Abstract Geological disposal provides the safe long-term management solution for higher-activity radioactive waste. The development of a repository (or geological disposal facility) requires a systematic and integrated approach, taking into account the characteristics of the waste to be emplaced, the enclosing engineered barriers, and the host rock and its geological setting. Clays and clayey material are important in the development of many national geological disposal systems. Clays exhibit many interesting properties, and are proposed both as host rocks and as material for engineered barriers. Whatever their use, clays present various characteristics that make them high-quality barriers to the migration of radionuclides and chemical contaminants. As host rocks, clays are, in addition, hydrogeologically, geochemically and mechanically stable over geological timescales (i.e. millions of years).

Abstract Due to their particularly good mechanical and self-healing properties combined with exceptionally efficient cation adsorbents and exchanger capacities, clay minerals and clay rock formations are considered as suitable geological barriers for radioactive waste disposal. The Middle Jurassic Opalinus Clay Formation has been identified as a potential host rock. Logging data were measured at the Benken borehole drilled through this formation in northern Switzerland. This paper presents a statistical methodology to improve the description of the physical properties of the clay rock based on the well-log data. The methodology involves the classification of a set of local statistics, calculated from a reduced number of principal components computed from well-log properties. The use of a kernel-based method to calculate local statistics allows an analysis of spatial variability to be carried out at different scales, and with different scale effects. The first-order layering was found to be robust and independent of kernel size (i.e. observation scale), while preserving small-scale heterogeneities that are useful for further interpretation. The log units can be more clearly interpreted in terms of stationary or transitional log units, depending on the behaviour of local statistics. Finally, the derived spatial variability of the log-units properties are compared with earlier lithological descriptions and stratigraphic data. Supplementary material: A spreadsheet summary with the determination of clustering parameters for a kernel size of 3 m is available at https://doi.org/10.6084/m9.figshare.c.4315991

Abstract Stratigraphic sequences in boreholes are commonly estimated by interpreting combinations of well logs. The interpretation is generally tedious and is made some time after log completion, which often leads to a loss of valuable first-hand information gathered on-site. This may lead to delayed or potentially poor on-site decisions. To make things worse, the standard interpretation of well logs is, at least to a certain degree, subjective and based on the manipulation of data, which may be difficult to trace in the long term. Small changes in lithology are often disregarded and alternating thin layers presenting different lithologies are often combined in one single (notably thicker) stratigraphic unit. Therefore an automatic parameter-based and thus traceable and objective quick look at the lithology immediately after log completion represents both a valuable tool to help with on-site decisions and a solid, mathematically based starting point for further physically based interpretations carried out by log analysts. We present a workflow for the interpretation of well logs defined as an optimization problem. The workflow is applied to the characterization of metre- to decametre-scale stratigraphic units along 13 boreholes in northern Switzerland (one-dimensional resolution) and to millimetre-scale features over a wall at the Mont Terri underground rock laboratory in Switzerland (two-dimensional resolution). The results show that: (1) the workflow accurately maps lithological changes; (2) the interaction with the analyst is minimized, which reduces the subjectivity of the interpretation; and (3) outputs are available for on-site decisions.

Abstract A prediction–evaluation approach is developed to assess the propagation of parameter, conceptual and scenario uncertainties in the estimated near-field temperatures of the full-scale emplacement experiment at the Mont Terri rock laboratory. The uncertainty assessment is performed using a three-dimensional thermo-hydraulic numerical model of the full-scale emplacement experiment that represents the emplaced materials and surrounding Opalinus Clay and accounts for heat generation at the heaters. The propagation of parametric uncertainties is assessed using a first-order second-moment method supplemented by Monte Carlo simulations sampling the uncertain parameter space. The risk of uncertain parameters resulting in the failure of the maximum temperature criteria is evaluated with a first-order reliability method. Conceptual and scenario uncertainties are evaluated with deterministic simulation variants. After the conclusion of predictive modelling, a mid-term evaluation of the temperature predictions is performed through a comparison with measurements after 2.5 years of heating. The comparison indicates that the best estimates of temperature agree well with the measurements and that the 95% error bands assessed with parametric uncertainty envelope the measured values in almost all locations. Additional comparison with the measured degree of water saturation and the relative humidity is performed to assess the hydraulic behaviour and set the ground for the long-term evaluation, which will include predictions of the near-field pore pressures.

