Quintessa

The coupled Thermo-Hydro-Mechanical (THM) behavior of the Callovo-Oxfordian claystone (COx) is of great importance for the design and safety calculations of the high-level radioactive waste disposal project in this potential host rock in France. The heat emitted by the waste causes a pore pressure increase within the surrounding rock essentially due to the differential thermal expansion of the pore water and the solid skeleton. The low permeability of the COx and its relative rigidity inhibits the discharge of the induced pressure build-up. Moreover, thermal loading may provoke thermo-mechanical stresses within the formation due to mechanical confinement by the rigidity of the surrounding host rock. An important research program has been conducted by the French National Radioactive Waste Management Agency (Andra) since 2003 in order to investigate the THM response of the COx under thermal loading, through laboratory tests, in-situ experiments, model development and numerical modeling. Within Task E of the DECOVALEX-2019 project, five research teams investigated upscaling THM modeling from a small-scale in-situ experiment (TED) to a full-scale in-situ experiment (ALC). The upscaling modeling started with a verification test to validate the numerical codes. Then, an interpretative modeling of the TED experiment was performed to calibrate the THM parameters of the COx. Finally, the calibrated THM parameters were used for a blind prediction of the ALC experiment. The modeling teams each adopted a thermo-poro-elastic approach which yielded satisfactory results. The blind prediction of the temperature field showed an overestimation of less than 2 °C which was considered acceptable. On the other hand, pore pressure was well predicted only in the direction parallel to the bedding whereas the slow dissipation of the pore pressure in the direction perpendicular to the bedding was not captured by any of the modeling teams – which remains an open question of the present study.

Running impact forces expose the body to biomechanical loads leading to beneficial adaptations, but also risk of injury. Highintensity running tasks, especially, are deemed highly demanding for the musculoskeletal system, but loads experienced during these actions are not well understood. To eventually predict GRF and understand the biomechanical loads experienced during such activities in greater detail, this study aimed to (1) examine the feasibility of using a simple two mass-spring-damper model, based on eight model parameters, to reproduce ground reaction forces (GRFs) for high-intensity running tasks and (2) verify whether the required model parameters were physically meaningful. This model was used to reproduce GRFs for rapid accelerations and decelerations, constant speed running and maximal sprints. GRF profiles and impulses could be reproduced with low to very low errors across tasks, but subtler loading characteristics (impact peaks, loading rate) were modelled less accurately. Moreover, required model parameters varied strongly between trials and had minimal physical meaning. These results show that although a two mass-spring-damper model can be used to reproduce overall GRFs for high-intensity running tasks, the application of this simple model for predicting GRFs in the field and/or understanding the biomechanical demands of training in greater detail is likely limited.

This paper is devoted to the study of the Thermo-Hydro-Mechanical (THM) responses of a porous rock with low permeability under thermal loading in the context of deep geological disposal of radioactive waste. To this aim, numerical simulations of a benchmark exercise of a hypothetical high-level radioactive waste (HLW) repository were performed. This benchmark exercise considered as a host formation the Callovo-Oxfordian claystone (COx), which has been selected for a deep geological disposal in France. Within the framework of the DECOVALEX-2019 project, five modelling teams (Andra, LBNL, NWMO, Quintessa, UFZ/BGR) adopted a thermo-poro-elastic approach and proposed different 3D representations of the HLW repository. The differences between the teams consisted mostly in the simplification of the geometrical model and the interpretation of the boundary conditions. Numerical results for temperature, pore pressure, and effective stress evolution in the far field (i.e., at the mid-distance of two HLW cells) were compared between the teams, to quantify the impact of modelling simplifications/assumptions for the assessment of the HLW repository. The THM behaviour of the COx formation in the near field (i.e., excavation damaged zone around the HLW cells) is not the objective of this study. Moreover, plane strain conditions were considered and evaluated in comparison to 3D modelling. Key parameters influencing the THM responses of the HLW repository were assessed by both mono- and multi-parametric analyses. Spatial variability analyses of THM parameters were also carried out to study the influence of the spatial correlation length on the Terzaghi effective stress and to estimate its probability distribution. The conclusions of this study provide reliable numerical techniques for modelling large-scale deep geological disposals and deduce the main behavior of the HLW repository.

