Clay Rock Research Articles

Diffusive transport of uranium and americium through clay rock down to ultra-trace levels

Abstract. Clay rock is a potential host rock for the final disposal of nuclear waste in deep geological formations. In the scenario of ground water intrusion into the nuclear waste repository and subsequent corrosion of canisters and waste, possibly released actinides, such as uranium (U) and americium (Am), would be transported through the engineered barrier and clay host rock mainly by diffusion. Actinides are known to exhibit low solubility and are strongly sorbing under the reducing conditions of deep geological formations. Diffusion experiments are therefore difficult to perform due to analytical constraints. To our knowledge, the diffusion of U in clay rock has not been investigated below concentrations of 10-7molkg-1 clay (e.g., Joseph et al., 2013) and for Am, no diffusion experiments have been performed in a clay rock, considered suitable as host rock, such as Opalinus clay (OPA). This work aimed at the investigation of the diffusive behavior of U and Am down to ultra-trace concentrations (≪10-7molkg-1) in OPA. Laboratory-scale diffusion experiments were conducted with samples of OPA, obtained from the Mont Terri underground laboratory, Switzerland for up to 240 d. The OPA samples were placed in contact with synthetic pore water (I=0.22molL-1, pH=7.24), spiked with 5×10-9molL-1 of 233U and 243Am, respectively. After termination of the experiment, the OPA samples were segmented into thin layers of 20–400 µm. The obtained clay segments were analyzed for the 233U and 243Am content with accelerator mass spectrometry. Concentration profiles were determined down to 10-12molkg-1 clay for 233U and 5×10-10molkg-1 clay for 243Am. U showed a typical diffusion profile which can be interpreted by applying Fick's laws, while the Am profile was more complex, requiring further investigation of transport modes. These results will provide relevant insights into the transport behavior of U and Am in OPA over a wide range of concentrations down to ultra-trace levels.

AREHS: effects of changing boundary conditions on the development of hydrogeological systems: numerical long-term modelling considering thermal–hydraulic–mechanical(–chemical) coupled effects

Abstract. Within the framework of the “Gesetz zur Suche und Auswahl eines Standortes für ein Endlager für hochradioaktive Abfälle” (Repository Site Selection Act – StandAG), the geoscientific and planning requirements and criteria for the site selection for a repository for high-active nuclear waste are specified. This includes, among others, the modelling of hydrogeological scenarios such as how future cold and warm periods and associated glaciation events can change the (petro-)physical properties specified in the StandAG as well as the natural hydrogeological properties of the overall system through, for example, reactivation of faults or changes in hydraulic gradients and consequently flow directions. The main objective of the AREHS (Effects of Changing Boundary Conditions on the Development of Hydrogeological Systems) project, funded by BASE (Federal Office for the Safety of Nuclear Waste Management; FKZ 4719F10402), is to model the effects of changing external boundary conditions on the hydrogeologically relevant parameters and effects (e.g. hydraulic permeability, porosity, migration pathways, fluid availability, hydraulic gradients) of a generic geological repository in Germany in all three potential host rocks (clay, salt and crystalline rocks) and its surrounding hydrogeological setting (Table 1). Special attention is paid to the cyclic mechanical loading and unloading due to glaciation events and the resulting stress changes (M), as well as induced temperature effects (T) due to permafrost and warm periods. As such processes can cause changes in the coupled far-field regime with groundwater flow and groundwater supply (H), as well as fluid transport due to thermal (T) and chemical (C) gradients, and reactivate faults/fractures (M) and thus create new/additional pathways, they are particularly relevant to the integrity of a repository over a period of 1 million years and must be properly captured with coupled THM(C) modelling. Before a model is set up for the different host rocks, a detailed assessment of relevant processes has been conducted based on NEA-2019 FEP catalogue (NEA, 2019) for high-level waste repositories. The modelling is performed using generic 3D models of typical host rock formations satisfying the StandAG criteria. Although the models for salt and clay rock have been adapted from generic models from recent research projects, for crystalline rock a new generic model had to be developed (Fig. 1) considering discontinuities of different scales that have to be incorporated into the THM(C) models explicitly as DFN (Discrete Fracture Network) networks. This is done by coupling two numerical codes: DFN-lab and 3DEC. A central phase in the overall modelling process is the benchmarking of the models with data from existing models and with field-scale studies. This is done separately for all three host rocks. In addition to extending the modelling capacities for glaciation processes and verifying by corresponding benchmarking tests (analytical solutions and literature comparisons), automated workflows have been developed to generate OpenGeoSys models from GOCAD structure models. Script-based automated workflows improve software quality for site investigation, especially in a sense of modularization as well as reproducibility. The generic workflow concept is currently being tested for the literature-based benchmarks and will, therefore, support a persistent and sustainable benchmarking procedure in the future.

