Deep Geological Disposal Research Articles

Scalable block preconditioners for saturated thermo-hydro-mechanics problems

We are interested in the modelling of saturated thermo-hydro-mechanical (THM) problems that describe the behaviour of a soil in which a weakly compressible fluid evolves. It is used for the evaluation of the THM impact of high-level activity radioactive waste exothermicity within a deep geological disposal facility. We shall present the definition of a block preconditioner with nested Krylov solvers for the fully coupled THM equations. Numerical results reflect the good performance of the proposed preconditioners that show to be weakly scalable until more than 2000 cores and more than 1 billion degrees of freedom. Thanks to their performance and robustness, a real waste storage problem on a scale, to our knowledge, unprecedented in the field, can be addressed.

Characterization and modeling of ground motion at depth in soft sedimentary rocks: Application to the Swiss Molasse Basin

The level of earthquake motion observed on the ground surface differs from the shaking at locations buried at a depth. Characterization of the underground shaking is important for the local seismic hazard assessment at depth which is an insufficiently investigated topic so far. Advancements in the prediction of the ground motion at depth are especially important for the design of deep geological disposals of nuclear waste, buildings with deep foundations, or studies focused on soil-structure interactions. In this paper, we analyze site-specific transfer functions that relate ground motions on the surface to that at a depth at sites located within soft sedimentary rocks of the Swiss Molasse Basin. We first evaluate empirical surface-to-borehole amplification functions at six borehole sites, compare them with predictions by a theoretical model, and characterize the high-frequency surface-to-borehole amplification. Next, we perform full-waveform prediction of ground motion at depth from surface recordings of seven significant regional earthquakes with ML > 4. The comparison of predicted and observed acceleration waveforms shows a high level of similarity in a broad frequency range and well-predicted values of PGA, PGV, and response spectra. Finally, we model a scenario earthquake for 9975 years return period at a test site, and we predict associated broadband waveforms at depth. By the latter, we demonstrate that our method to predict ground motions at depth can be used as a basis for the site-specific seismic hazard of deep geological disposals of nuclear waste.

KAERI underground research laboratory: Overview of in-situ experiments

Social concerns regarding the safety of high-level radioactive waste have increased with growing public awareness of environmental issues and nuclear power. The performance assessment of deep geological disposal systems is crucial to reduce the uncertainties associated with high-level radioactive waste disposal and enhance the overall public confidence in nuclear safety. Accordingly, the Korea Atomic Energy Research Institute (KAERI) has undertaken various studies on the development of a deep geological disposal system for high-level waste and disposal safety evaluation. The KAERI Underground Research Tunnel (KURT), South Korea's only underground research laboratory dedicated to radioactive waste disposal, was constructed in 2006 and expanded in 2015. Since its construction, numerous in-situ experiments have been conducted and are currently underway in the KURT. The KURT plays a significant role in assessing the feasibility, safety, stability and appropriateness of a deep geological disposal system in South Korea and also provides an opportunity to revitalize industrial-academic-scientific cooperation between related institutions. This report summarizes two key in-situ experiments and international joint research conducted between 2007 and 2017 to assess the performance of the engineered and natural barriers of the KURT. The research experiences from the in-situ tests conducted at the KURT will provide crucial information on the safety and feasibility validation of the deep geological disposal system and will be an important contributor to the success of the Korean high-level radioactive waste disposal program in the future.