Abstract Boda Claystone is a very tight clayey rock with extreme low porosity and permeability, nano-size pores and small amounts of swelling clays. Due to this character it is ideal as a potential host rock for research into the possibilities of high-level waste deposition in geological formation. Though the research started more than 30 years ago, the genesis, the geotectonic history of the Boda Claystone Formation (BCF) and the geology of surrounding areas has only been sketched out recently. On the basis of research of the past few years the process of sedimentation of different blocks was able to be reconstructed. Equipment and methodological developments were needed for the investigation of reservoir geological and hydrodynamic behaviour of this rock, which began in the early 2000s. Based on them the pore structure and reservoir could be characterized in detail. Only theoretical approaches were available for the chemical composition of free porewater. Traditional water-extracting methods were not adaptable because of excessively low porosity and nano-scale pore size distribution. Hence, new ways have to be found for getting enough water for analysis. These new results of BCF research help to prepare more sophisticated and directed experiments, in which there is a great interest internationally.

Abstract Deep geological disposal of high-level radioactive waste (HLW) in a repository with a system of engineered and natural barriers has been recognized as an appropriate disposal concept by Chinese authorities since 2003, and both crystalline rocks and argillaceous rocks are considered as the candidate host rocks for HLW disposal repository. The 1:200 000 regional survey indicated that there are potential clay formations in Mesozoic–Cenozoic sedimentary basins in NW China. Five candidate areas have been suggested with potential clay formations including the Tamusu and Suhongtu areas with upper K 1 Bayingebi clay formations in the east Bayingebi Basin, in the Inner Mongolia Autonomous Region. On the basis of a detailed ground geological, hydrological and geophysical surveys, two test boreholes drilled to a depth of 800 m in the Tamusu area revealed that there are three lacustrine-facies clay formations (K 1 b 2-3 , K 1 b 2-2 and K 1 b 2-1 ). The thickness of the K 1 b 2-3 and K 1 b 2-2 clay formations is about 300–600 m with sandstone and siltstone interbeds, while the thickness of the K 1 b 2-1 homogeneous clay formation is more than 200 m with the depth of 450 m below the surface. The spatial extension of the clay formations could meet the fundamental criteria to ensure the long-term safety of the repository. Initial mineralogical studies on core samples indicated that the mineral assemblage is dominated by analcite, kaolinite, illite and dolomite. The homogeneous argillaceous rocks rich in analcite in Tamusu area could be a new type of host rock for a HLW disposal repository.

Abstract Hydraulic testing has revealed dramatic underpressures in Paleozoic shales and carbonates at the Bruce nuclear site in Ontario. Although evidence from both laboratory and field studies suggests that a small amount of gas-phase methane could be present in the shale, previous studies examining causal linkages between the gas phase and the underpressure have been inconclusive. To better elucidate processes in such a system, we used a highly simplified 1D representation of the site to test, by using iTOUGH2-EOS7C, the effects of various factors on the evolution of gas-phase methane and pressures within the system. Heterogeneity was represented by three stratigraphic regions with slightly different capillary pressure characteristics and, in one case, three thin distinct zones with very different characteristics. Underpressure occurred only when gas pressures set as an initial condition required it, and even in this case it was geologically short-lived. We conclude that the presence of multiple fluid phases is unlikely to explain the underpressure at the site; we suggest that the influence of gas-phase methane on porewater flow is minimal. This is consistent with prior conceptualizations of the underpressured section as a thick aquiclude, in which solute transport occurs extremely slowly, bounded by aquifers of significantly higher permeability.

Abstract In a deep geological repository (DGR) for the long-term containment of radioactive waste, gases could be generated through a number of processes. If gas production exceeds the containment capacity of the engineered barriers or host rock, these gases could migrate through these barriers and potentially expose people and the environment to radioactivity. Expansive soils, such as bentonite-based materials, are currently the preferred choice of seal materials. Understanding the long-term performance of these seals as barriers against gas migration is an important component in the design and long-term safety assessment of a DGR. This study proposes a hydro-mechanical linear poro-elastic visco-capillary mathematical model for advective-diffusive controlled two-phase flow through a low-permeability expansive soil. It is based on the theoretical framework of poromechanics, incorporates Darcy’s Law for both the porewater and poregas, and a modified Bishop’s effective stress principle. Using the finite element method (FEM), the model was used to numerically simulate 1D flow through a low-permeability expansive soil. The results were verified against experimental results found in the current literature. Parametric studies were performed to determine the influence on the flow behaviour. Based on the results, the mathematical model looks promising and will be improved to model flow through preferential pathways.