Measurement Methods for Above-ground Site Investigation Programmes according to StandAG (Project “MessEr”) – an Overview

Abstract. In Germany, the site selection process for a repository for high-level radioactive waste is stipulated by the Site Selection Act (Standortauswahlgesetz – StandAG). During the site selection process, the implementer has to identify and propose siting regions. The Federal Office for the Safety of Nuclear Waste Management (Bundesamt für die Sicherheit der nuklearen Entsorgung – BASE) as the responsible federal authority has to review the implementer's proposal and finally the siting regions remaining in the site selection process will be laid down by federal law. The corresponding above-ground site investigation programmes submitted by the implementer will also be reviewed and then determined by BASE. These programmes and subsequent revisions have to be published by BASE in the Federal Gazette (§ 15 StandAG). When reviewing as well as determining above-ground site investigation programmes, state-of-the-art science and technology have to be considered – especially in the field of applied measurement methods. For this reason, it is convenient to have a catalogue of relevant and available measurement methods. This overview shows the findings and results from the project “MessEr”, which was carried out by GRS gGmbH and initiated and funded by BASE (funding code/FKZ 4717F01202). To derive such a catalogue for the above-ground site investigation within the site selection process the project “MessEr” (Messmethoden für übertägige Erkundungsprogramme gemäß StandAG – Measurement methods for surface-based exploration programmes in accordance with the StandAG) was initiated by BASE. This catalogue should combine two key features: (i) it should be understandable not only for experts but also for interested members of the general public and (ii) it also should provide a basis to evaluate proposed site investigation programmes. Accordingly, the main task within the project was to compile above-ground measurement methods considering the three host rocks according to the StandAG – rock salt, clay rock and crystalline rock – and to classify them with regard to the kind of investigation, measuring principles and achievable knowledge. In a second step, every single measurement method had to be characterised and described in detail: advantages, disadvantages and efforts, as well as limitations and uncertainties. Furthermore, aspects such as applicability (with regard to the host rock), the combination of different methods and any possible needs of research and development were discussed. For further information, references such as standard textbooks, weblinks, journals or reports from other international repository projects were compiled for each method. The final step of “MessEr” was to evaluate Appendices 1 to 12 to § 24 StandAG (“geoscientific consideration criteria”) with respect to their significance and requirements for above-ground site investigation methods. This resulted in three matrices showing the correlations of required parameters and necessary knowledge according to the StandAG with the compiled measurement methods.

Identification and evaluation of processes for joint disposal of high level radioactive waste and low to intermediate level radioactive waste

Abstract. The on-going research project „Identification and evaluation of processes that can arise by disposing of both high level radioactive waste (HAW) and low to intermediate level radioactive waste (LAW/MAW) at the same site“ (GemEnd, FKZ 4719F10401), commissioned by the Federal Office for the Safety of Nuclear Waste Management (BASE), is concerned with the question which thermal, hydraulic, mechanical, chemical and biological (THMCB) processes could be of importance for the long-term safety of the geological repository for high level radioactive waste. The focus of the project is on mutual influences between the HAW and LAW/MAW repositories, which should be constructed separately according to the Safety Regulations (Federal Ministry for the Environment, Nature Conservation and Nuclear Safety, BMU, 2020). A second point of emphasis is on processes that could result from the disposal of small amounts of LAW/MAW within the HAW repository. The analyses carried out for each of the potential host rocks clay rock, rock salt and crystalline rock as well as for a combination of clay rock above crystalline rock at a generic site are divided into a qualitative and a quantitative part. As for the qualitative analyses, all potentially relevant processes are identified and evaluated as to whether they are negligible or principally relevant for the exemplary repository configurations considered and according to the current state of knowledge. With regard to the quantitative analyses, the possible extent of potentially safety-relevant processes is illustrated by means of coupled numerical simulations. Of special interest are the effects of particularly sensitive model approaches and/or parameters and notably of the distances between the HAW and LAW/MAW repositories in the different exemplary repository configurations considered. From the results of the quantitative and qualitative analyses, knowledge gaps will be identified and the possibility of their reduction by research and development activities will be discussed. Furthermore, aspects of the transferability of the results to the German site selection procedure will be illuminated. At the interdisciplinary research symposium safeND selected preliminary results of both the qualitative and quantitative analyses will be presented.