Assessing relevant transport processes in Opalinus Clay at the Mont Terri rock laboratory using excess-pressure, concentration and temperature profiles

In the context of studies on deep geological disposal of radioactive waste, the overpressures whose origin is still debated raise the question of the relevant transport processes to be taken into account in clay media. An anomaly of about 13 bars was identified during the Deep Borehole (DB) experiment at the Mont Terri URL (Switzerland) whose objective was to identify the impact of hydraulic and chemical transient behavior on fluid and solute transfers. The first stage of DB experiment consisted in drilling BDB-1, a 250 m long-inclined borehole across the entire Opalinus Clay (OPA). The second stage of the work aimed at determining the driving forces (pressure, temperature, and salinity gradients) and transport coefficients (diffusion, hydraulic conductivity, chemical and thermal osmotic coefficients) using a multi-packer hydraulic system. The last step of this fully experimental and modelling integrated study was the development of 1D transport models either in fully transient or in pseudo-steady state to reproduce the measured pressure and salinity profiles. Pure hydraulic calculations evaluated a Darcy velocity in the order of 10−14 m s−1, related to a downward flow. However the best agreement with the data was obtained by introducing osmotic processes. Chemical osmosis slightly increased the advective flow in the same direction, but its effect is minor compared to that of thermo-osmosis which inverts the water movement with a mean Darcy velocity of about 10−13 m s−1. However, the Peclet number still confirmed the dominance of diffusion with respect to advection at Mont Terri.

Benchmarking a new TH2M implementation in OGS-6 with regard to processes relevant for nuclear waste disposal

In this paper, thermo-hydro-mechanically (THM) coupled processes triggered during the construction, operation and closure of a deep geological repository for heat generating, high level radioactive waste are discussed based on a generic disposal concept. For this purpose, we are using the numerical non-isothermal two-phase–two-component flow in deformable porous media (TH2M) implementation (Grunwald et al. in Geomech Geophys Geo-energy Geo-resour, 2022) in the open-source software OpenGeoSys (Bilke et al. in Transport Porous Media 130(1):337–361, 2019, https://doi.org/10.1007/s11242-019-01310-1). THM coupled effects covered in this work focus on single and two-phase-flow phenomena, gas and heat generation as well as poro-elastic medium deformation. A suitable set of benchmarks covering aforementioned THM-effects, devised in the scope of the BenVaSim benchmarking project (Lux et al. in Synthesis report. BenVaSim—International Benchmarking for Verification and Validation of TH2M Simulators with Special Consideration of Fluid Dynamical Processes in Radioactive Waste Repository Systems. Tech. rep., 2021, https://doi.org/10.13140/RG.2.2.28998.34887) is chosen and one additional benchmark is presented, allowing for the demonstration and comparison of the OGS-6 TH2M implementation against results obtained by other well-established codes used in the field. Apart from the code comparison, the benchmarks also serve as means to analyze THM coupled processes in a repository based on very simplified geometries. Therefore, they can help to improve the process understanding, but any quantitative results should not be interpreted as predictions of the behaviour of a real repository. The results obtained in this work agree well with the results presented by the project partners in BenVaSim—both in single phasic, fully liquid saturated cases and in partially saturated two phase regions. Hence, the suitability of the OGS-6 TH2M implementation for the application in the field of radioactive waste management, supporting the safety case and analyzing the integrity of the geological and geotechnical barrier systems is demonstrated. Finally, a detailed discussion of observed phenomena in the benchmarks increases our understanding and confidence in the prediction of the behaviour of TH2M coupled systems in the context of deep geological radioactive waste disposal.

Influence of the water content on the complex conductivity of bentonite

Compacted bentonite is considered as a potential buffer material for deep geological disposals of high-level nuclear wastes. Methodologies to non-intrusively monitor the water content of such sealing materials are important in the context of the safety of these storage facilities and for various engineering applications as well. Induced polarization is a non-intrusive geophysical method sensitive to the water content of porous media. We investigated the complex conductivity spectra of 69 samples made of 2 distinct MX80 bentonites, one in the form of powder from crushed pellets (Type I) and the other in the form of a granulated bentonite mixture (GBM, type II). The samples are prepared at different compaction states and saturations. The pore water conductivity of the porous samples is estimated to be ∼2.5 S m−1 (25 °C) by two different methods. The complex conductivity spectra were obtained at room temperature (25 ± 2 °C) in the frequency range 1 Hz-45 kHz. In-phase and quadrature conductivities reflect conduction (electromigration) and polarization processes, respectively. At a given frequency, both the in-phase and quadrature conductivities increase with the water content along a trend that is independent of the compaction state. An induced polarization model based on the dynamic Stern layer model is used to explain these results. The first Archie's exponent m is inferred from the formation factor F using the in-phase conductivity data versus the pore water conductivity data at different salinities (NaCl, 25 °C). The dynamic Stern layer model applied to polarization correctly predicts the dependence of the in-phase conductivity, quadrature conductivity, and normalized chargeability with the water content and the Cation Exchange Capacity (CEC). This petrophysical work can easily be applied to time-domain induced polarization data collected in field conditions to monitor hydraulic barriers.