Abstract A key component of the site comparison planned for the deep geological disposal of spent fuel and high-level waste (SF/HLW) in Switzerland is the assessment of the evolution of repository-induced perturbations in the repository nearfield associated with thermal effects from heat production due to radioactive decay of radionuclides, as well as gas pressures developing in the backfilled underground structures from the anaerobic corrosion of the steel waste canisters and tunnel support materials. The assessment of such effects is integrated in the site comparison through safety indicators used to evaluate repository performance. In this context, probabilistic assessments need to integrate the uncertainty of the entire ensemble of input parameters, and estimate the propagation to these indicators in a reliable and computationally efficient manner. This paper presents the development of a methodology for an indicator-based assessment of heat- and gas-induced effects in a SF/HLW repository in Opalinus Clay integrating a probabilistic treatment of parametric uncertainty. The workflow is demonstrated using preliminary data, repository configurations and indicators. Complementary simulations are presented to demonstrate the feedback to the optimization of repository design in order to mitigate repository-induced effects that can potentially compromise the safety function of the engineered and natural barriers.

Abstract Piping and erosion phenomena are serious problems affecting the integrity of buffer materials, which are an element of engineered barrier systems in the geological disposal of high-level radioactive waste. In this study, the outflow behaviour and the condition of buffer materials are investigated using a test pit drilled into host rock at the Horonobe Underground Research Laboratory to consider countermeasures to contain the outflow of the buffer material. The results are as follows: (1) Piping and erosion phenomena occur irrespective of the injection flow rate. However, when the rate is small, the buffer material is considered to be self-repairing and the outflow of the buffer material can be suppressed. (2) When the injection water contains large amounts of electrolytes, the surface of the buffer material peels off and precipitates, probably decreasing the waterproof performance. (3) Bentonite pellets are likely to be an effective countermeasure against piping and erosion.

Abstract Colloid concentration is an important parameter in models of colloid-facilitated transport. The purpose of the present study is to characterize colloid concentrations and colloid stability in natural groundwater from the Horonobe Underground Research Laboratory (URL) in Hokkaido, Japan. The particle sizes of colloids in groundwaters from the Horonobe URL range from several nanometres to c . 500 nm, with a mode particle size of c . 120 nm. Evaluation of colloid stability by DLVO theory suggests that larger colloids (i.e. >100 nm in diameter) would be more stable than smaller colloids in some groundwaters. The estimated colloid particle concentrations when considering the results of DLVO calculations ranged from 2.33 × 10 6 to 1.12 × 10 8 particles/ml, and mass concentrations were estimated to range from 45 to 1540 µg l −1 for diameters greater than 100 nm. Colloids in Horonobe groundwaters appear to be less stable, with a moderate potential for transport, than colloids investigated in similar international studies. This reduced stability may be due to relatively higher ionic strengths and moderate dissolved organic concentrations in Horonobe groundwaters compared to their international counterparts.

Abstract Bentonite-based buffer materials play an important safety role in engineered barriers planned for use in geological disposal repositories for radioactive high-level waste (HLW) in Japan. The effectiveness of buffer materials is dependent on the status of groundwater saturation during resaturation of the repository. Accordingly, it is important to determine the behaviour of buffer materials during saturation and predict post-saturation conditions such as the distribution of residual dry density and chemical alteration. In this study, the rate of groundwater uptake into a buffer material was determined to clarify the behaviour of the material during the saturation process. As mechanical changes and chemical alteration of buffer materials are generated by groundwater permeation, knowledge of the water uptake rate is necessary for the prediction of post-permeation conditions. In the experiment reported here, one-dimensional permeation by distilled water and a NaCl water solution at a constant rate was monitored over a period of more than seven years. The results indicated that the seepage and saturation front moved in proportion to the square root of the seepage time. The coefficient of the relationships between the seepage and the saturation fronts with time of the reference bentonite used in Japan was determined.