Radionuclide geochemistry: solubility and thermodynamics in an HLW repository

Abstract. Deep geological disposal is the internationally favoured option to isolate high-level nuclear waste (HLW) from the biosphere and to minimise the potential radiological risk for future generations. Potentially contacting aqueous solutions such as groundwater may, however, lead to the corrosion of the solid casks containing the nuclear waste, and the formation of aqueous radionuclide systems in the near-field of the emplacement rooms. As dissolved species, radionuclides can in principle further migrate into the far-field and finally reach the biosphere on medium and long timescales. Like all chemical species, the radionuclides are subject to fundamental (geo)chemical laws. Relevant reactions that control retention and release, and hence, the migration behaviour and fate of radionuclides in a repository, are solubility equilibria, formation of soluble complexes, redox reactions, sorption on and incorporation into mineral surfaces, transport phenomena etc. These processes depend directly on the (geo)chemical boundary conditions, and, consequently, can differ greatly for various host rock systems such as clay rock, rock salt, and crystalline rock. Many of the radionuclides in HLW are heavy metals that are sparingly soluble under various repository-relevant conditions, e.g. actinides, lanthanides, transition metals, so that only partial dissolution (mobilisation) from the solid waste matrices is expected. This underlines the importance of evaluating the radionuclide solubility within a geochemically based safety assessment for repositories as it provides reliable upper-limit concentrations of the mobile, potentially migrating radionuclide fraction in the near-field. In this contribution, we discuss relevant aspects related to the topic radionuclide solubility and thermodynamics in a HLW repository. This includes a summary of recent laboratory studies on the solubility behaviour and speciation of key radionuclides in repository-relevant solutions, which are an important basis for obtaining (geo)chemical information and models, and the corresponding fundamental thermodynamic constants on aqueous radionuclide systems. National and international thermodynamic database projects, where quality-assured thermodynamic data (solubility products, complex formation constants, and ion-interaction parameters) are evaluated and compiled, e.g. the Nuclear Energy Agency Thermochemical Database (http://www.oecd-nea.org, last access: 1 November 2021) or the Thermodynamic Reference Database (http://www.thereda.de, last access: 1 November 2021), are highlighted and the main remaining uncertainties discussed. The experimental information and the quantitative thermodynamic data are applied within a generic case study to demonstrate the impact of different geochemical solution conditions representing different host rock systems considered as HLW repositories in Germany on the solubility and speciation of selected radionuclides.

On the temperature in a final disposal site for high-level radioactive waste

Abstract. The Site Selection Act stipulates a precautionary temperature limit of 100 ∘C on the outer surface of the containers with high-level radioactive waste (HLRW) in the final disposal site. This precautionary temperature limit should be applied in preliminary safety analyses provided that the maximum physically possible temperatures in the respective host rocks have not yet been determined due to pending research. Increasing temperatures in the deep geological underground, caused by the heat generation of the HLRW, can trigger thermal, hydraulic, mechanical, chemical and biological processes (THMCB) in the respective host rocks of a final disposal site and thus endanger safety. Furthermore, high temperatures may hamper the feasibility to retrieve and recover HLRW from a final disposal site. Such processes are described in detail in databases for features, events and processes (FEP) databases. Single components or barriers of a final disposal facility may require specific design temperatures for the preservation of their features once a concept for long-term safety of a final disposal site is established; however, the interactions of all relevant processes of a concept for a final disposal site must be considered when a specific temperature limit for the outer surface of the containers is derived. This temperature limit may vary for particular safety and final disposal concepts in the host rock: salt, clay and crystalline rock. The conclusion is that temperature limits regarding the outer surface of the containers should be derived specifically for each safety and disposal concept and should be supported by a solid safety analysis. Temperature limits without reference to specific safety concepts or the particular design of the final disposal site likely narrow down the possibilities for optimisation and could adversely affect the site selection process in finding the best suitable site.