Safety functions for the clay buffer in a repository for high-level radioactive waste (HLW) are fulfilled if the presence of montmorillonite with high swelling capacity and low permeability is maintained in the long-term. The transformation of montmorillonite to the non-swelling mineral illite is addressed in most safety assessments by using simple semi-empirical kinetic models, but this approach contrasts with all other near-field geochemical modelling activities that employ a full description of thermodynamic and kinetic mineral-fluid processes. The consistency of these two modelling approaches has been studied by simulating the montmorillonite to illite transformation in the marine sediment profile penetrated by the Ocean Drilling Program (ODP) Site 1174, offshore Japan. Illite in mixed-layer smectite-illite increases from 20% at <700 m below seafloor (mbsf) to 89% at 1100 mbsf. Illitization of smectite at Site 1174 using the semi-empirical approach has been shown by previous authors to provide a satisfactory match to the gradual change of illite content with depth, albeit with significant differences between model variants. In comparison, the approach used in the current study was to simulate the mineralogical and fluid chemical evolution of a ‘packet’ of a typical fluid-saturated sediment using a model involving full kinetic and thermodynamic treatment of mineral dissolution-precipitation reactions, along a temperature-time burial curve defined by published thermal conductivity data for Site 1174. The results of these simulations showed that the onset of illitization at a depth of 700 mbsf could be matched, but that the overall rate of conversion was significantly more rapid than observed, or as modelled by the simple semi-empirical kinetic approach. The onset and rate of illite growth was strongly linked to the rate of transformation of amorphous silica to quartz. Geochemical model simulations therefore err on the side of conservatism, but may produce unrealistic estimates of illitization. This comparison demonstrates that models involving full kinetic and thermodynamic treatment of mineral dissolution-precipitation reactions must be carefully applied to simulate other transformation reactions of montmorillonite relevant to the geological disposal of HLW, such as those arising from the interaction of montmorillonite with iron/steel, cement and/or groundwater.

Andra performs a wide range of in-situ experiments at its Meuse/Haute-Marne Underground Research Laboratory (MHM URL). The purpose of these experiments is to study the feasibility of a radioactive waste repository in the Callovo-Oxfordian claystone formation (COx). An important research program has been conducted by Andra since 2005 to investigate THM response of the COx to a thermal load through laboratory and in situ experimentations. A step-by-step approach is followed, which starts with small scale heating boreholes (TED experiment) and extends to full-scale (ALC experiment). Modelling and interpretation of the TED and ALC experiments are conducted in the context of the Task E within the DECOVALEX-2019 framework. DECOVALEX-2019 is a multidisciplinary, co-operative international research effort focused on modelling coupled Thermal-Hydraulic-Mechanical-Chemical (THMC) processes. Based on the TED experiment measurements, the THM parameters of the COx are calibrated through a fully-coupled THM model using COMSOL. Then, the ALC experiment is successfully blind predicted and interpretatively modelled using the calibrated THM parameters of the COx and the proposed coupled THM model.

Spherical carbonate concretions are commonly observed in marine clayey sedimentary strata and often contain well preserved fossils. Previous studies revealed that the spherical concretions are formed by the very rapid reaction with decomposed organic matter from inside and Ca²⁺ ion of seawater. However, the detailed mass transport process during concretion formation has not been completely understood. Here two different size of spherical concretions, cm size of tusk-shell concretions and metre size of Moeraki boulders, are re-examined to understand the diffusion oriented formation process. Field observations, and detailed mineralogical (XRD) and geochemical analyses (SXAM, XRF, δ¹³C) revealed diffusive transport of HCO3⁻ from decaying organic matter and Ca²⁺ from surrounding pore-water of marine origin led to solid carbonate precipitation reactions that progressed from the margin of a concretion. Based on the compositional gradients across the concretions, a diffusion based diagram has been applied to estimate the growth rates of the different size of spherical concretions. The process and rate estimation indicate that even gigantic spherical concretions can form quite rapidly in the muddy matrices under a diffusion-controlled transport regime.

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The work presented in this article is a part of the international European Commission FORGE project which studied the behaviour of repository gases in the context of radioactive waste disposal. Experimental work is essential to understand the main transport mechanisms for gas and to determine the main geophysical parameters. However, while laboratory and rock experiments can help to investigate the short- and partly the middle-term behaviour of gas in a rock formation, long-term predictions have to be based on numerical simulations. Numerical simulation of long-term gas behaviour in a deep geological repository was one of the aims in the FORGE project. The objectives of the FORGE modelling were to describe the state-of-the-art consideration of gas in performance assessment, and to propose an updated treatment of gas issues in long-term safety assessments for high-level radioactive waste and spent nuclear fuel. Three benchmark exercises for a hypothetical geological repository in clay host rock ranging in scale from a single disposal cell to the whole repository were defined. To provide added value to this benchmark, a feature not yet well represented in typical gas simulations was introduced: the explicit representation of the interfaces between the excavation disturbed zone and the engineered elements within the excavation, such as waste canisters, bentonite plugs, and access drifts. In order to assess gas transport behaviour at the whole repository scale, models were developed with ‘mathematical’ or ‘numerical’ upscaling techniques for small-scale features. The assessment across different modelling scales revealed the main long-term gas migration pathways and led to the conclusion that the explicit representation of interfaces is not important.

