Abstract Geochemical data from the deep aquifer system below the Boom Clay in NE Belgium have been collected and reinterpreted. The data were obtained between 1980 and 2014 within the framework of radioactive waste disposal studies. Currently, groundwater in the deep aquifer system mainly ranges between NaHCO 3 type and NaCl type. Because of the low groundwater velocity in this aquifer system, re-equilibration with the mineralogical composition of the host formations generally occurs. The main geochemical indicators point out that the current formation waters are a mixture between the original saline marine porewaters and freshwater recharge. SE–NW gradients of increasing ion concentrations are observed and can be explained in agreement with the pattern of natural groundwater flow. Calcite dissolution and cation exchange are still ongoing in this freshening aquifer system. A low sulphate content in the deep aquifer system indicates sulphate reduction associated with organic matter degradation. Inverse modelling along a flow path using PHREEQC generally confirms the reactions that have been derived in the data analysis.

Abstract In Denmark, mapping and preliminary investigations of Palaeogene and Early Neogene clay deposits have been performed over the past 5 years. The goal was to locate potential host rocks for the final disposal of low- and intermediate-level radioactive waste from the Danish Research Centre Risø, which has to be decommissioned within the next 5–8 years. Five areas with low-permeability Paleocene and Oligocene clay formations, situated in northern Jylland, NE Fyn and southern Lolland, not far from the Baltic coast at Femern Belt, have been suggested. The clay formations are between 75 and 150 m thick, and have large lateral distribution. They are covered by thin layers of glacial clayey tills and the shallow depth to the clay formations is attractive, partly because they are easy to access and partly because there are no groundwater aquifers situated above or below the clay deposits. The paper gives an overall review of the characteristics of the six different clay formations within four of the areas.

Abstract In 2009, ONDRAF/NIRAS, the Belgian Agency for Radioactive Waste and Enriched Fissile Materials, established a six-year research, development and demonstration (RD&D) programme to explore the feasibility of its concept for the geological disposal of Category B and C radioactive waste in Belgium. This programme, generally referred to as the ‘B&C Techno project’, aims to acquire evidence and build arguments in order to demonstrate that the proposed disposal system can be constructed, operated and progressively closed, taking into account long-term safety requirements and operational safety requirements. Galson Sciences Ltd (GSL) has been supporting the co-ordination of the activities involved in this project. This support has included synthesis of research outputs, and the development of various tools and methods to demonstrate that the Belgian geological disposal concept is feasible to implement. Two examples of studies conducted by GSL for ONDRAF/NIRAS as part of the B&C Techno project to substantiate feasibility are described in this paper: A review of other national geological disposal concepts to identify transferable experience relating to the feasibility of constructing, and assembling comparable disposal system components. Development of a storyboard illustrating the different steps involved in disposing of Category B and C waste.

Abstract The Full-Scale Demonstration Of Plugs And Seals (DOPAS) Project is a European Commission programme of work jointly funded by the Euratom Seventh Framework Programme and European nuclear waste management organizations (WMOs). The DOPAS Project aims to improve the industrial feasibility of plugs and seals, the measurement of their characteristics, the control of their behaviour over time in repository conditions, and their hydraulic performance with respect to safety objectives. Within the project, an approach has been developed and applied to assess the compliance of two of the full-scale experiments to their design bases. The approach involves a review of each requirement in the design basis and the strategy used to demonstrate compliance of the experiment with that requirement. Feedback in the form of proposed updates to the design statements is captured by this approach. Learning points on plugs and seals from the compliance assessment are also noted for consideration in the DOPAS Project outcomes. This developed compliance and assessment approach can be employed as part of, or in conjunction with, other more generic approaches used by WMOs, including monitoring, full-scale testing and the documentation of construction procedures.