Geochemical benchmark tests to validate the conversion of thermodynamic data for TOUGHREACT

Abstract. According to the ongoing site selection process for a repository for high-level radioactive waste in Germany, rock salt, clay and crystalline rock are possible host rocks. The pore water of these rocks contains saline solutions with high ionic strengths. To model the speciation and/or migration of radionuclides in long-term safety analyses for nuclear waste disposal, a geochemical code that includes thermodynamic data suitable for saline solutions is needed. Thermodynamic equilibrium in saline solutions with high ionic strengths is usually modelled using the Pitzer approach (Pitzer, 1991). Within the context of nuclear waste disposal, the THEREDA project (Moog et al., 2015) provides thermodynamic data for some widely used geochemical codes (PHREEQC, Geochemist's Workbench, ChemApp, and EQ 3/6) using the Pitzer approach; however, for modelling in long-term safety analyses for nuclear waste disposal, another geochemical code, TOUGHREACT, is used. Therefore, scripts were developed to convert thermodynamic data of the THEREDA project to be applicable in TOUGHREACT. The scripts were validated by benchmark tests and by comparing calculations using PHREEQC and TOUGHREACT (Weyand et al., 2021). In total, 50 different benchmark tests were performed considering 3 specific geochemical systems, which are relevant to long-term safety analyses: (1) oceanic salt system, polythermal: K, Mg, Ca, Cl, SO4, H2O(l), (2) actinide system, isothermal: Am(III), Cm(III), Nd(III), Na, Mg, Ca, Cl, OH, H2O(l) and (3) carbonate system, isothermal: Na, K, Mg, Ca, Cl, SO4, HCO3/CO2(g), H2O(l). Each benchmark test considered specific ion concentrations in solution and in gaseous phases in the presence of specific minerals. The benchmark tests derived the geochemical equilibria and the results of both codes were compared to each other and to experimental data. The results of the calculations using both codes showed a good correlation. Remaining deviations can be explained by technical differences of the codes.

Site selection for the best clay-hosted repository in Switzerland

Abstract. The site selection process for the deep geological repositories in Switzerland is in its final phase. All three remaining sites fulfil the requirements of the repositories for low level waste (LLW) as well as for high level waste (HLW) and spent fuel. Using surface-based exploration methods, including 3D seismic studies and deep bore holes, Nagra has recently collected the necessary data to select the most suitable site. The sites will be compared based on 13 technical criteria defined in 2008 and already used in the 2 previous phases of the selection process. The criteria comprise safety-related aspects including the barrier properties and their long-term stability as well as the construction suitability of the repository and its access facilities. If one site offers optimal conditions for both HLW and LLW and the space requirements can be met at that site, a co-disposal facility will be proposed. This facility will then comprise separate emplacement areas with specific safety concepts for the different waste categories. In our contribution we present the overall approach for the surface-based exploration in the different stages of the site selection process. We show how we integrated third party data, seismic surveys, surface mapping as well as deep and shallow bore holes to drive the site selection process. Example data sets from the ongoing deep drilling campaign (clay content, hydrotest data, porewater isotope profiles etc.) and recent 3D seismic surveys are used to illustrate our approach to distinguish the remaining sites according to long-term safety and to underline the large safety margin of the selected clay host rock in long-term evolution scenarios.

Deciphering porosity clogging at barrier interfaces in deep geological repositories for radioactive waste

