Tournemire Underground Research Laboratory Research Articles

DIGIT: An In Situ Experiment for Studying the Diffusion of Water and Solutes under Thermal Gradient in the Toarcian Clay Rock at the Tournemire Underground Research Laboratory: Part 1—Goals, Scoping Calculations, Installation and First Results under Unheated Conditions

DIGIT: An In Situ Experiment for Studying the Diffusion of Water and Solutes under Thermal Gradient in the Toarcian Clay Rock at the Tournemire Underground Research Laboratory: Part 1—Goals, Scoping Calculations, Installation and First Results under Unheated Conditions

Physical properties variations in a shaly formation across a fault core

Summary Faults in general, and in clay materials in particular, have complex structures that can be linked to both a polyphased tectonic history and to the anisotropic nature of the intact rock. Drilling through faults in shaly materials allows measuring properties such as the structure, mineralogical composition, stress orientation and physical properties. We combine different petrophysical measurements on core samples retrieved from a borehole drilled perpendicularly to a fault zone affecting Toarcian shales from the Tournemire underground research laboratory (France). The borehole is cross-cutting the entire fault thickness which is of the order of ten meters. We perform several types of measurements: density, porosity, saturation directly in the field, and P-wave velocities together with P-waves anisotropy on core samples taken at regular intervals. Special protocols were developed to preserve as much as possible the saturation state of the samples. From our measurements, we were able to track the increase of damage, characterized by a smooth decrease in elastic moduli from the intact zone to the fault core. We then calculated Thomsen's parameters to quantify the elastic anisotropy evolution across the fault. Our results show strong variations of the elastic anisotropy with the distance to the fault core as well as the occurrence of anisotropy reversal.

Impact of initial structural heterogeneity on long-term swelling behavior of MX80 bentonite pellet/powder mixtures

To better understand results of SEALEX in situ tests carried out at Tournemire Underground Research Laboratory, the hydromechanical behavior of a pellet/powder MX80 bentonite mixtures prepared at a dry density of 1.49 Mg/m3 were investigated by means of microfocus X-ray computed tomography (μ-CT) observations and laboratory small scale infiltration tests. Radial and axial swelling pressures as well as relative humidity were monitored while wetting. Two configurations were considered: for the first, a pellet/powder mixture was prepared following a specific protocol to minimize initial structural heterogeneity; the second one was specially designed to study a strong heterogeneous mixture distribution. μ-CT observations performed on the two samples during hydration revealed an apparently homogeneous sample for the first mixture after 100 days of hydration. For the second specimen, several voids were still observed after 40 days of hydration. A comparison was made between the in situ and mock-up tests. It was observed that the evolutions of radial and axial swelling pressures depend on the initial heterogeneous distribution of the mixture. This heterogeneity is due to the different dry density values at the vicinity of the different sensors. The final values of axial swelling pressures were different for both configurations for the same global dry density.

Potential of Surface-to-Tunnel Seismic Tomography to Detect Vertical Faults: Application to the Tournemire Underground Research Laboratory, France

The Tournemire Underground Research Laboratory (Aveyron, France), developed by the French Institute for Radiological Protection and Nuclear Safety, is composed of a tunnel excavated in an argillaceous geological layer belonging to a limestone–argillite–limestone sub-horizontal sedimentary sequence. Sub-vertical fault zones are intercepted by drifts and boreholes excavated from the tunnel in the argillaceous layer at a depth of about 250 m. These fault zones are characterized by velocities lower than those of the embedding rocks. In this study, we assess the capacity of first arrival traveltime tomography to detect and characterize such fault zones in a surface-to-depth transmission configuration with a limited angle aperture. The source at the surface is a sledgehammer and turns out to be very repetitive. The source trigger is transmitted to the acquisition bay via a VHF transmitter. Receivers are located at depth in underground works. First arrivals are clear and traveltime picking is straightforward. Before inverting real data, we conduct synthetic tests with the real acquisition geometry to assess limits and validity of first arrival traveltime tomography to detect a lower velocity fault zone using different initial configurations and inversion constraints during the inversion. We show that introducing a priori information on velocities in the shallow subsurface, limestones and argillaceous layers and constraints on maximum velocities greatly helps improving results and subsequent interpretation. Velocities obtained from the real data display a main lower velocity zone that extends from the galleries in the argillaceous layer right up to the surface in limestones. The location and extension of this lower velocity zone are consistent with fault and fracture zones mapped from geological observations.