Adsorption and diffusion of selenite on Boda Claystone Formation

This study provides adsorption and diffusion data of selenite on Boda Claystone Formation (BCF) which is a potential host rock of a deep geological disposal of high-level radioactive waste. The experiments were performed on two diverse core samples: one albitic claystone sample characteristic for the entire BCF and one pyrite containing sample sparsely occurring in BCF. The experiments were carried out under atmospheric conditions. Batch experiments were carried out to study the kinetics of adsorption at a high initial concentration (1.2 × 10−3 M), the adsorption isotherms and reversibility were investigated in the 10−10–10−3 M concentration range. Adsorption onto petrographic thin sections was done to study the elemental distribution on the microscale and the oxidation state of selenium. The maximum of the distribution coefficient was found as Kd ≈ 200 L/kg and a decrease was experienced around 10−6–10−7 M equilibrium concentration, which showed similarities to other argillaceous rocks. Isotopic exchange experiments revealed reversibility of selenite adsorption. Diffusion was studied with through-diffusion and in-diffusion experiments. Using X-ray fluorescence, despite a low initial concentration of 2.3 × 10−5 M in the in-diffusion experiment, a meaningful diffusion profile of selenium could be obtained, from which the selenite apparent diffusion coefficient Dappselenite = (1.5–4.3) × 10−14 m2/s and the selenite rock capacity factor αselenite = 1.4–2.2 were determined. As selenium species are redox sensitive the oxidation state of adsorbed species was studied with X-ray absorption near edge structure spectroscopy on SeK edge. Adsorbed selenium remained in +IV oxidation state, however reduction was experienced on the pyritic sample.

Geochemical interactions at the steel-bentonite interface caused by a hydrothermal gradient

Bentonites are used in deep geological disposal facilities as an engineered barrier to isolate high level radioactive waste, contained in metallic canisters. The present study, performed at laboratory scale, evaluated the behaviour of MX-80 (Na-bentonite) and FEBEX (Ca-Mg-Na-bentonite) in contact with carbon steel, subjected to a hydrothermal gradient. A dominant Na-Cl-SO4 saline solution was injected towards the compacted bentonite from the top, while a heater, located at the bottom in contact with the steel disc, maintained a constant temperature of 100 °C. The cells were studied after one and six months of interaction. Changes in the physical (water content and specific surface area) and chemical (cation exchange capacity and element distribution) properties of the bentonite were observed, as well as the formation of a corrosion layer on the steel, at the interface with bentonite, mainly composed of magnetite, maghemite and hematite. The bentonites were mainly altered at the mm scale, being enriched in iron content, and changing their ion distribution to Ca-dominant smectite (in MX-80 bentonite).

Experimental evidence on the cracking and sealing mechanisms of compacted bentonite by using microfocus X-ray computed tomography