Abstract. The disposal of spent nuclear fuels and high-level radioactive wastes in deep geological repositories represents one of the greatest scientific-technical and societal challenges of our times. Most disposal concepts rely on a multibarrier system, consisting of a combination of engineered materials, geotechnical and geological barriers to provide a safe containment of the radioactive waste to protect humans and the environment against dangers arising from ionizing radiation. A reliable safety assessment of a deep geological repository over assessment time scales of several 100 000 years requires a profound and comprehensive understanding of the complex coupled physical (thermal, hydraulic, mechanical), chemical and biogeochemical (THM/CB) processes that govern the long-term evolution of the repository system. As a result of thermal and chemical gradients at the interfaces of different components and materials of the multi-barrier system (e.g. interfaces between metallic waste containers and bentonite backfill or between structural concrete and clay host rock), mineral dissolution and precipitation reactions are promoted; thus the (local) porosity, the volume filled with gas and/or water, can increase or decrease leading to changes in the macroscopic transport properties of the respective media. Although a reduction of the porosity (porosity clogging) appears to be desirable to inhibit radionuclide migration, it can also be detrimental, particularly in the case of gas pressure build-up due to canister corrosion or bacterial activity. So far, porosity clogging at barrier interfaces and associated consequences on solute or gas transport remain poorly understood; currently used mathematical descriptions of porosity clogging in reactive transport codes usually fail to capture respective experimental observations (Chagneau et al., 2015; Deng et al., 2021). In this context, we are developing a “lab-on-a-chip” set-up, which combines time lapse optical microscopy imaging and in operando Raman spectroscopy (Poonoosamy et al., 2019, 2020) to determine (i) whether complete clogging is possible and permanent, (ii) which parameters control the porosity clogging and (iii) which changes in transport properties of porous media are induced due to porosity clogging. Our approach comprises micronized counterdiffusion experiments with in situ visualization and monitoring of the evolution of mineralogy and microstructure/pore architecture with time. Complementary pore scale modelling will be used to derive key relationships that describe changes in transport properties due to mineral precipitation-induced porosity clogging. This approach will help to improve reactive transport codes and their predictive capabilities thus enhancing confidence and reduce uncertainties in long-term predictions, leading to more realistic descriptions of the evolution of complex repository systems.

Sub-areas in steep rock salt formations – what do we know about the geological composition of North German salt structures?

Abstract. Suitable host rocks for a repository for high-level radioactive waste (HLW) in Germany include not only clay and crystalline rocks but also rock salt formations in so-called flat and steep bedding (StandAG, 2017). Favourable repository relevant properties of rock salt are, e.g. the high heat conductivity, low porosity and permeability, and its viscoplastic deformation behaviour. Thicker salt deposits can be attributed to the formation of approx. 700 salt structures that have formed under various geological conditions in the North German Basin (NGB) over the last 250 million years. According to their shape and genesis, salt structures are classified as salt pillows (considered as flat bedding) or salt diapirs (steep bedding). Out of a total of 74 sub-areas in rock salt, 60 sub-areas in salt diapirs consisting of Permian evaporates were identified within the first phase of the German site selection procedure (BGE, 2020). At the current stage of the site selection process, a conservative approach has been adopted and the internal structure of the salt structures have not yet been taken into account for further classification of the identified sub-areas. However, the interior of the salt structures not only consists of rock salt but also of varying proportions of clay, carbonate and anhydrite rocks, as well as potassium salts formed by progressive evaporation of marine brines. Multi-phase salt tectonics has led to the folding of these differently composed layers and to complex internal structures. Therefore, detailed knowledge of the salt structure compositions is necessary to identify suitable rock salt areas for the designation of the containment providing rock zone. As a result of decades of research through extensive salt and potash mining, cavern storage and exploration for final waste disposal, Permian salt rocks represent a well-studied host rock in Germany. The use of different exploration methods and multidisciplinary data evaluation have led to a comprehensive understanding of the internal composition of some well-studied salt structures. Systematic studies have shown that several factors have influenced the formation of salt structures as well as their external shapes, sizes, and spatial distribution (e.g., Pollok et al., 2020). Furthermore, the amount and distribution of suitable host rocks varies greatly in different salt structures and is closely related to their internal structure. Since the interior of salt structures has not been considered in the site selection process so far, a classification of salt structures (or sub-areas) into certain types with varying internal composition and complexity is presented. By examining their lithofacial composition, genesis, and structural geological position within the basin, it is possible to narrow down these types to specific areas in the NGB. Without the acquisition of additional exploration data in this phase of the site selection process, this salt structure classification provides important data for the legally demanded assessment of the overall favourable geological situation.