Isolated spherical carbonate concretions are frequently observed in finer grained marine sediments of widely varying geological age. Recent studies on various kinds of spherical carbonate (CaCO3) concretions revealed that they formed very rapidly under tightly constrained conditions. However, the formation ages of the isolated spherical carbonate concretions have never been determined. Here we use 87Sr/86Sr ratios to determine the ages of these spherical concretions. The studied concretions formed in the Yatsuo Group of Miocene age in central Japan. Some formed post-mortem around tusk-shells (Fissidentalium spp.), while other concretions have no shell fossils inside. The deformation of sedimentary layers around the concretions, combined with geochemical analyses, reveal that Sr was incorporated into the CaCO3 concretions during their rapid formation. Strontium isotopic stratigraphy using 87Sr/86Sr ratios of all concretions indicates an age of 17.02 ± 0.27 Ma, with higher accuracy than the ages estimated using micro-fossils from the Yatsuo Group. The results imply that the 87Sr/86Sr ratio of isolated spherical carbonate concretions can be applied generally to determine the numerical ages of marine sediments, when concretions formed soon after sedimentation. The 87Sr/86Sr age determinations have high accuracy, even in cases without any fossils evidence.

Isolated spherical carbonate concretions observed in marine sediments are fascinating natural objet trouve because of their rounded shapes and distinct sharp boundaries. They occur in varied matrices and often contain well preserved fossils. The formation process of such concretions has been explained by diffusion and rapid syn-depositional reactions with organic solutes and other pore water constituents. However, the rates, conditions and formation process of syngenetic spherical concretions are still not fully clear. Based on the examination of different kinds of spherical concretions from several locations in Japan, a diffusion based growth diagram was applied to define the generalized growth conditions of spherical concretions formed around decaying organic matter. All analytical data imply that the spherical concretions formed very rapidly, at least three to four orders of magnitude faster than previously estimated timescales. The values indicate that spherical concretions are preferentially grown within clay- to silt-grade marine sediments deposited in relatively deep (a few tens of metres) environments dominated by diffusive solute transport, very early in diagenesis.

In this paper, a comparative modelling exercise from the DECOVALEX-2015 project is presented. The exercise is based on in situ experiments, performed at the Tournemire Underground Research Laboratory (URL), run by the IRSN (Institut de Radioprotection et de Sûreté Nucléaire), in France. These experiments aim at identifying conditions (e.g. technical specifications, design, construction, and defects) that will affect the long-term performance of swelling clay-based sealing systems, which is of key importance for the safety of underground nuclear waste disposal facilities. A number of materials are being considered as seals; the current work focusses on a 70/30 MX80 bentonite–sand mixture initially compacted at a dry density of 1.94 Mg/m³. The performance of the sealing plug involves at least three different important components, which are the hydro-mechanical behaviour of the bentonite–sand core, the overall permeability of the surrounding argillite, and the influence of the technological gap between the core and the argillite. Two particular tests have been selected for a comparative modelling exercise: the WT-1 test, which was designed to study the rock mass permeability, and the PT-A1 test, which aimed at quantifying the evolution of the hydro-mechanical field within the bentonite–sand core. A number of independent teams have worked towards modelling these experiments, using different codes and input parameters calibrated on additional small-scale laboratory experiments. Their results are compared and discussed.

The geological formation immediately surrounding a nuclear waste disposal facility has the potential to undergo a complex set of physical and chemical processes starting from construction and continuing many years after closure. The DECOVALEX project (DEvelopment of COupled models and their VALidation against EXperiments) was established and maintained by a variety of waste management organisations, regulators and research organisations to help improve capabilities in experimental interpretation, numerical modelling and blind prediction of complex coupled systems. In the present round of DECOVALEX (D-2015), one component of Task C1 has considered the detailed experimental work of Yasuhara et al. (Earth Planet Sci Lett 244:186–200, 2006), wherein a single artificial fracture in novaculite (micro- or crypto-crystalline quartz) is subject to variable fluid flows, mechanical confining pressure and different applied temperatures. This paper presents a synthesis of the completed work of six separate research teams. A range of approaches are presented including 2D and 3D high-resolution coupled thermo–hydro–mechanical–chemical models. The results of the work show that while good, physically plausible representations of the experiment can be obtained using a range of approaches, there is considerable uncertainty in the relative importance of the various processes, and that the parameterisation of these processes can be closely linked to the interpretation of the fracture surface topography at different spatial scales. http://rdcu.be/pax6

