC) has been proposed to streamline the disposal of spent nuclear fuel. However, there are scenarios where direct disposal of DPCs may result in temperatures in excess of the specified upper temperature limits for some engineered barrier system (EBS) materials, which may cause alteration within EBS materials dependent on local conditions such as host rock composition, chemistry of the saturating groundwaters, and interactions between barrier materials themselves. Here we report the results of hydrothermal experiments reacting EBS materials—bentonite buffer and steel—with an analogue crystalline host rock and groundwater at 250 °C. Experiment series explored the effect of reaction time on the final products and the effects of the mineral and fluid reactants on different steel types. Post-mortem X-ray diffraction, electron microprobe, and scanning electron microscopy analyses showed characteristic alteration of both bentonite and steel, including the formation of secondary zeolite and calcium silicate hydrate minerals within the bentonite matrix and the formation of iron-bearing clays and metal oxides at the steel surfaces. Swelling clays in the bentonite matrix were not quantitatively altered to non-swelling clay species by the hydrothermal conditions. The combined results of the solution chemistry over time and post-mortem mineralogy suggest that EBS alteration is more sensitive to initial groundwater chemistry than the presence of host rock, where limited potassium concentration in the solution prohibits conversion of the smectite minerals in the bentonite matrix to non-swelling clay species.

Investigations of effect of underground excavations using hydrologic and tracer transport modeling

Understanding the effect of underground excavations and barrier capability of the near field are relevant to the geologic disposal of nuclear waste. For a repository in crystalline host rock characterization of the naturally fractured rock and the excavated disturbed zone are of importance to the study of migration of radionuclides. Excavations could weaken the repository components that are part of the engineered barrier system. Permeable excavated disturbed zones could also develop around the repository walls that could create conduits for radionuclides away from the repository. Experimental hydrology and tracer transport data from the Mizunami Underground Research Laboratory in Central Japan have been used to develop a site-scale model for the study area. Upscaled permeability and porosity data of a previously developed discrete fracture network model were used. In this study development and utilization of the model is presented. The model was used to reproduce hydrology and non-reactive transport in the modeling area. The modeling work includes prediction of inflow of water into the research tunnel during excavation of tunnel parts and related draw-down. The modeling work also includes prediction of pressure recovery and changes in tracer distribution as a result of water filling of a tunnel chamber. Modeling results were successfully compared with the project hydrology and tracer concentration histories. Modeling results of inflow and recovery were complementary and together with fracture characterization combined to give a better constraint on the system. The study provides experimental and modeling methods to study the effect of excavations in an underground research laboratory.

Exploring the governing transport mechanisms of corrosive agents in a Canadian deep geological repository

All nuclear energy producing nations face a common challenge associated with the long-term solution for their used nuclear fuel. After decades of research, many nuclear safety agencies worldwide agree that deep geological repositories (DGRs) are appropriate long-term solutions to protect the biosphere. The Canadian DGR is planned in either stable crystalline or sedimentary host rock (depending on the final site location) to house the used nuclear fuel in copper-coated used fuel containers (UFCs) surrounded by highly compacted bentonite. The copper-coating and bentonite provide robust protection against many corrosion processes anticipated in the DGR. However, it is possible that bisulfide (HS−) produced near the host rock-bentonite interface may transport through the bentonite and corrode the UFCs during the DGR design life (i.e., one million years); although container performance assessments typically account for this process, while maintaining container integrity. Because the DGR design life far exceeds those of practical experimentation, there is a need for robust numerical models to forecast HS− transport. In this paper we present the development of a coupled 3D thermal-hydraulic-chemical model to explore the impact of key coupled physics on HS− transport in the proposed Canadian DGR. These simulations reveal that, although saturation delayed and heating accelerated HS− transport over the first 100s and 10,000s of years, respectively, these times of influence were small compared to the long DGR design life. Consequently, the influence from heating only increased total projected HS− corrosion by <20% and the influence from saturation had a negligible impact (<1%). By comparing the corrosion rate results with a simplified model, it was shown that nearly-steady DGR design parameters governed most of the projected HS− corrosion. Therefore, those parameters need to be carefully resolved to reliably forecast the extent of HS− corrosion.