Suitability of flat bedded salt formations in Germany as the site for a repository for heat-producing radioactive waste

Abstract. The search for a site of a final repository for highly radioactive waste in Germany was renewed when the Site Selection Act (StandAG) came into force in 2013. In Germany the development of concepts for a final repository and safety analyses for a repository in a salt dome was prioritized for many decades, whereas repository concepts in clay and crystalline rock were first considered only two decades ago. The aim of a comparative site selection procedure is to find a site before 2031, which provides the best possible safety for the enclosure of highly radioactive waste over a time period of 1 million years. The fundamental safety-related consideration is the enclosure of toxic waste in a so-called containment effective geological area (ewG). The main aspect of a long-term proof of safety is logically the systematic proof of safe long-term enclosure of the deposited waste. The approach developed within the framework of appropriate research projects (e.g. Eickemeier et al., 2013) is essentially based on the proof of geotechnical integrity of the ewG as the fundamental geological barrier as well as the geotechnical barriers. Due to their unique characteristics, including imperviousness and plastic deformability, salt rocks have been used for decades in Germany and worldwide in mining and especially for energy storage. Whereas halite in salt domes (type steep​​​​​​​ inclined salt) is distributed particularly in northwest Germany, flatly deposited salt rock (type flat bedded salt​​​​​​​) dominates in middle Germany and salt pillows (type salt pillows) in parts of northeast Germany. Both types of “bedded salt”​​​​​​​ widely reflect in their lateral extension sedimentation-related deposition conditions, apart from diagenetically related alterations. Beginning with the presentation of the host rock-specific boundary conditions of the various rock salts, this article focuses on the appropriate procedures for the proof of integrity of the geological barrier rock salt, based on the available experiences, corresponding reference studies and analogous examples. In the results it is shown that repository concepts in bedded salt formations and especially in the constellation of salt pillows provide substantial safety-related advantages due to a site-specific multibarrier system with alternate deposition of salt and saliferous clay as well as an intact overlying rock covering.

Evolution of anisotropy with saturation and its implications for the elastoplastic responses of clay rocks

AbstractMany clay rocks have distinct bedding planes. Experimental studies have shown that their mechanical properties evolve with the degree of saturation (DOS), often with higher stiffness and strength after drying. For transversely isotropic rocks, the effects of saturation can differ between the bed‐normal (BN) and bed‐parallel (BP) directions, which gives rise to saturation‐dependent stiffness and strength anisotropy. Accurate prediction of the mechanical behavior of clay rocks under partially saturated conditions requires numerical models that can capture the evolving elastic and plastic anisotropy with DOS. In this study, we present an anisotropy framework for coupled solid deformation‐fluid flow in unsaturated elastoplastic media. We incorporate saturation‐dependent strength anisotropy into an anisotropic modified Cam‐Clay (MCC) model and consider the evolving anisotropy in both the elastic and plastic responses. The model was calibrated using experimental data from triaxial tests to demonstrate its capability in capturing strength anisotropy at various levels of saturation. Through numerical simulations, we demonstrate the role of evolving stiffness and strength anisotropy in the mechanical behavior of clay rocks. Plane strain simulations of triaxial compression tests were also conducted to demonstrate the impacts of material anisotropy and DOS on the mechanical and fluid flow responses.

Simulation of non-linear heterogeneity-based isotherms for characterizing anaerobic and aerobic sorption of Se and Cs on various clay rocks

Simulation of non-linear heterogeneity-based isotherms for characterizing anaerobic and aerobic sorption of Se and Cs on various clay rocks

A case study on TBM cutterhead temperature monitoring and mud cake formation discrimination method

The mud cake is easily formed during the tunnel boring machine (TBM) excavation in clay soils or rocks containing clay minerals. Mud cake will lead to soil disturbance of tunnel face, clogging cutterhead and even affect the construction efficiency and personnel safety. In this study, a discrimination method of mud cake formation based on cutterhead temperature was proposed. An online monitoring system was designed and installed on the slurry balance TBM. The results show that: (a) the cutterhead temperature data can be reliably detected and transmitted by the system; (b) in a tunneling cycle, the temperature at some positions of the cutterhead will increase first and then decrease; (c) during the field test, the temperature variation is around 2.5 °C under the normal condition, but the temperature variation will increase more than 50 °C due to the mud cake or geological change; (d) compared with the cooling rate, mud cake formation can be accurately discriminated.