Hydraulic seals using compacted sand–bentonite blocks are an important part of the closure phase of deep geological disposal facilities for the isolation of many categories of radioactive wastes. An understanding of the hydro-mechanical behaviour of these seals and the ability to model their behaviour is a key contribution to safety cases and licence applications. This work reports the development of a hydro-mechanically coupled model and its application to the simulation of a range of test conditions investigated in the SEALEX experiments conducted by IRSN at Tournemire URL. The work has been conducted as part of the recently completed DECOVALEX-2015 project. Richards’ equation for unsaturated fluid flow is coupled to a nonlinear elastic strain-dependent mechanical model that incorporates a moving finite element mesh, and calibrated against laboratory experiments. Stress and volumetric dependencies of the water retention behaviour are incorporated through the Dueck suction concept extended to take into account permanent changes in water retention behaviour during consolidation. Plastic collapse in laboratory results is modelled with the application of a source term activated by a threshold defined in terms of the net axial stress and net suction. The model is used to simulate both a 1/10 scale mock-up laboratory test and full-scale in situ performance test and is capable of reproducing the major trends in the data with just nine mechanical parameters and an experimentally defined stress threshold.

The geological formation immediately surrounding a nuclear waste disposal facility has the potential to undergo a complex set of physical and chemical processes starting from construction and continuing many years after closure. The DECOVALEX project (DEvelopment of COupled models and their VALidation against EXperiments) was established and maintained by a variety of waste management organizations, regulators and research organizations to help improve capabilities in experimental interpretation, numerical modelling and blind prediction of complex coupled systems. In the present round of DECOVALEX (D-2015), one component of Task C1 has considered the detailed experimental work of Yasuhara et al. (Appl Geochem 26:2074–2088, 2011), wherein three natural fractures in Mizunami granite are subject to variable fluid flows, mechanical confining pressure and different applied temperatures. This paper presents a synthesis of the completed work of six separate research teams, building on work considering a single synthetic fracture in novaculite. A range of approaches are presented including full geochemical reactive transport modelling and 2D and 3D high-resolution coupled thermo–hydro–mechanical–chemical (THMC) models. The work shows that reasonable fits can be obtained to the experimental data using a variety of approaches, but considerable uncertainty remains as to the relative importance of competing process sets. The work also illustrates that a good understanding of fracture topography, interaction with the granite matrix, a good understanding of the geochemistry and the associated multi-scale THMC process behaviours is a necessary pre-cursor to considering predictive models of such a system. http://rdcu.be/kHBx

A comparative modelling exercise involving several independent teams from the DECOVALEX-2015 project is presented in this paper. The exercise is based on various laboratory experiments that have been carried out in the framework of a French research programme called SEALEX and conducted by the IRSN. The programme focuses on the long-term performance of swelling clay-based sealing systems that provide an important contribution to the safety of underground nuclear waste disposal facilities. A number of materials are being considered in the sealing systems; the current work focuses on a 70/30 MX80 bentonite–sand mixture compacted at dry densities between 1.67 and 1.97 Mg/m3. The improved understanding of the full set of hydro-mechanical processes affecting the behaviour of an in situ sealing system requires both experiments ranging from small-scale laboratory tests to full-scale field emplacement studies and coupled hydro-mechanical models that are able to explain the observations in the experiments. The approach was to build models of increasing complexity starting for the simplest laboratory experiments and building towards the full-scale in situ experiments. Following this approach, two sets of small-scale laboratory experiments have been performed and modelled. The first set of experiments involves characterizing the hydro-mechanical behaviour of the bentonite–sand mixture by means of (1) water retention tests under both constant volume and free swell conditions, (2) infiltration test under constant volume condition, and (3) swelling and compression tests under suction control conditions. The second, more complex, experiment is a 1/10th scale mock-up of a larger-scale in situ experiment. Modelling of the full-scale experiment is described in a companion paper. A number of independent teams have worked towards modelling these experiments using different conceptual models, codes, and input parameters. Their results are compared and discussed. This exercise has enabled an improved modelling of the bentonite–sand mixture behaviour, in particular accounting for the dependence of its retention curve on the dry density. Moreover, it has shown the importance of the technological voids on the short-term behaviour of the sealing system.