Abstract Self-sealing tests were carried out on cylindrical samples artificially cracked on one-third of the diameter with a perfectly controlled aperture. Water was then injected into the crack. An innovative cell was used that had been developed, the body of which is transparent to X-rays. The sample could fully rotate in the nanotomograph, allowing a 3D reconstruction of images before, during and after tests, a visualization of the evolution of the cracked zone, and a quantification of the variations in crack volume during self-sealing. Permeability measurements were made to quantify the influence of self-sealing on flows. In the present work, two facies of claystone with different CaCO 3 contents were tested. In the clay-rich sample, an important but not total, reduction in volume was observed, as well as a large decrease in permeability, even if a safe claystone value was not recovered and a two-phase kinematic occurred. On the CaCO 3 -rich samples, a small volume reduction of the fracture was observed with a small decrease in water permeability. The influence of the mineralogy on the self-sealing capacity of the claystone was demonstrated and a threshold of carbonate content of around 40% was exhibited to discriminate samples able to self-seal from those that were not.

Abstract The speciation of selenium (Se) in clay-rich host rocks is important within the framework of geological disposal of radioactive waste since it affects its migration. Removal of selenite from formation water can be caused by reduction and adsorption. Reduction could potentially be inhibited or delayed by adsorption. Here, the interplay of adsorption and reduction of selenite was investigated in batch experiments with Boom Clay and its separated size fractions. In all experiments, dissolved Se concentrations (Se aq ) showed a fast initial decrease that was followed by a slower decline until removal was almost complete. X-ray absorption spectroscopy indicated that adsorption of selenite accounted for the fast removal of Se aq followed by slower selenite reduction. Eventually, almost all solid-bound Se IV became reduced to Se 0 in all experiments. The progress of Se aq removal and Se IV reduction to Se 0 could be described by a kinetic model involving reversible adsorption on clay minerals and reduction by pyrite. This implies that the reduction of selenite to Se 0 is not significantly hindered or delayed by selenite adsorption on clay minerals. Pyrite is probably the most relevant reductant for selenite in Boom Clay, although reduction by Fe II structurally bound in clay minerals might provide an additional pathway for selenite reduction in clay rocks. Supplementary Material: X-ray diffractograms of separated clay-size, silt-size and total BC material are available as Supplementary Material . Also provided are particle size distributions of all materials and extra information on XANES and EXAFS results, Se concentrations through time for experiments with standard clay minerals and figures of the sensitivity analysis of the kinetic model. The information is available at https://doi.org/10.6084/m9.figshare.c.4363826

Abstract The Prototype Repository (PR) tunnel is located at the Äspö Hard Rock Laboratory near Oskarshamn in the southeast of Sweden. In the PR tunnel, six full-sized deposition holes (8.37 m deep and 1.75 m in diameter) have been constructed. Each deposition hole is designed to mimic the Swedish reference system for the disposal of nuclear fuel, KBS-3V. The PR experiment is designed to provide a full-scale simulation of the emplacement of heat-generating waste. There are three phases to the experiment: (1) the open tunnel phase following construction, where both the tunnel and deposition holes are open to atmospheric conditions; (2) the emplacement of canisters (containing heaters), backfill and seal in the first section of the tunnel; and (3) the emplacement of canisters, backfill and seal in the second section of the tunnel. This work describes the numerical modelling, performed as part of the engineered barrier systems (EBS) Task Force, to understand the thermo-hydraulic (TH) evolution of the PR experiment and to provide a better understanding of the interaction between the fractured rock and bentonite surrounding the canister at the scale of a single deposition tunnel. A coupled integrated TH model for predicting the wetting and the temperature of bentonite emplaced in fractured rock was developed, accounting for the heterogeneity of the fractured rock. In this model, geometrical uncertainties of fracture locations are modelled by using several stochastic realizations of the fracture network. The modelling methodology utilized information available at early stages of site characterization and included site statistics for fracture occurrence and properties, as well as proposed installation properties of the bentonite. The adopted approach provides an evaluation of the predictive capability of models, it gives an insight of the uncertainties to data and demonstrates that a simplified equivalent homogeneous description of the fractured host rock is insufficient to represent the bentonite resaturation.