Compacted bentonite plays important roles in multi-barrier systems for the deep geological disposal of radioactive waste; one such role is to absorb groundwater infiltrating from the surrounding rock and to expand to seal technological voids. In the self-sealing process, the expansive deformation occurs along with a cracking phenomenon, which may lead to the performance degradation of engineered barrier. In this paper, Gaomiaozi bentonite was compacted into a dense sample to conduct a hydration test, accompanied with a simulated radial technological void. Microfocus X-ray computed tomography was used to scan the sample at various hydration moments to observe the evolutions of external appearance and internal structure. Consistent and significant results were obtained through qualitative observations and quantitative analyses. The self-sealing process underwent a gradual slowdown, manifested as a linear correlation between sample volume and hydration time on the semi-logarithmic coordinate. The sealing order of technological void was from bottom to top, consistent with the wetting path. Simultaneously, the hydration cracks developed following the same sequence. The maximum height of crack propagation was always less than that of the sample expansion. Moreover, the cracking direction of bentonite was always perpendicular to the tensile stress, resulting in a vertical strike for most cracks. However, there still existed a horizontal crack, which may be attributable to the weakness of the stratified interface. Under persistent hydration, the bentonite adjacent to the cracks was supplied with water, thereby strengthening the expansion and gradually closing the cracks. The hydration, expansion, and cracking were closely interrelated, and the relevant mechanisms are revealed in detail.

Thermo-hydro-mechanical analysis of the complete lifetime of the bentonite barrier in the FEBEX in-situ test

The FEBEX test was a large-scale demonstration project for the deep geological disposal concept of nuclear waste involving bentonite seals that lasted 18 years. One of the objectives of the test was to evaluate the capabilities of numerical methods to provide reliable predictions of the physical processes in a geological repository. Although previous studies have demonstrated the performance of current models of water, vapour and heat flow to capture the evolution of temperature and relative humidity, some uncertainties remain in the capabilities of constitutive models to predict and interpret the stress–strain behaviour of the bentonite. In this paper a recently developed thermo-hydro-mechanical (THM) elastoplastic constitutive model is used to analyse the bentonite barrier of the FEBEX test by means of the Finite Element method. The model features a two-way hydro-mechanical coupling and includes thermo-plasticity. The associated water retention formulation distinguishes the behaviour of adsorbed water and free water. The predictive capabilities of the model are tested by calibrating the material parameters on the sole basis of laboratory tests. Good predictions of total stress, dry density and water content are obtained and the analysis of the computed THM stress paths provides new insights on the causes of the final heterogeneous state of the bentonite barrier.

Modelling the density homogenisation of a block and granular bentonite buffer upon non‐isothermal saturation

AbstractThis paper presents a numerical analysis of the mechanical performance of a bentonite clay buffer for the containment of nuclear waste in the context of deep geological disposal. The design of the buffer is based on the Swiss concept where the waste canisters are emplaced on pedestals of compacted bentonite blocks and the remaining space between the tunnel and the canister is backfilled with grains of highly compacted bentonite. A complete analysis of the long‐term performance of the repository requires a good understanding of the mechanical evolution of the bentonite upon heating from the radioactive waste and hydration from the host rock. Despite its importance, the implications of the initial heterogeneous bentonite layout, characterised by blocks and grains, on the final dry density at the repository scale in the steady state have not been previously studied. The present study aims to shed light into these processes by means of finite element modelling using an advanced constitutive model for the bentonite behaviour that considers several thermo‐hydro‐mechanical couplings. The constitutive model is shown to be able to reproduce several laboratory tests involving saturation of block and pellets at different dry densities. The model predictions, extended up to 100,000 years, indicate that the bentonite blocks and grains tend to homogenise in terms of dry density as the buffer reaches full saturation. Due to the different swelling potential of the block pedestal and the granular backfill, the canister is subjected to movements, although these remain relatively small. The impact of initial segregation of the granular bentonite is also studied and it is seen to not to affect substantially the mechanical evolution of the buffer, although it might reduce canister displacements.