Understanding oxidizing transient conditions in clayey rocks

Large amounts of carbon steels are foreseen in most deep geological disposals of high-level radioactive waste. The disposal cells will be initially aerated and the ventilation will renew oxygen at their front edge for years to decades. Excavation damaged zones (Edz) along underground works (tunnels, drifts) may promote oxygen migration. The diffusion and reactivity of oxygen within such zones is still poorly known despite that steel corrosion is significantly enhanced in the presence of oxygen. With this respect, four tests were implemented in the Tournemire Underground Research Laboratory, France. In each test, the oxygen partial pressure and the total pressure within a chamber isolated from the atmosphere by a packer were recorded over 90–150 days. Oxygen was never completely depleted in the chambers, likely due to oxygen diffusion through the Edz, whereas pyrite oxidation within the Edz was the main oxygen sink. The four in situ tests are different in terms of geometries (volume and surface of chambers) and drilling modes (compressed air flushing vs. pressurized argon flushing). A sequential global optimization of the relevant parameters (diffusion and reaction) has been developed, aiming at reproducing as closely as possible the measured four in situ tests data by modelling. The key controlling parameters are shown to be the Edz thickness and the ratio Dekre (effective diffusion coefficient over oxygen consumption first-order effective rate), whose optimum was found between 23 m2 and 45 m2. The Edz extent active for oxygen diffusion ranges between 0.6 and 1.0 borehole radius, with mean water saturation of 0.7, a mean De of 6 × 10−7 m2·s−1 and a mean kre of 2 × 10−8 s−1 (or kr = 6 × 10−7 s−1 per equivalent pore volume). With respect to the safety assessment of radioactive waste disposals, any plug system that would not intercept the Edz could not ultimately prevent oxygen entry into the structure, which could have an impact on dimensioning carbon steel components of disposal concepts. The proposed optimization methodology could be used to quantify diffusion and reaction kinetics in other contexts where pyrite reactivity and/or oxygen diffusion in water unsaturated media play a key role, such as the leaching of mining ores and acid mine drainage.

Fault imprint in clay units: Magnetic fabric, p-wave velocity, structural and mineralogical signatures

We investigated the evolution of the structure, physical properties and fluid flow properties across a fault zone within the Toarcian shale formation from the Tournemire Underground Research Laboratory (France). With this purpose, an integrated approach was used based on the analysis of petrofabric (anisotropy of magnetic susceptibility, AMS, and of p-wave velocity, APV) and the mineralogy of two cores intercepting the F2 fault, a strike-slip fault with a small vertical offset.The fracture patterns indicated that the fault registered a multiphase tectonic evolution. Two strike-slip regimes, with approximate NE-SW and NW-SE compression, and a normal regime with a WNW-ESE direction of extension were identified. Most of the N-S fault planes collected in TF1 core, have a shallow dip with values between 5° to 45°.We further evidenced a close relationship between the fault architecture and petrophysical properties. A preferential NW-SE orientation of the K1 axes was observed that is in agreement with the extensional direction of the dextral movement of the fault. Within the fault core, an important decrease in anisotropy was observed and indicates clay platelets rotations related to tectonic deformation. A weaker anisotropic change was observed in ASM1. This difference may be related to the local variability of the fault zone and suggests the need to study the characteristics of a fault zone both across its different architectural zones and along the main fracture plan.Finally, we identified evidence of minerals alteration related to fault reactivation and fluid circulation. Fluid circulation enabled the transformation of siderite in magnetite, a neo-formation of kaolinite and a precipitation of calcite were at vicinity of the damage zone/fault core boundary.In TF1, while the major mineralogical changes occur in the damage zone. In ASM1, a higher illite-smectite mixed layers and calcite content was observed in the eastern side of the fault.

Porosity distribution in a heterogeneous clay-rich fault core by image processing of 14C-PMMA autoradiographs and Scanning Electron Microscopy

AbstractShale formations are considered by a number of countries as the most suitable media to dispose of high-level radioactive waste. This is mainly due to the impermeable, self-sealing, chemical reducing, and sorption properties that tend to retard radionuclide migration. However, shale formations can also contain highly connected fault zones with permeabilities that can differ of several orders of magnitudes with respect to the undeformed host rock. The objective of this work is to use the 14C-PMMA autoradiography method combined with SEM-EDS measurements to understand the porosity variations in and around fault gouges and to define their relationship to mechano-chemical processes. The studied samples were taken from a low permeability shale in a small-scale vertical strike-slip fault at the Tournemire underground research laboratory. Results display significant variations in porosity and mineralogy along the studied gouge zone due to polyphased tectonics and paleo-fluid circulations.