Abstract Nuclear waste disposal in geological formations relies on a multi-barrier concept that includes engineered components – which, in many cases, include a bentonite buffer surrounding waste packages – and the host rock. Contrasts in materials, together with gradients across the interface between the engineered and natural barriers, lead to complex interactions between these two subsystems. Numerical modelling, combined with monitoring and testing data, can be used to improve our overall understanding of rock–bentonite interactions and to predict the performance of this coupled system. Although established methods exist to examine the prediction uncertainties due to uncertainties in the input parameters, the impact of conceptual model decisions on the quantitative and qualitative modelling results is more difficult to assess. A Swedish Nuclear Fuel and Waste Management Company Task Force project facilitated such an assessment. In this project, 11 teams used different conceptualizations and modelling tools to analyse the Bentonite Rock Interaction Experiment (BRIE) conducted at the Äspö Hard Rock Laboratory in Sweden. The exercise showed that prior system understanding along with the features implemented in the available simulators affect the processes included in the conceptual model. For some of these features, sufficient characterization data are available to obtain defensible results and interpretations, whereas others are less supported. The exercise also helped to identify the conceptual uncertainties that led to different assessments of the relative importance of the engineered and natural barrier subsystems. The range of predicted bentonite wetting times encompassed by the ensemble results were considerably larger than the ranges derived from individual models. This is a consequence of conceptual uncertainties, demonstrating the relevance of using a multi-model approach involving alternative conceptualizations.

Abstract A geological disposal facility (GDF) is the widely accepted long-term solution for the management of higher-activity radioactive waste. It consists of an engineered facility constructed in a suitable host rock. The facility is designed to inhibit the release of radioactivity by using a system consisting of engineered and natural barriers. The engineered barriers include the wasteform, used to immobilize the waste, the waste disposal container and any buffer material used to protect the container. The natural barrier includes the rocks in which the facility is constructed. The careful design of this multi-barrier system enables the harmful effects of the radioactivity on humans and biota in the surface environment to be reduced to safe levels. Bentonite is an important buffer material used as a component of a multi-barrier disposal system. For example, compacted bentonite rings and blocks are used to protect the copper container, used for the disposal of spent fuel, in the KBS-3 disposal system. As the bentonite saturates, through contact with groundwater from the host rock, it swells and provides a low hydraulic conductivity barrier, enabling the container to be protected from deleterious processes, such as corrosion. The characteristic swelling behaviour of bentonite is due to the presence of significant quantities of sodium montmorillonite. Recently, there have been detailed in situ experiments designed to understand how bentonite performs under natural conditions. One such experiment is the Buffer–Rock Interaction Experiment (BRIE), performed at the Äspö Hard Rock Laboratory near Oskarshamn in the SE of Sweden. This experiment is designed to further understand the wetting of bentonite from the groundwater flow in a fractured granite host rock. In this paper, the observations from the BRIE are explained using an integrated model that is able to describe the saturation of bentonite emplaced in a heterogeneous fractured rock. It provides a framework to understand the key processes in both the rock and bentonite. The predictive capability of these models was investigated within the context of uncertainties in the data and the consequence for predictions of the wetting of emplaced bentonite. For example, to predict the wetting of emplaced bentonite requires an understanding of the distribution of fracture transmissivity intersecting the bentonite. A consequence of these findings is that the characterization of the fractured rock local to the bentonite is critical to understanding the subsequent wetting profiles. In particular, prediction of the time taken to achieve full saturation of bentonite using a simplified equivalent homogeneous description of the fractured host rock will tend to be too short.

Abstract Geological disposal is the most realistic option for high-level radioactive waste in Japan. In considering long-term stability for geological disposal, several types of materials have been studied as engineered barriers with a host rock. We focused our study on metal and bentonite as engineered barrier materials and investigated the long-term corrosion tendency of the metal exposed to bentonite. An electrochemical method for inducing accelerated corrosion was studied in a laboratory, and we analysed some field samples from a FEBEX dismantling project (FEBEX-DP) in Switzerland for comparison with our experimental results.

Abstract A safety concept and a safety demonstration concept for the disposal of high-level radioactive waste in German clay formations have been developed. The main safety objective is to retain the radionuclides inside a ‘Containment Providing Rock Zone’. Thus, the radionuclide transport should be restrained by adequate safety functions of the geological and geotechnical barriers. The compliance with legal dose constraints has to be demonstrated for probable evolutions and less probable evolutions. As a basis for system analysis, generic geological reference models, disposal concepts and repository designs have been developed for northern and southern Germany. All data relevant for future system evolution were compiled in two FEP (features, events and processes) catalogues. They provide information on FEP characteristics, their probabilities of occurrence, their interactions and identify ‘initial FEP’ that impair the safety functions of relevant barriers. A probable reference scenario has been deduced systematically from the probable ‘initial FEP’, and from probable processes relevant for radionuclide mobilization and transport. Four different starting points to develop alternative scenarios (i.e. less probable evolutions) were identified. The scenario development methodology is applicable to different kinds of host rock and therefore may be a basis for the preliminary safety analyses necessary in the future site selection process in Germany.