Effect of Spatial Variability of the Elastic Modulus in Composite Ground on the Structural Performance of Large-diameter Tunnels

The spatial variability of soil properties is often neglected in design and construction, resulting in underestimating or overestimating the tunnel structural performance. Furthermore, the risks of ignoring the spatial variability of ground properties increase significantly as the tunnel diameter increases. This study investigates the effects of ground spatial variability on the convergence of shield tunnels in the composite ground, based on a real case of a cross-sea tunnel typically embedded in a ground mixture of soft clay and hard rock in Shenzhen, China. The influence of scale of fluctuation (SOF) for the soil and rock elastic modulus on tunnel convergence is investigated. It is found that the mean value of the tunnel convergence is less affected. In contrast, the tunnel convergence coefficient of variation is strongly affected by changing the elastic modulus SOF of the soil or rock layer. Moreover, the results indicate that as the elastic modulus follows the lognormal distribution, the vertical and horizontal convergence of the tunnel agrees well with the lognormal distribution.

Investigation of europium retention on Callovo-Oxfordian clay rock (France) by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and percolation experiments in microcells

Clay-rock formations are under investigation in several countries as a potential host of radioactive waste repositories. In order to assess the long-term safety of these concepts, retention distribution data (Rd) are needed to describe the interaction between radionuclides and natural rock. While Rd values obtained from batch-type experiments with dispersed material are essential for the development of retention models, it is also important to generate retention data under more realistic conditions, in particular with compact samples. Specifically, the literature data on strongly retained radionuclides proves to be very scarce. The present study focuses on the retention of europium (Eu) at trace concentrations on clay samples extracted from the Callovo-Oxfordian (COx) formation in France. Rd values obtained with compact COx samples are assessed by a methodology coupling percolation experiments (in pressurized microcells), performed using submillimeter-sized samples in thickness, and Laser Ablation Inductively Coupled Plasma Mass Spectrometry measurements. The Rd found at trace concentrations under non-transient conditions are high (Rd > 104 mL g−1) and in agreement with those recorded under classical batch-type conditions or those deduced from the distribution of naturally-occurring Eu between the pore water and the clay rock. They can be described by modeling with existing sorption databases, considering an adsorption process only on the clay fraction. This was validated by solid state LA-ICP-MS analyses of one of the Eu-spiked COx samples; the added europium was found exclusively in the COx clay fraction.

Sand production and building of borehole storage facilities in Russia’s Far North

The environmental problems that arise in the course of gas–condensate and oil field development in the conditions of the Russian Far North are discussed. It is urgent to boost theoretical advancement in the area of design as well as structural and process decision-making when justifying parameters for building underground storage facilities for liquid and solid waste of drilling. Building of borehole storage facilities belongs in the class of physicochemical geotechnologies, which supposes that a part of natural rock mass is removed via a borehole drilled from ground surface (borehole hydraulic technology). The main processes involved in the building are thermal destruction of permafrost sandy and clay rocks and the lift of sand and return water from bottomhole to ground surface. The lifting parameters should ensure removal of sand and water from boreholes at the preset flow rate and at the controlled water–air interface position in compliance with the underground storage facility building program. It is found that the average washout capacity in permafrost sandy rocks during construction of an underground storage facility is linearly dependent on the product of average reduced rate of thermal destruction and average temperature of water in borehole, which proves the accepted washout design procedure. In the zone of a future underground storage facility in permafrost sandy rocks, it is typical that rock properties are highly variable both along and across sand strata. The air-lift parameters depend on the air-lift capacity and on the density of fluid to be lifted. The stable air-lift of sandy slurry is possible when the slurry density is not higher than 1.4 g/cm3, which conforms to the water content round 54 %.