Development of a viscoelastoplastic model for a bedded argillaceous rock from laboratory triaxial tests

Argillaceous rocks are candidate host and (or) cap formations for the geological disposal of nuclear wastes in many countries, including Canada, France, and Switzerland. The understanding of the long-term mechanical behaviour of such rocks is an essential requirement for the assessment of their performance as a barrier against radionuclide migration. The French Institute for Radiological Protection and Nuclear Safety (IRSN) operates an Underground Research Laboratory (URL) in Tournemire, France, in a rock formation known as the Tournemire shale. Many types of experiments are conducted at the Tournemire URL to better understand the physical and chemical behaviour of this shale and its interaction with seal materials intended to be used in the geological disposal of radioactive wastes. The Canadian Nuclear Safety Commission (CNSC) collaborates with the IRSN and CanmetMINING laboratories to perform experimental and theoretical research on the mechanical behaviour of the Tournemire shale. Using data from creep tests, and monotonic and cyclic triaxial tests performed at CanmetMINING Laboratories, we developed constitutive relationships for the mechanical behaviour of the Tournemire shale. The model is based on the theory of plasticity, and takes into consideration the inherent anisotropy due to the existence of bedding planes, hardening behaviour before the peak strength, and viscosity.

Comparative modelling approaches of hydro-mechanical processes in sealing experiments at the Tournemire URL

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 Surete Nucleaire), 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.

Effect of technological macro voids on the performance of compacted bentonite/sand seals for deep geological repositories

Three in-situ performance tests have been installed in the Tournemire Underground Research Laboratory to investigate the impact of technological voids on the long-term performance of the bentonite-based seals. These tests are still in progress, but the collected set of data provides already valuable information on the hydro-mechanical behaviour of the tested seals. The swelling cores consisted of pre-compacted blocks of a natural sodic Wyoming bentonite (MX80 type) mixed with quartz sand in a ratio of 70/30 (in dry mass) with different geometries (monolithic disks or four jointed disks). Two types of technological macro voids existed within the experiments: annular voids between the bentonite-based blocks and the rock and voids between the blocks. Artificially injected water volume, relative humidity, and swelling pressure in both radial and axial directions were monitored. Comparison of the results showed that the presence of technological voids constituted new hydration sources (annular voids) and flow paths (voids between the blocks) that changed the saturation kinetics. The effect of technological voids was also evidenced when comparing the swelling pressure kinetics. The existence of an additional void for one test between the packer and the host-rock greatly influenced the axial swelling pressure evolution, since it constituted a free volume into which the bentonite could extrude. Compared to monolithic disks, the evolution rates of radial swelling pressure were significantly lower in case of four jointed disks. For each test, different evolution of radial swelling pressure was observed, suggesting a heterogeneous structural distribution of bentonite/sand mixture within the blocks.

Hydro-mechanical analysis of SEALEX in-situ tests — Impact of technological gaps on long term performance of repository seals

The paper describes the observations and numerical analysis of the first SEALEX performance test PT-N1 installed in Tournemire Underground Research Laboratory (URL). The aim of this large-scale in-situ test is to investigate the impact of technological gaps on the long term performance of bentonite based seals. The swelling core consists of pre-compacted blocks of MX80 bentonite/sand mixture (70/30 in dry mass). An annular technological gap with variable width exists between the bentonite-based core and the host rock. The test is extensively instrumented for monitoring the main Hydro-Mechanical (HM) variables. Material parameters are determined from an extensive laboratory program carried out in the context of SEALEX project. To ensure an adequate interpretation of the test, the annular technological gap has been appropriately represented. A coupled HM formulation that incorporates the relevant processes involved in the problem under consideration has been adopted to analyse the evolution of the test. The paper presents and discusses the hydraulic and mechanical observations in the bentonite based core. Special attention is paid to the effect of technological gap on bentonite homogenisation as hydration progresses. The model was able to predict correctly the global HM behaviour of the bentonite based core considering the uncertainties and heterogeneities associated to the fabrication and installation processes of the bentonite based blocks. Technological gap was demonstrated to have an impact on dry density distribution.