Preliminary safety assessments in the high-level radioactive waste site selection procedure in Germany

Abstract. The Federal Company for Radioactive Waste Disposal (BGE) is a German waste management organization responsible for implementing the search for a site with the best possible safety for the disposal of high-level radioactive waste for at least 1 million years, following the amendments of the Repository Site Selection Act in 2017. The selection procedure is meant to be a participatory, transparent, learning and self-questioning process based on scientific expertise. This contribution provides an overview of the methodology of the forthcoming preliminary safety assessments as a major part of the next steps in the site selection procedure. This procedure overall consists of three phases with increasing levels of detail for identification of the best site. The first phase consists of two steps. The objective of the first step was to determine sub-areas in the three considered host rocks, salt (halite), claystone and crystalline host rock, by applying legally defined exclusion criteria, minimum requirements and geoscientific weighing criteria. A total of 90 sub-areas that cover approximately 54 % of the area of Germany were identified due to their general suitable geological conditions. The results were published in September 2020. The second step of phase one is currently in progress and includes representative preliminary safety assessments that aim to assess the extent to which the safe containment of the radioactive waste can be expected in the investigated sub-area. The requirements for conducting preliminary safety assessments in the site selection procedure are defined by a governmental directive released in October 2020. Representative preliminary safety assessments have to be performed for each sub-area and consist of the compilation of all geoscientific information relevant to the safety of a repository, the development of preliminary safety and repository concepts and the analysis of the disposal system. In addition, a systematic identification and characterization of uncertainties has to be undertaken and the need for exploration, research and development must be determined. The application of the representative preliminary safety assessments as well as the following renewed application of geoscientific weighting criteria will lead to the identification of siting regions within the larger sub-areas identified in step one. These regions will be considered, first for surface-based geoscientific and geophysical exploration, including e.g. seismic exploration and drilling of boreholes. Subsequently, the last phase of the site selection will proceed with subsurface exploration. Finally, all suitable sites will be proposed and the German government and parliament will decide the actual site. This process is expected to be finalized in 2031.

A systematic approach for surface exploration of sites – analysis of international exploration programs

Abstract. The site selection procedure for a high-level radioactive waste repository in Germany is based on the Repository Site Selection Act (StandAG, 2017), which comprises three phases. In phase 2 the Federal Company for Radioactive Waste Disposal (BGE) will conduct surface exploration. Based on the exploratory findings, the further developed preliminary safety analyses, the common requirements and criteria, and potential socioeconomic analyses will be applied feeding into proposed sites for underground exploration. Commissioned by the BGE, the Federal Institute for Geosciences and Natural Resources (BGR) contributes to this procedure with the projects GeoMePS and ZuBeMErk, which collate and assess geoscientific and geophysical methods and programs for surface exploration. Their common goal is to develop recommendations for surface exploration of siting regions. For this purpose, the BGR has developed a systematic approach that includes (1) deducing exploration targets, (2) compilation of geoscientific and geophysical exploration methods in a database structure, and (3) analysis of case studies of national and international exploration programs for high-level radioactive waste disposal. Exploration targets are based on the common criteria and requirements as defined by the StandAG. The identified exploration targets (Kneuker et al., 2020) together with a large number of geoscientific and geophysical exploration methods were integrated and linked within the BGR database “GeM-DB”. All methods were evaluated according to their suitability and applicability for (a) the three defined host rocks (crystalline rock, claystone, rock salt) and (b) the previously defined exploration targets. In step (3) the BGR reviews national and international waste disposal programs exploring for crystalline rock, claystone, and rock salt. Here, the focus is on nondestructive and minimally invasive surface exploration techniques, such as geophysical airborne and ground-based methods or investigations in drill holes and on drill cores. The aims are to identify gaps in the method catalogue of the GeM-DB and to infer exploration directives for surface exploration during phase 2. An example is the analysis of the Swedish site selection process, especially the site investigation program. There, the site investigations are, e.g. the basis for the discipline-specific site descriptive models, which were applied for design and safety assessments (SKB, 2001). The Swedish site investigation program along with programs of other countries considering crystalline host rocks, such as Finland and Canada, show a common ground, which could be adapted for surface exploration of crystalline host rock regions in Germany. The assessment and evaluation of selected programs exploring for rock salt and claystone is currently in progress. The entire systematic approach of the projects GeoMePS and ZuBeMErk aims to develop recommendations for a nondestructive and minimally invasive surface exploration program of siting regions in Germany, regarding the lithological, structural, mechanical, and hydrogeological characterization of the different host rock formations.