Abstract The international DOPAS Project (Demonstration Of Plugs And Seals), which involves the participation of 14 European organizations (Posiva, ANDRA, DBE-TEC, GRS, NAGRA, RWA, SÚRAO, SKB, CTU, NRG, GSL, BTECH, VTT and ÚJV Řež), is concerned with the structural solution for sealing plugs to be used in deep geological radioactive waste repositories. The project is funded by the 7th Framework Programme – EURATOM. The Czech contribution to the DOPAS project (the EPSP experiment – Experimental Pressure and Sealing Plug) consists of an experiment that is being conducted at the Josef Regional Underground Research Centre (Josef URC, Čelina-Mokrsko). The aims of the EPSP experiment are to develop, monitor and verify the functionality of such plugs, and to determine a detailed characterization of the materials from which the plug is constructed. The EPSP experiment is being conducted by a Czech consortium made up of SÚRAO, CTU and ÚJV Řež, a. s.

Abstract The aim of the DOPAS project is to address the design basis, reference designs and strategies for the plugs and seals to be used in geological disposal facilities. The Czech ‘Experimental Pressure and Sealing Plug’ (EPSP) experiment has the following objectives: to develop, monitor and verify the functionality of an in situ experimental plug, and to determine and describe in detail the materials to be used with respect to experimental plug construction. The functionality of the experimental plug will be verified by means of conducting pressure tests after the construction phase of EPSP. The internal bentonite section of the EPSP will be constructed from bentonite pellets with a dry density of 1850 kg m −3 ; the pellets will disintegrate following saturation and swelling to form a homogeneous material with a dry density of around 1400 kg m −3 . This process leads to the question of the pore space of the bentonite material: that is, whether it will be possible to employ a numerical model for homogeneous compacted bentonite powder porosity instead of that of the double pore/void space of the bentonite pellets used in the experiment. For this reason, a laboratory physical model (PHM – physical hydraulic model) was incorporated into the DOPAS project for the purpose of describing the saturation and disintegration of the bentonite pellets. The aim of this paper is to describe the results of the space distribution of relative humidity in a sample consisting of bentonite pellets with a view to subsequent numerical modelling and to extend the current knowledge concerning the behaviour of the bentonite pellets used in the EPSP experiment. The results show that following water saturation and the swelling of the bentonite the pellets form a homogeneous mass in the same way as does the compacted bentonite powder. After a period of 160 days, the pellets attained 100% relative humidity at a distance of 12.5 cm and swelling pressure recorded at the end of the model was seen to slowly increase over this time period.

Abstract In the underground laboratory of Meuse/Haute-Marne (Bure) in France, different fracture types have been intensively investigated. Within a co-operative project between BGR and ANDRA, geophysical measurements and borehole gas tests were conducted in three well-designed boreholes in the gallery GRM to characterize the fracture structure and to determine the gas tracer velocity within fracture networks. On the basis of seismic measurements and nitrogen gas interference tests, helium was injected into an interval of borehole OHZ3003, where an unloading joint has been identified. The injection took place in the form of a 10 min ‘pulse’ with an injection pressure of 2 bar. The other two boreholes, OHZ3002, where an upper part of the shear-mode fractures (‘chevron’ pattern) dominate, and OHZ3001, where shear-mode fractures (subvertical ‘oblique’ fractures and ‘chevron’ pattern) exist, served as observation holes. To maintain the pressure gradient between the injection hole and the observation holes, nitrogen gas was subsequently flushed into the injection hole. Two breakthrough curves of helium concentration and pressure developments in the two observation holes were continuously monitored using two helium leakage detectors and pressure gauges. To interpret the measured pressure and concentration data, numerical models were constructed. A 3D model was used to simulate nitrogen gas flow and 2D models were applied to simulate a helium transport process. The real volume of the injection interval was considered in the model and the experimental process was simulated. Using the calibrated transport parameter data for a helium tracer from previous studies in the Mont Terri Rock Laboratory, the calculated breakthrough curves agreed well with those obtained from measuring the variation in permeability. The permeability derived from the helium tracer test agrees well with the estimation obtained from the nitrogen gas tests.