SEALEX in-situ experiments-performance tests of repository seals: experimental observations and modelling

The paper describes observations and numerical analysis of SEALEX performance tests installed in Tournemire Underground Research Laboratory (URL). One of the objectives of the large scale in-situ tests is to investigate the impact of technological gaps on the long term performance of bentonite based seals. The swelling cores consist of pre-compacted blocks of a natural sodic Wyoming bentonite (MX80 type) mixed with quartz sand in a ratio of 70/30 (in dry mass) with different geometries (monolithic disks or four jointed disks). Several technological gaps exist within the in situ tests: Gaps between the blocks and annular gap with variable width between the bentonite-based core and the host rock. All the tests are extensively instrumented for monitoring the main Hydro-Mechanical (HM) variables. Comparison of the experimental results showed that the presence of technological gaps constituted new hydration sources (annular gaps) and flow paths (gaps between the blocks) that changed the saturation kinetics. A coupled HM formulation that incorporates the relevant processes involved in the problem under consideration has been adopted to analyse the effect of the annular technological gap on dry density homogenization of the bentonite based core as hydration progresses. Technological gaps were demonstrated to have an impact on dry density distribution.

Laboratory investigation on the mechanical behaviour of Tournemire argillite

This paper presents the results of a laboratory testing program that was designed to investigate the mechanical behaviour of the Tournemire argillite. Eighty rock samples were obtained from boreholes drilled at different angles in the walls and floor of an existing gallery at the Tournemire Underground Research Laboratory (URL), France. The experimental program consists of the measurement of the physical properties of the argillite and its mechanical response to loading during uniaxial tests, triaxial tests with various confining pressures, unconfined and confined cyclic tests, and Brazilian tests. Since the Tournemire argillite is characterized by the presence of closely spaced bedding planes, the rock specimens were loaded in different directions to bedding planes (i.e., loading orientation angle, θ = 0°, 30°, 45°, 60°, and 90°). Acoustic emission data were also recorded to detect the initiation and propagation of micro-cracks during the uniaxial tests. Most of the tests were performed at the natural moisture content of the rock specimens as delivered to CANMET Laboratories in Ottawa, Canada, where the experiments were conducted. The main objective of the testing program is to identify the mechanical properties of the Tournemire argillite. This paper mainly focuses on the description and interpretation of the test results. The development of an elastoplastic-damage model to describe the mechanical behaviour of the Tournemire argillite is the subject of another paper.

Characterization of low-molecular-weight organic acids isolated from the Toarcian argillite pore water (Tournemire site, France)

Bacterial activity in the near-field environment of radioactive waste disposal facilities in deep argillaceous rocks is currently under investigation. Bacterial development could be enhanced by the availability of low-molecular-weight organic acids (LMWOA) dissolved in the pore water of clayey rocks. This study firstly aimed at isolating and characterizing the LMWOA of the Toarcian argillite from the Tournemire Underground Research Laboratory (France, Aveyron). It also aimed at assessing the disturbing effects that could be induced by a disposal facility on the type and release of LMWOA (exposure to oxygen, alkaline pH, temperature increase). Under the experimental conditions investigated at 60°C, the mean dissolved organic carbon (DOC) is about 15 mg L−1 with a LMWOA proportion of 35 %. The main LMWOA are composed of formate, fumarate, propionate, acetate, and lactate. DOC is mainly influenced by both temperature and pH increases, whereas the LMWOA type nature and release are more specifically affected by the exposure to oxygen.

Temporal behavior of a ventilated claystone at the Tournemire URL: Cross-spectral analyses focused on daily harmonics