An approach for simulating thermal, hydraulic, mechanical and chemical (THMC) coupled processes in single rock fractures has been developed under the framework of a self-developed numerical method, i.e., an elasto-plastic cellular automaton. The balance equations of multi-physics problems to describe the THMC process in single rock fracture are solved by using cellular automaton technique on space scale and finite difference method on time scale, respectively. Using the concept of cellular automaton, a single rock fracture surface is discretized into a system composed of cell elements. Different apertures, i.e., 0 for contact and nonzero for void, are assigned to each cell element based on the fracture surface topography. The fluid flow, stress-dependent chemical reaction and solute transport are simulated by using a cellular automaton updating rule, in which only local cell balance equation is considered. The contribution of cell elements in contact to cell’s transmissivity and convection can be ignored conveniently. The Lagrangian method is used to simulate the particle transport. Special treatment for particle transport to outer boundaries and internal boundaries is adopted. As a result, the local behaviors, such as the formation of local contact, dead ends in the fracture and the local aperture change, are conveniently updated dynamically. The approach is used to simulate the coupled THMC process in a single novaculite fracture. The behaviors of pressure dissolution caused by effective stress, free-face dissolution/precipitation, thermal-dependent fluid flow and ion transport are well reproduced by using the developed approach, subject to parameter calibration. The robustness of the general approach to such complex problems is demonstrated by comparing with experimental data.

Fluid migration in the subsurface has the potential to induce changes in fluid pressure distribution, temperature distribution, mechanical stresses and the chemistry of both the fluid and the natural geological material it is flowing through. In many situations, the change in all of these processes gives a coupled response, in that one process feeds back to another. When trying to understand fluid flow through naturally and artificially fractured systems, it is important to be able to identify the relative importance of the processes occurring and the degree of interactions between them. Modelling of such highly nonlinear coupled flow is complex. Current and predicted computational ability is not able to simulate discretely all the known and physically described processes operating. One approach to coping with this complexity is to identify the relative importance and impact of relevant processes, dependent on the application of interest. Addressing such complexity can be particularly important when the characteristics of natural and disturbed geological materials are being evaluated in the context of disposal of radioactive waste or other geo-engineering systems where an understanding of the long-term evolution is required. Based on a series of coupled (THMC—Thermal, Hydraulic, Mechanical and Chemical) experimental investigations on the flow of fluid through fractured novaculite and granite crystalline rock samples, several couplings are examined where there is both a significant kinetic chemical control as well as mechanical and temperature control on the fluid flow behaviour. These interactions can be shown both in the literature and experimentally to have a significant effect on the rate of fluid flow through fractures. A new discrete numerical approach and a new homogenous approach are used to model the experimental results of coupled flow through fractures. The results of these modelling approaches are benchmarked both against one another and against the experimental results, and then the processes included in the approaches are ranked in order of impact.

Bentonite barriers perform safety critical functions in many radioactive waste disposal concepts, but it is challenging to accurately predict bentonite resaturation behaviour in repository settings. Coupled models of the hydro-mechanical response of bentonite are used to demonstrate understanding of bentonite behaviour in experiments and to predict the response of bentonite in a repository environment. Following trials of a range of numerical approaches, a new model is presented, referred to as the Internal Limit Model, which makes use of key observations on limiting stresses supported in bentonite samples in experimental data. This model is based on the Modified Cam Clay model, and uses the observation that for a given dry density of bentonite, there is a limiting stress that the sample can support, be that stress due to swelling, compaction or suction, to explicitly couple the hydraulic and mechanical models. The model is applied to experimental data from the SEALEX experiments, involving a 70/30 by mass mixture of MX80 bentonite and sand. The model is able to reproduce the experimental data using a single set of parameters for all the experiments considered. This builds confidence that the model will be useful in the future for predictive modelling given appropriate data to characterise the bentonite material being used.

Low-permeability clay formations provide good candidate host rocks for geological disposal of radioactive waste, because there is expected to be limited movement of gas or water through the formation. However, when constructing tunnels, the stress state in the formation around the tunnel will change, which can lead to damage to the formation, changing the bulk hydraulic properties of the formation close to the tunnel. There is the potential for this damaged zone to act as a preferential pathway for fluid flow and radionuclide transport. A water injection experiment is ongoing at the Tournemire underground rock laboratory to investigate the hydraulic properties of the Toarcian argillite in which the laboratory is constructed. Water is injected into the formation at the end of a sealed borehole and moves preferentially in the damaged zone along the borehole walls. The rate of water injection into the rock changed over the first year of the experiment and the causes of this change are investigated in this paper by numerical modelling. The study demonstrates that the change in water injection rate into the argillite can be explained by the evolving hydraulic properties of the damaged zone around the borehole. The findings of the modelling study are discussed in the context of long-term radioactive waste disposal.