Stability Analysis of Host Claystone under T-M Coupling of HLW Disposal Repository in China: Experiments and Numerical Simulation

This paper focuses on the stability of the high-level radioactive waste (HLW) in the proposed clay rock in Tamusu area of China. The in-situ stress as well as the variational characteristics of ambient temperature caused by nuclide decay during HLW storage should be noticeable. A series of thermal property tests and thermo-mechanical coupled strength (T-M) tests of rock samples in the target formation are carried out. Then the stability of surrounding rock of an HLW under the combination of heat release from HLW and in-situ stress is simulated and analyzed by numerical method. Thermal properties of Tamusu clay rock samples are obtained by testing their thermal conductivity. In order to obtain the characteristics and the failure modes of rock samples at different temperatures, the T-M coupling experiments in the temperature range of 100°C are conducted. Numerical model for simulating the state of operation of the nuclear waste tank buried in the tunnel within 100 years is constructed. A thermal boundary by the heat release equation of HLW and the real in-situ stress level in Tamusu area are considered in the model. While, the variation law of surrounding rock’s temperature, stress, and deformation corresponding to the embedding time is obtained from the numerical calculation. Finally, the stability of the deep geological repository is comprehensively evaluated. The results show that the temperature has a significant impact on the T-M coupling characteristics of Tamusu clay rock, and the proposed repository numerical model has no large deformation and failure problems in 100 years. However, the temperature of the surrounding rock of the repository may exceed the safety standard value during the operation period.

Improving the Hardened Properties of Nonautoclaved Silicate Materials Using Nanodispersed Mine Waste

The development trend of building materials science directs at on energy and resource conservation, as well as the creation of green composites. An important step in this direction is the disposal of waste of various origins, including mine waste. This study is aimed to improve the hardened properties of nonautoclaved silicate materials using nanodispersed mine waste for the use of the structural formation of wall silicate products. Binder composites obtained from sand-clay rocks of an incomplete stage of clay formation were prepared in the form of new compositions, using a modified calcareous-sandy-clay binder, based on which silicate products with improved strength properties and durability characteristics were developed. Using differential thermal analysis (DTA) and X-ray diffraction patterns, as well as scanning electron microscopy images, the synthesis of hydration products in the system sandy clay rocks–calcium hydroxide–water without a traditional autoclave treatment was also studied. The features of hydration phase formation and hardening of modified binders were investigated taking into account the chemical, structural, and morphological features of sandy-clay rocks. Meanwhile, the microstructural, morphological, and thermal properties of the silicate materials at 2-day were also determined, and their work investigated their compressive strength and water absorption. The results show that the polymineral composition of the studied sandy-argillaceous rocks, which are nanodispersed mine waste, determines the possibility of interaction with quicklime with the formation of binders during hydrothermal treatment without pressure. Accordingly, it contributes to the production of low energy consumption silicate wall materials. Cementitious composites of unautoclaved silicate materials based on sandy clay rocks have hydraulic properties that are caused by the process of further formation and recrystallization of calcium hydrosilicates, which optimizes the ratio between gel and crystalline components and compacts the structure of the cementitious composites, which leads to an increase in performance. The use of sand-clay rocks instead of traditional quartz sand in the production of silicate materials improves the process of forming the raw mix, increases the strength of silicate products by 4–11 times, and optimizes the weather resistance of wall materials.

Interpretation of Subsurface Structure Based on the Magnetic Data at Semurup Geothermal Area Kerinci

The interpretation of subsurface structures in Semurup geothermal area has been carried out using the geomagnetic method. Data were collected in an area of 1500 m × 1400 m consisting of 160 points. The magnetic anomaly value obtained was derived from the total magnetic induction value that has been corrected by IGRF and diurnal variation, then transformed by reduction to equator and upward continuation to remove noise and separate local and regional anomalies. The results of data processing showed the total magnetic field values in the study area ranged from -1730.4 nT to 1909.0 nT. Magnetic anomalies in this study area are dominated by negative values that may be caused by demagnetised rocks (a result of hydrothermal alteration). The results of 2D modeling, it has 5 rock layers that can be classified into 3 main parts of the geothermal system The first and second layers are caprock with a depth of up to 850 meters consisting of sedimentary rock, clay, and sandstone. The third layer is indicated as a reservoir with a depth from 850 to 1450 m and is dominated by sandstone and clay alteration Hot rock in the fourth and fifth layers is dominated by basalt igneous rock and the presence of dacitic lava intrusion from the northeast of the study area at depths below 1450 m, and the Siulak fault as a outflow zone for geothermal fluid. The presence of the caprock, reservoir, hot rock, and fault zones indicates that the Semurup area has geothermal potential and is suitable for further exploration.