The main topic of this communication is the presentation of study, auscultation and supervision procedures of deep geological radioactive waste storage repositories using natural harmonic forcings. In this paper, the effects of natural ventilation on the macroscopic behavior of a clayrock are investigated by means of time series recorded underground over a period of two years in the eastern part of Gallery 1996 at the Tournemire Underground Research Laboratory (URL). This study is based on time series acquired in the atmosphere, at the gallery wall surface, and inside the rock mass. It includes measured signals from 6 thermo-hygrometers, 5 crack-meters (measuring the displacement of 2 shrinkage cracks and 3 tectonic fractures), and a 1 meter-FDR (Frequency Domain Reflectometry) profile probe equipped with 6 sensors for measuring the volumetric pore-water content into the rock mass. Auto-spectral and cross-spectral analyses using the concept of Singular Spectrum Harmonics (SSHs and cross-SSHs) are developed. Our analyses and interpretations focus here mainly on the solar diurnal atmospheric tide (denoted S1). This tide corresponds to the insolation cycle of the Earth atmosphere during a mean Solar Day (24h 00min). This component is tracked throughout the various measured signals (“Spectral Tracking” of tide fluctuations across signals). This is equivalent, in a way, to analyzing the temporal behavior of the URL during a “Mean Solar Day on Earth”. Results indicate that the daily natural forcing caused mainly by a combination of barometric and temperature related fluctuations, is the most important effect overall on our various signals. The daily harmonic induces the fluctuations of gallery air temperature, relative and absolute air humidity, and it leads to desaturation of the claystone, which in turn leads to the claystone deformation and damage. The effects of the annual harmonic SA may also be significant (it was fully analyzed in the more complete version of this work); however the SA results are not presented here because the spectral resolution on the annual harmonic is insufficient at this stage with only 2years and 2months of clean recorded data. Focusing the present study on daily fluctuations, the cross-spectral time shifts were obtained from the phase spectrum, for the daily component S1, for various pairs of pore-water content sensors located at different distances from the gallery wall. These time shifts were then used to quantify a “spectral velocity” which is found to be on the same order than the hydraulic conductivities deduced previously from pneumatic tests performed in the Excavation Damaged Zone of Gallery 1996.

Tournemire argillite/carbon steel evolution under bacterial influence after 10 years of in situ interaction

The purpose of the present study was to characterise the physicochemical evolution of argillite/carbon steel coupons after 10 years of interaction inside the Tournemire Underground Research Laboratory (France). Carbon steel coupons were introduced in boreholes drilled in the geological formation in contact with the argillite, and were overcored 10 years later to study the interfaces. The iron release by steel corrosion induced mineralogical dissolutions (Ca phases) and precipitations (iron oxides and hydroxides), leading to a partial clogging of the argillite porosity. A sulphur enrichment was observed in the corrosion area, linked to a potential microbial activity. The biodiversities differed depending on the steel type, indicating the influence of iron–clay interactions. This analysis has shown the presence of bacteria known for their involvement in corrosion processes and isolates able to develop at elevated temperatures.

First corrosion stages in Tournemire claystone/steel interaction: In situ experiment and modelling approach

Steels are possible materials for high-level radioactive waste containers used in long-term geological disposal in claystone environments. The French Institute for Radioprotection and Nuclear Safety (IRSN) is conducting an experimental programme on claystone/steel interactions in underground natural conditions at its Tournemire Underground Research Laboratory (URL, Aveyron, France). In this programme, several experimental conditions have been tested: i) the location in the geological formation, ii) the nature of the steel, iii) the claystone texture, and iv) the water content. This programme has investigated, after six years of experimentation, the effect of these parameters on the degree of steel corrosion, the spatial transport of iron and the claystone/steel interaction processes. Whatever the context results indicate corrosion rates of about 3μm/year for carbon steels and about 0.1μm/year for stainless steels. A detailed petrological study of the claystone/steel interface reveals a clear influence of free water content and claystone texture on the iron transport processes. All the data obtained from this study demonstrate that, during the initial oxic stage of steel corrosion, oxygen entrapped in the claystone porosity reacts with iron to form goethite/lepidocrocite minerals. Even after six years of reaction, there is no observed evidence of the anoxic corrosion of steel, irrespective of the context. The precipitation of iron hydroxide within the claystone causes intense calcite dissolution and a slight dissolution of mixed-layered illite–smectite. The minimum amount of oxygen required to form goethite in our experiments is estimated by thermodynamic simulations. This amount of oxygen is nearly the same as the amount of iron corroded. Moreover, additional simulations show that siderite forms when goethite becomes unstable under anoxic condition, and that the Tournemire mineral assemblage buffers the pore-water pH at a value close to neutrality, which prevents magnetite formation. Conclusions underline the important role played by oxygen entrapped in the near-field formation on the first iron corrosion stage. The quantification of the amount of oxygen present in the near field is a critical parameter that has to be taken into account in the nuclear waste storage safety assessment.