Abstract A laboratory programme of uniaxial, triaxial, cyclic and Brazilian tests was conducted to investigate the anisotropic mechanical behaviour of the Tournemire argillite, with different axial loading orientations with respect to the bedding planes (i.e. loading orientation angle, θ=0°, 30°, 45°, 60° and 90°). The experimental results show that both strength and deformation of the argillite are direction-dependent. Failure occurs in a brittle manner with a sudden collapse of the material strength. The failure mode exhibits localization along distinct failure planes and also depends on the loading orientation. This paper summarizes the experimental results and describes constitutive relationships that were developed in order to simulate the stress–strain behaviour of the Tournemire argillite. A microstructure tensor approach is adopted in order to take into account the anisotropic behaviour of the argillite. The identification procedure for material function and parameters is outlined, and the model is applied to simulate the set of triaxial tests performed at different levels of confining pressure and orientation of the bedding planes. It is demonstrated that the model adequately reproduces the anisotropy, the pre-peak stress–strain response and the onset of material collapse in those tests.

Abstract Convergence measurements recorded for one representative micro-tunnel (diameter c. 0.7 m) in Callovo-Oxfordian claystone were analysed. The micro-tunnel was excavated in the direction of the horizontal principal major stress. In situ observations showed anisotropic convergence with the maximum and minimum values in the horizontal and vertical directions, respectively. The horizontal closure of walls was fitted on the basis of a semi-empirical convergence law. This law is a predictive model reflecting the global response of the ground to excavation works. As the convergence measurements were performed after the end of excavation, their evolution in time can only be related to the time-dependent behaviour of the ground and the effect of the face advance cannot be captured. It is shown that some parameters of the semi-empirical law did not change along the micro-tunnel. An easy and efficient method is thus proposed for the long-term prediction of wall closure by the fitting of a single parameter on recorded data. Comparison with a drift (diameter c. 5 m) highlighted the influence of the support installation and the rate of excavation on the variation in the parameter values of the semi-empirical law. The vertical closure of the micro-tunnel walls, which showed a very weak evolution over time, was analysed based on the rate of convergence.

Abstract The excavation damaged zone (EDZ) around the backfilled tunnels of a geological repository represents a possible release path for radionuclides, corrosion and degradation gases that needs to be adequately addressed by safety assessment (SA) modelling tools. The hydromechanical phenomena associated with the creation and temporal evolution of the EDZ are of high complexity, precluding detailed representations of the EDZ in conventional SA. Thus, simplified EDZ models mimicking the safety-relevant features of the EDZ are required. In this context, a heuristic modelling approach has been developed to represent the creation and evolution of the EDZ in an abstracted and simplified manner. The key features addressed are the stochastic character of the excavation-induced fracture network and the self-sealing processes associated with the re-saturation after backfilling of the tunnels. The approach has been applied to a range of generic repository settings to investigate the impact of repository depth and in situ conditions on the hydraulic significance of the EDZ after repository closure. The model has been benchmarked with a dataset from a self-sealing experiment at the Mont Terri underground rock laboratory (URL), demonstrating the ability of the approach to mimic the evolution of the hydraulic significance of the EDZ during the re-saturation phase.