Shear strength and shear stiffness analysis of compacted Wyoming-type bentonite

Wyoming-type bentonite is one of the materials to be used as part of the Engineered Barrier System (EBS) in deep geological disposal facilities for the safe disposal of spent nuclear fuel.In the KBS-3 type repository, the canisters containing the spent fuel assemblies are surrounded by bentonite buffer. Therefore, it is necessary to understand the bentonite behaviour in shearing conditions for the long-term performance assessment of the canisters. Bentonite shearing could arise, for example, due to potential post-glacial earthquakes.The deviatoric stress at failure and the shear modulus of a compacted Wyoming-type bentonite were measured using different shearing tests. Tests were carried out on unsaturated samples at three laboratories. The shearing tests on unsaturated samples were unconfined and triaxial compression tests, hollow cylinder tests and simple shear tests. Unconfined and triaxial compression tests were also carried out on saturated samples. The failure was quite ductile when tests were carried out with confining pressure.The samples were compacted at different dry densities and degrees of saturation and tested at different confining pressures and strain levels to study their influence on the shear properties. Shear strength and shear modulus were studied based on models that consider the double porosity structure of the bentonites. Previously published triaxial tests and resonant column test results were also included in this study.In combination with these models, several alternatives for the definition of the constitutive stress were checked for the case of bentonite. A novel, approach, involving the results of mercury intrusion porosimetry in estimating the constitutive stress, was introduced.The shear strength calculated by the models showed a good agreement with the measured values. In some cases, unrealistic large tensile stresses were predicted. Furthermore, the models can assess the tendency of the shear modulus as a function of the shear strain, degree of saturation and dry density.

Real-time permeability evolution of limestone under high temperature and triaxial stresses

In deep underground geological disposal of nuclear waste, geothermal energy development, underground coal gasification, geothermal resources development and other underground high temperature projects, they are all closely related to the permeability and thermal expansion characteristics of rocks in high temperature high-pressure real-time state. In this experiment, the real-time evolution of permeability of limestone under high-temperature and triaxial stresses condition was studied using the HTHP rock mechanics triaxial test system and compared with the permeability of limestone after high temperature cooling treatment. The test results show that: the effect of temperature on limestone permeability facilitation under high temperature cooling condition is significant, and the permeability increases by 1–2 orders of magnitude relative to the initial permeability for every 100 °C increase in the preheat temperature after 300 °C. Under high-temperature and high-pressure in real time condition, the difference values of permeability at different temperature points relative to room temperature always do not more than one order of magnitude. During the heating process at 500 °C, the permeability curves of the triaxial pressures range from 15 to 35 MPa are very different. Under the pressure condition of 15 MPa, the permeability curve increases monotonically and rapidly with the increasing temperature; When the pressure reaches 25 MPa pressure, there is a peak value area in the permeability curve; the permeability decreases monotonically with temperature when the pressure up to 35 MPa. Compared with the only thermal effect, the thermal- mechanical coupling effect has a significant inhibitory effect on limestone permeability development, and the permeability disparity between the two types of heat treatment rocks become more and more significant with increasing temperature and hydrostatic pressure.

Development of Biosphere Assessment Modeling Strategy for Deep Geological Disposal in Generic Site of the Korean Peninsula

Development of Biosphere Assessment Modeling Strategy for Deep Geological Disposal in Generic Site of the Korean Peninsula

High-efficiency deep geological repository system for spent nuclear fuel in Korea with optimized decay heat in a disposal canister and increased thermal limit of bentonite