In situ investigations and reactive transport modelling of cement paste/argillite interactions in a saturated context and outside an excavated disturbed zone

The interactions between cementitious materials and a clayey deep formation were investigated by studying the specific in situ context of the Tournemire Underground Research Laboratory (URL) of the French Institute for Radioprotection and Nuclear Safety and by reactive transport modelling using the HYTEC code. The study forms part of the safety assessment framework for the deep geological disposal of high to intermediate level long-lived radioactive waste. The in situ context investigated in the Tournemire URL corresponds to an engineered cemented borehole crosscutting the Toarcian argillite formation. The argillite/CEM II cement paste contacts have been in place over 18a and were sampled in a saturated context outside the excavated disturbed zone (EDZ). Studies of the mineralogy (XRD, carbonatometry, SEM and TEM), petrophysical properties (BET) and geochemistry (TOC, Sr contents, C, O and Sr isotopes, EDS analyses) were carried out both on the argillite and on the cement paste in contact. Alteration of the cement paste is clearly expressed by decalcification and the opening of macroporosity. These modifications are mainly due to the dissolution of portlandite. The neoformation of C–S–H phases was identified in the first few micrometre next to the argillite interface, along with secondary carbonates at the outermost contact. Geochemical measurements argue for the introduction of a sedimentary fluid into the macroporosity of the cement paste to explain the formation of part of these secondary phases. This hypothesis is considered and tested using the HYTEC code, which indicates that such transport could have occurred near the argillite/cement paste contact at a very early stage. After this stage, the transport was reversed and ‘cementitious’ fluids flowed from the cement paste to the argillite. The changes brought about by these fluids are observed over a thickness of 11–13mm in a so-called ‘black rim’, in which carbonates and C–S–H secondary phases are identified in the matrix of the sediment. An illitization process may also be observed in this altered rim, reaching its maximum development towards the inner part. Geochemical analyses show that the argillite disturbances are strictly confined to the black rim. Theoretical mineralogical profiles based on thermodynamic equilibria defined by the HYTEC code are in good agreement with the observations, and are used to achieve a better understanding of transport processes.

Origin of Cl− and High δ37Cl Values in Radiocarbon Dated-Fracture Groundwaters at the Tournemire URL (France)

Fracture groundwaters collected from the indurated Toarcian/Domerian clayrock in the IRSN Underground Research Laboratory (URL) at Tournemire (south of France) are rare and limited in volume. The present study aims at resolving an apparent contradiction observed for these groundwaters: how can these groundwaters in transmissive fractures contain indicators of both geologically young and old residence times? Young residence times on the order of a few thousand years are indicated by the presence of 14C, and old residence times appear to be indicated by the extraordinarily positive δ37Cl values, between 5 and 8‰ vs SMOC. These highly enriched δ37Cl values might be attributed to a long diffusive transport (on the order of tens of million years) between the semi-permeable clayrock sequence and its surrounding aquifers, as estimated by modeling in a companion paper [1].Knowledge of the geometry of the fractures and the diffusion parameters in the intact clayrock allowed us to propose a 2D modeling for the transport of chloride and its stable isotopes using the Hytec code. Simulations applied to borehole M3 data show that the measured Cl values in the fracture groundwater are consistent with a diffusive exchange between the unaltered argillite porewater and meteoric waters that infiltrated in the fracture about 26 ka ago and have become isolated since then. This result is in good agreement with previously estimated maximum 14C residence times for these fracture groundwaters. The modeled δ37Cl value at 26 ka would be enriched by only 0.02‰ with respect to the initial imposed value for the argillite, which remains in the analytical uncertainty of ±0.2‰. These simulations provide evidence that the aqueous chloride and high δ37Cl values may reflect values of the unaltered argillite porewater.

Ten Years of Toarcian Argillite - Carbon Steel In Situ Interaction

In situ interaction experiments over periods of 2, 6, and 10 years between Toarcian argillite and carbon steel discs were carried out in the Tournemire Underground Research Laboratory (URL), yielding a dataset of the materials’ geochemical evolution under conditions representative of the future geological disposal of high-level long-lived radioactive wastes. The carbon steel discs were exposed to corrosion due to trapped oxygen. The corrosion rates indicate that the oxidizing transient lasted between 2 and 6 years. A systematic dissolution of calcium phases (Ca- smectite sheets in I/S and calcite) was observed in the iron diffusion halos. The iron release induced mineralogical dissolution and precipitation reactions, which partly clogged the argillite porosity.