Abstract Permeability and its spatial distribution around an underground opening in a geological formation are important for the interpretation of thermal, hydraulic and mechanical findings from an in situ demonstration experiment. Within the site characterization programme of the Full-scale Emplacement (FE) experiment, permeability measurements with nitrogen gas have been conducted from six short boreholes. Four of them were located in a section without shotcrete support and two in a section with a three-layer-shotcrete lining. As expected, the extension of the zone with an increased permeability was larger (up to 2 m) in the area without shotcrete support than that in the section with a shotcrete lining (less than 1.5 m). The water content in the sections with or without shotcrete linings also showed different behaviour over long-term monitoring. The water content in the deep borehole section in the area with a shotcrete lining stayed almost constant, while the water content in the deep borehole section in the area without shotcrete tended to continuously decrease. In general, the water content close to the tunnel is influenced by the seasonal change in the temperature and relative humidity within the tunnel, especially in the section without a shotcrete lining. Analysis of the abovementioned observations/findings was done by performing FEM (finite-element method) calculations with OpenGeoSys (OGS) software using a coupled hydromechanical model. Owing to the high stiffness of shotcrete, the displacement in the section with a shotcrete lining was smaller. This, in turn, results in a smaller extension in the excavation damaged zone (EDZ). However, shotcrete has a relatively high suction capacity and high initial water content: thus, the interface between the shotcrete and the Opalinus Clay becomes more saturated. Therefore, the excavation-induced fractures in the Opalinus Clay close to the shotcrete can be sealed by swelling. The water content decreases continuously, as a result of desaturation occurring during the operational phase and the associated change in porewater pressure.

Abstract In the context of the underground storage of radioactive waste, the aim of this experimental study is to characterize the effect of damage on transport and water retention properties of Callovo-Oxfordian (COx) argillite. The originality of the study is to simultaneously investigate the pore-size distribution, water retention, the dry, effective and relative gas permeability, and the gas breakthrough pressure (GBP) of damaged COx argillite. These different properties are all relevant to characterizing the fluid transport ability of COx argillite. Results show that the damage has a significant impact on the properties of the COx argillite. It induces a decrease in its water retention capacity and GBP, and it increases its gas permeability and apparent porosity available to water owing to the creation of micro-cracks. Another objective is to show which of these properties is the most suitable to detect early damage states in COx argillite, with a potential use being to identify them in situ . GBP appears to be the best ‘detector’ of damage because of its sensitivity to damage even under high confinement pressures. Gas permeability could be a good indicator of damage, as it increases significantly (one or several orders of magnitude) after the damage. Finally, the water permeability curve is a poor indicator of COx argillite damage.

Abstract As part of its ongoing project on repositories for high-activity, long-lived radioactive waste, a 2000 m deep borehole was drilled by the French Nuclear Waste Agency (ANDRA) in the layered structure of alternating aquifers and aquitards of the Eastern Paris Basin. Among the information retrieved from this borehole, the vertical distribution of chloride in porewaters showed that, in addition to vertical diffusion, lateral advection in the aquifers plays a major part in transporting chlorine away from the study area. Helium concentrations were also measured in porewaters along the borehole. Because the helium input function is different from that of chlorine, it represents an excellent alternative tracer to further constrain transport characteristics. We applied an advection–diffusion model to the helium profiles with the appropriate source term for 4 He based on U–Th measured concentrations of uranium and thorium. 40 Ar/ 36 Ar data, which were available along the whole sequence, were also simulated. The modelled and measured 4 He profiles were in good agreement, indicating that the transport parameters used for the chlorine simulations were robust. 40 Ar/ 36 Ar simulations also gave coherent results and confirmed that most of the radiogenic 40 Ar remained trapped in the rocks (primarily in clays and feldspars).

Abstract The present work is focused on the study of strontium transport through crushed granite in the presence of bentonite colloids under dynamic arrangement. The aim of the experiments was to investigate the effect of bentonite colloids on strontium migration in crushed granite. The tracer behaviour was studied in a column set-up under aerobic conditions with a continuous inlet of the liquid phase of a constant tracer concentration (activity) and flow rate. Defined volumes of liquid phase were sampled at periodic time intervals at the column outlet for the measurements of tracer concentrations (activity). The transport was described by breakthrough curves. The stepwise approach included these steps: (1) an evaluation of the hydrodynamic column properties by the non-sorbing tracer 3 H; (2) a column experiment with bentonite colloids in deionized water was performed; (3) migration of 85 Sr solution in two liquid phases (deionized and synthetic granitic water); and (4) the transport of a radiocolloid suspension in deionized water was studied. Results showed different behaviour of bentonite colloids and strontium in the column. Bentonite colloids behaved as a non-sorbing tracer: conversely, strontium showed strong sorption on granitic material. The strontium transport in the presence of bentonite colloids differed from strontium transport itself. The strontium transport in the presence of colloids was faster than transport without the bentonite colloids. The observed retention of strontium on granite suggests a higher affinity of strontium towards granitic rock than towards bentonite colloids, and showed the reversibility of the sorption of strontium on bentonite colloids.