Discussion of parameters used to distinguish suitable from less suitable HLRW bentonites

Abstract. Bentonites will be used in the construction of some high-level radioactive waste (HLRW) repositories mostly in combination with crystalline host rocks. They will be used both as a geotechnical barrier (compacted bentonite blocks) around the canisters and for backfilling. The bentonite should be stable in contact with cement pore water, minimize metal corrosion, be stable against erosion and various salt solutions, retard radionuclides, prevent canister displacement, possess high thermal conductivity, be stable against radioactive radiation, keep its swelling capacity even when dried, and, most importantly, should have a low hydraulic conductivity. Bentonites are natural materials (clays) which are dominated by swelling clay minerals called smectites. All bentonites, therefore, possess high water uptake capacity, swelling, and cation exchange properties. Different bentonites from different deposits worldwide differ with respect to their chemical and mineralogical composition, composition and charge distribution of the smectites, particle size and morphology, microstructure (arrangement of particles relative to each other), and interlayer population. All these parameters affect the performances of bentonites in different applications. The bentonite industry, therefore, compares different bentonites based on empirical investigations to produce superior products. Specifications which could be used to select a suitable HLRW bentonite were discussed by Kaufhold and Dohrmann (2016). Additional information has been published later (Kaufhold et al., 2020a, b). First of all, some of the above listed desired bentonite properties depend more on the degree to which it is compacted compared to the natural variability. High compaction decreases the hydraulic conductivity and increases thermal conductivity. In order to prevent canister displacement only a small swelling pressure is needed which is easily achieved by compaction with all bentonites. Generally, the type of exchangeable cation is the most important parameter determining bentonite properties such as swelling and rheology. Large scale deposition tests, however, proved that the cation population will readily equilibrate with the surrounding water. The initial type of exchangeable cation is, therefore, less relevant. More important is the Fe content which negatively affects the thermal and chemical stability. Structural Fe of the smectites can be reduced or oxidized by bacteria and radiation. The Fe content of the bentonite should therefore be low. Highly charged smectites proved to be less corrosive in combination with iron canisters because they provide more reducing conditions compared to low charged ones. Bentonites containing highly charged smectites should be preferred if Fe canisters are used. In the case of Cu canisters no effect of the charge could be found. Also, soluble or at least partly soluble components such as sulphates, sulphides, carbonates, and organic matter should be absent since their possible dissolution would decrease the dry density and hence the swelling pressure. The presence of reactive silica in some bentonites proved to buffer the dissolution reactions at the cement bentonite interface and hence could have a beneficial effect.

The anisotropic properties of granites – effects of tectonic emplacement mode on potential crystalline host rocks for nuclear waste deposits in Germany

Abstract. For permanent nuclear waste disposal sites, crystalline rocks, especially granitic/granodioritic batholiths, are considered an appropriate host rock. Principally, three types of granitic plutons occur in the extra-alpine crystalline basement of Germany that were consolidated during the late Paleozoic Variscan orogeny of Central Europe: (i) Pre-Variscan voluminous granodiorites that are hardly affected by the subsequent continent–continent collision; (ii) voluminous granites in various tectonic settings intruded during the late orogenic stage of the Variscides; (iii) post-orogenic granites related to vast Permian intracontinental extension. Thus, in terms of the syn-intrusive tectonic setting and post-intrusive processes there are significant differences. Although it can be expected that different tectonic environments caused significant differences in the material properties, for Germany, however, there is no systematic study regarding the fabric of such plutonites. In order to find the most suitable “granite” we investigate the primary anisotropy of granites evolved during the emplacement and crystallization of the melt. For this we sample rocks of all three principal types and various syn-intrusive tectonic settings, i.e., compression, extension, strike-slip, transtension, and transpression. By means of combined measurements of the “Anisotropy of the Magnetic Susceptibility” and the “Shape Preferred Orientation” we characterize the syn-intrusive flow pattern, i.e., the magmatic foliation and lineation. The Crystallographic Preferred Orientation is analyzed by a combination of neutron time-of-flight experiments and electron backscatter diffraction measurements at the Frank Laboratory of Neutron Physics at JINR, Dubna, Russia, and the TU Bergakademie Freiberg respectively. Furthermore, special attention is given to the systematic mapping of annealed microcracks evolved during late magmatic fluid escape and/or post-crystallization hydrothermal activity. In a second step we compare the primary anisotropy with the post-magmatic fracture pattern of the particular granites. Those fractures constitute probable fluid pathways and, thus, the first-order risk for a potential permanent nuclear waste disposal. All datasets are organized in a Geological Information System allowing for a complete traceability of the different investigation steps. The results of this study will serve as a basis for a future detailed exploration.