To use nuclear energy sustainably, spent nuclear fuel, classified as high-level radioactive waste and inevitably discharged after electricity generation by nuclear power plants, must be managed safely and isolated from the human environment. In Korea, the land area is limited and the amount of high-level radioactive waste, including spent nuclear fuels to be disposed, is relatively large. Thus, it is particularly necessary to maximize disposal efficiency. In this study, a high-efficiency deep geological repository concept was developed to enhance disposal efficiency. To this end, design strategies and requirements for a high-efficiency deep geological repository system were established, and engineered barrier modules with a disposal canister for pressurized water reactor (PWR)-type and pressurized heavy water reactor type Canada deuterium uranium (CANDU) plants were developed. Thermal and structural stability assessments were conducted for the repository system; it was confirmed that the system was suitable for the established strategies and requirements. In addition, the results of the nuclear safety assessment showed that the radiological safety of the new system met the Korean safety standards for disposal of high-level radioactive waste in terms of radiological dose. To evaluate disposal efficiency in terms of the disposal area, the layout of the developed disposal areas was assessed in terms of thermal limits. The estimated disposal areas were 2.51 km2 and 1.82 km2 (existing repository system: 4.57 km2) and the excavated host rock volumes were 2.7 Mm3 and 2.0 Mm3 (existing repository system: 4.5 Mm3) for thermal limits of 100 °C and 130 °C, respectively. These results indicated that the area and the excavated volume of the new repository system were reduced by 40–60% compared to the existing repository system. In addition, methods to further improve the efficiency were derived for the disposal area for deep geological disposal of spent nuclear fuel. The results of this study are expected to be useful in establishing a national high-level radioactive waste management policy, and for the design of a commercial deep geological repository system for spent nuclear fuels.

Evolution characteristics of acoustic emission and strain energy for deep granite under different damage stages

The first underground research laboratory (URL) as the research platform for the deep geological disposal of high-level radioactive waste (HLW) in China is being built in a deep granite rock mass. The damaged characteristics and mechanical properties of the deep granite under loading seriously influence the URL’s construction safety and disposal function for HLW. Therefore, this paper investigates the damaged characteristics and mechanical properties of the Beishan deep granite used in various confining pressure levels through a series of triaxial compression and acoustic emission (AE) tests. Based on the crack volumetric strain model, the crack stress thresholds of the deep granite specimens under various confining pressure levels were acquired. The damage development of the specimens used in five confining pressure conditions was analyzed, and the evolution of AE parameters and strain energy of the deep rock under different damage stages were explored. The studied results shown that the crack stress thresholds of the deep granite rose linearly with the increasing confining pressure, but their stress ratios remained relatively constant under various confining pressure levels. Under the rock loading tests, the designed confining pressure levels influenced the AE hit count and AE accumulative count development under the rock damage process. The high-frequency points of the AE hit count coincide with the AE accumulative count increasing, and the maximum value of the AE hit count of the specimens used in different confining pressure conditions appears in the postpeak stress‒strain stage. Additionally, with the axial strain of the loading rock rising in the prepeak stress‒strain stage, all the strain energy (including total energy U, elastic energy Ue and dissipated energy Ud) continues to rise, and in the postpeak stress‒strain stage, Ue stored inside the rock is quickly released, resulting in the Ue/U decreasing and Ud/U increasing.

Modelling simultaneous transport of natural and anthropogenic radionuclides in fractured media — Diffusion into the heterogeneous layered rock matrix for an arbitrary length decay chain

Transport models used for assessing the safety of radioactive waste repositories hosted in fractured bedrock typically do not consider fluxes of naturally occurring radionuclides in the rock and their further migration in flow-bearing fractures. A consistent model that simultaneously describes the transport of radionuclides from both natural and anthropogenic sources has been developed, where decay chains and rock heterogeneity are accounted for. The model accounts for advective flow in the fracture, a decay chain of arbitrary length, and diffusion into and out of the adjacent rock matrix composed of different geological layers. The proposed solution has been verified against a previously published steady state case which considers a homogeneous rock matrix of infinite extent where porewater ingrowth is not accounted for. The model is also applied to some different calculation examples for both transient and limiting steady state cases to represent typical applications of the model as well as to illustrate the effect of different parameters and processes on the transport of natural radionuclides in fractured rocks. This study presents a novel and powerful tool to simulate migration of both anthropogenic and natural radionuclides in and from crystalline rocks to the biosphere. The presented modelling is essential in safety and performance assessment of deep geological disposal of radioactive waste in fractured rocks. The obtained analytical solution can be used to compare relative fluxes of natural and anthropogenic radionuclides, which is useful for validation of the radionuclide transport parameters obtained from field and laboratory experiments.