Abstract Through-diffusion experiments are conventional experiments to measure the transport parameters of radionuclides in clays. Typically, a regular replacement of the outlet volume by a tracer-free volume is performed. In the classical approach, this type of through-diffusion experiment is modelled by assuming a zero concentration in the outlet volume. Nonetheless, this assumption is not always correct, usually because the outlet volume is insufficiently large or the time between two consecutive replacements of the outlet volume is too long. Therefore, a model was developed disregarding this assumption and, instead, considers the tracer concentration in the outlet volume to evolve, as in the experiments: the flux into the outlet volume increases the tracer concentration and, at each replacement, the tracer concentration in the outlet volume is set to zero. The model was used to reproduce the diffusion of strontium (Sr) and tritiated water (HTO) in illite and Boom Clay. Model results yielded good matches with the tracer evolution in the inlet and the outlet, and the tracer profile in the core at the end of the experiment.

Abstract Modelling of radionuclide transport in clay can be simplified and accelerated by exploiting radial symmetry of laboratory experiments or nuclear waste repositories design. Consequently, the multispecies reactive transport code MCOTAC has been extended to radial symmetry, exploiting the advantages of its random walk transport description. Random walk in radial r -symmetry is mimicked by two-dimensional (2D) random walk projected to a radial coordinate for geochemical calculations. This guarantees fast 2D( x , y ) transport calculations, together with complex geochemical calculations in 1D( r ) only. The new model concept has been benchmarked for simple geochemical systems and applied to laboratory diffusion experiments with HTO, 22 Na and 36 Cl in Opalinus Clay using higher spatial resolution for modelling than reported in the literature. This makes it possible to distinguish between the transport properties of the sample and the filters, and yields more careful determination of radionuclide transport parameters: for example, there was a difference of a factor of 2 between the diffusion coefficient and log K d sorption coefficient in the case of 22 Na compared to the former best-fit analysis.

Abstract The transport of a radioactive solute during the transient thermo-hydraulic regime with gas generation in and around a disposal cell depends on complex multi-phase processes. Numerical simulations can improve the understanding of the system by providing detailed information on the temporal and spatial distribution of the radionuclides. In particular, their fluxes can be computed under the given transient conditions considering radionuclide, heat and gas release from the waste. However, such detailed multi-phase simulations are very demanding with respect to computational resources and time. Based on the knowledge gained from such complex simulations, we have developed a robust simplified single-phase approach for performance and safety assessment, the improved efficiency of which enables extensive parameter studies. The simplified approach comprises, on the one hand, homogenization of features of high detail and, on the other hand, the employment of two-phase simulation results that are used to deduce equivalent single-phase parameterizations. The results have been validated with various benchmark criteria at well-defined interfaces in the modelled disposal cells based on the simulated radionuclides fluxes.

Abstract This paper describes the thermohydromechanical (THM) simulation of engineered barrier systems (EBS) for the final disposal of nuclear spent fuel in Finland. The bentonite barriers were simulated with the Barcelona Basic Model and the model was calibrated from laboratory tests. The evolution of gap closure and the presence of a fracture intersecting the disposal were analysed. The simulations were performed in 2D axisymmetrical geometries. Full 3D simulations were carried out in order to check the effect of the third dimension. The time required for the barriers to reach full saturation, the maximum temperature, deformations and displacements at the buffer–backfill interface and the homogenization of components both locally and globally are the main interests. The effect of rock fracture and the hydraulic conductivity of the rock are subjected to 2D sensitivity analyses.