Potential impact of cementitious leachates on the buffer porewater chemistry in the Finnish repository for spent nuclear fuel – A reactive transport modelling assessment

Cementitious materials will be used during the construction and operation of a geological repository for spent nuclear fuel at Olkiluoto in Finland. Upon contacting water, cement dissolution will generate high-pH leachates, which might eventually reach the proximity of a canister deposition hole through an interconnected network of fractures in the crystalline host rock. Highly-alkaline conditions near the bentonite buffer surrounding the canister could affect the performance of the buffer safety functions. In this study, we evaluate the potential impact such cement leachates might have on the chemical composition of the buffer porewater over time-frames relevant for the safety assessment of the repository.Although a comprehensive mechanistic assessment of these interactions is not possible due to their complexity, we demonstrate that key processes and their impact on the chemical composition of the bentonite porewater can be bounded. To this end we apply a reactive transport modelling based on an analysis of processes and parameter values. The model considers a 3D geometry including: the canister, the bentonite buffer, and a discrete fracture in the rock intersecting the deposition hole. Cement leachates flow through the fracture around the deposition hole, while solutes exchange with the buffer porewater via advective and diffusive mass transfer.Modelling results indicate that a combination of restricted water flow within the fracture, slow diffusive solute transport in the buffer, and chemical reactions will act together to minimise the extent of buffer porewater perturbation, should cement leachates reach the vicinity of a deposition hole. The model pessimistically estimates a maximum pH variation to be below 0.1 unit, and a maximum concentration change of about factor 3 (relative to initial) for reacting components within most of the buffer volume. This is only about double the perturbation values predicted for “natural” evolution of the buffer porewater, in the absence of cement leachates.Model uncertainties are evaluated by a series of sensitivity cases. These calculations suggest that additional processes, not directly accounted for in the base model (such as leachate-groundwater mixing and dilution on transport through the fracture network, and porosity reduction in the buffer due to mineral precipitation), could significantly contribute to a further reduction of the magnitude of potential buffer porewater perturbation.

Impact of rock fracture geometry on geotechnical barrier integrity – A numerical study

The effect of fracture geometry on bentonite erosion for a generic repository site in crystalline host rock environment was investigated by means of 2-d numerical simulations. Fracture geometry was varied systematically using random aperture normal distributions with a mean aperture of 1 mm and standard deviations between 0 and 0.7 mm, respectively. Moreover, two aperture correlation lengths (0.2 m and 2 m) were applied. Based on the synthetic fracture aperture fields generated the cubic law in conjunction with the Darcy equation is used to simulate fracture flow fields for mean flow velocities in the fracture between 1 × 10−5 m/s and 1 × 10−7 m/s. These flow fields are used in a two-way coupling approach to bentonite erosion simulations.The results of the study clearly show the influence of variable fracture aperture on bentonite erosion behaviour and erosion rates (kg/a). Increasing fracture aperture standard deviation leads to increasing heterogeneous flow velocity distributions governing the erosion behaviour and erosion rates. Calculated steady state erosion rates are in the range of ~0.25 kg/a down to ~0.014 kg/a. The highest erosion rate is calculated for the highest mean flow velocity in conjunction with the highest standard deviation. The effect of aperture heterogeneity diminishes for the lowest flow velocities.In summary, the results show the effect of fracture heterogeneity on bentonite erosion, especially for high to medium mean flow velocities combined with high to medium fracture heterogeneity under the model boundary conditions and model capabilities and limitations considered. An increase of up to ~83% in erosion rate compared to the constant aperture case highlights the need to consider fracture aperture heterogeneity and its effect on the bentonite erosion in the assessment of the safety and evolution of a high-level nuclear waste repository.

Numerical analysis of coupled hydro-mechanical and thermo-hydro-mechanical behaviour in buffer materials at a geological repository for nuclear waste: Simulation of EB experiment at Mont Terri URL and FEBEX at Grimsel test site using Barcelona basic model

We investigated the applicability of a thermo-elasto-plastic model using a TOUGH2-MP/FLAC3D simulator to reproduce the coupled hydro-mechanical behaviour at the engineered barrier emplacement experiment in Opalinus Clay and the coupled thermo-hydro-mechanical behaviour at the full-scale engineered barrier experiment in a crystalline host rock, as part of the DECOVALEX-2019 project Task D. In the numerical models, we adopted Darcy's law, Fick's law, and Fourier's law for the advective flux of fluid, vapour diffusion, and conductive heat flux, respectively. We also used the mechanical constitutive law, which explicitly considers the effects of temperature, pore pressure, suction, and strain changes using the elastic model and the Barcelona basic model. In general, the numerical models in the simulator successfully represented the coupled behaviour in bentonite buffer materials at the two long-term in situ experiments in terms of evolution and spatial distribution for temperature, relative humidity, total stress, and displacement. The performance of the models for the dismantling of the in-situ experiments is globally satisfactory compared with the post-mortem results for saturations, water contents, and dry density.