Investigation of water-retention and uniaxial compression behaviour of MX80 bentonite pellet at various suctions

MX80 bentonite pellet/powder mixture has been regarded as a candidate sealing material in deep geological high-level radioactive waste disposal. As single pellet plays an important role in the global hydromechanical behaviour of the mixture, a series of single pellet compression tests were conducted on cylindershape MX80 bentonite 32-mm pellet after reaching suction equilibrium. The changes in volume and water content during the suction equilibration with the vapour equilibrium technique allowed the water-retention properties to be investigated. X-ray μCT observations were also made on pellets after reaching the target suctions. Results showed that upon drying the water content, void ratio and volumetric strain decreased slightly, but increased significantly upon wetting. Moreover, upon drying the network of cracks presented insignificant change and the cracks were mainly located in the marginal areas of pellet, while on wetting, cracks substantially propagated from the marginal areas to the core of pellet. The uniaxial compression tests on single pellets showed that the axial strain εaxial contributed to the contractancy, while the lateral strain εlateral to the dilatancy. The uniaxial shear strength σa, max, the crack closure (CC) stress σcc, the crack initiation (CI) stress σci, the crack damage (CD) stress σcd and the Young’s modulus E were found to decrease with suction decreasing, evidencing the suction effect through softening and cracking. Because of the combined effect of suction and wetting-induced cracks, the Poisson’s ratios v increased until the suction decreased to 38-25 MPa, and then decreased with further suction decrease. Uniaxial loading closed the horizontal wetting-induced cracks at suctions ≤ 9 MPa, leading to a drop of εlateral before CC.

An experimental methodology to assess the impact of desaturation on gas diffusion in clay based materials

The transport of gas in clay-based materials is dominated by diffusion. SCK CEN has studied and recorded an extensive data set of diffusion coefficients of gases in various saturated clays (Boom Clay, Opalinus Clay, Callovo-Oxfordian claystone and bentonites) used in Europe as host formations or in engineered barrier systems in the context of deep geological disposal of high and intermediate level nuclear wastes. However, partially saturated conditions may exist during the life cycle of a radioactive waste repository. The current study aims at establishing an experimental method to measure the diffusion coefficients of various gases in partially saturated clay-based materials by means of double through diffusion tests (cross diffusion of gases across the porous sample, driven by concentration gradients) and to assess the impact of desaturation on gas diffusivity. Historically, gas diffusion measurements have been performed under dissolved conditions with continuous water and dissolved gas flow around the clay sample. However, since the saturation degree of the sample has to be constant, a novel setup has been designed and optimized to operate under a pure gas phase under controlled relative humidity (RH). The experimental concept consists of cross diffusion of gas mixtures from two reservoirs at a fixed relative humidity on either side of the sample cell, namely, the upstream and downstream reservoirs. The RH is controlled using the same saturated salt solutions in both reservoirs. During the experiment, gases will diffuse from the upstream to the downstream reservoir. By measuring the change in gas composition in the downstream reservoir with a gas analyzer equipped with a mass spectrometer (which expresses the gas composition in terms of partial pressure), the gas flux and hence the diffusion coefficient is estimated by applying steady state Fick’s first law. The experimental methodology has been validated by performing an experiment on a sandy-clay sample of known petrophysical properties, at full saturation.

Suction in shales: consequences on triaxial testing

Significant interest has been devoted to claystones and shales in the context of geological radioactive waste disposal at great depth. The determination of their mechanical properties is needed for appropriate design of the underground galleries and tunnels. Given their high sensitivity to changes in water content, special care has to be taken so as to provide characteristics as close as possible to the (saturated) insitu ones. Based on the hydromechanical path followed by samples from coring to trimming in the lab, that most often lead to some degree of desaturation resulting from evaporation and drying of the samples, some procedures aimed at minimising the resulting perturbations are described. The considerations presented are based on data obtained on two (swelling) claystones considered in Europe for deep geological disposal, i.e., the Callovo-Oxfordian claystone (France) and the Opalinus Clay (Switzerland).