Abstract This paper describes how four scientific and safety relevant issues have been addressed in special-purpose research laboratories focusing on the geological disposal of high level and long-lived radioactive waste. These are: (a) the effects of heat on the engineered barriers and the geological environment; (b) the geochemical characterization of pore-water in argillaceous rocks; (c) the diffusion and retention of radionuclides; and (d) the full-size sealing of a waste emplacement. They are illustrated by experiments conducted in five underground research laboratories (URLs), three of which are in clay formations (Mol in Belgium, Centre de Meuse–Haute-Marne in France, and Mont Terri Rock Laboratory in Switzerland) and two in granite (Aspö Hard Rock Laboratory in Sweden and Grimsel Test Site in Switzerland). This paper highlights how the various types of experiments are related and how their results have been applied to foster progress. The most complex experiments have revealed artefacts and technical or methodological difficulties associated with interactions among multiple phenomena, the occurrence or intensity of which cannot be analysed by simple models. In turn, these difficulties have prompted experiments targeted at elementary phenomena, thereby encouraging the development of new investigation protocols and monitoring tools. More than 30 years of investigations in special-purpose URLs show the benefits of in-situ experimental programmes in the context of radioactive waste management. The laboratories have opened up avenues for research and advanced knowledge and technology. Thanks to a large component of international cooperation, they have made it possible to mobilize the financial and human resources required for this type of research. They have, above all, shared thoughts and promoted interdisciplinary studies around the same subject. They make common strategies possible at international level.

Abstract In this paper we describe the characteristics of the surface environment of the Boom Clay in the Campine area (outcrop and subcrop zone), which is regarded as a potential host formation for disposal of radioactive waste. A good description and understanding of the relationship between surface variables – the geomorphology, hydrography, vegetation, soils, land use and hydrology – is needed to evaluate the past evolution and assess the future evolution of the surface environment. Changing climatic conditions (glacials and interglacials), global sea-level variations and tectonic movements (uplift and subsidence) may cause the surface environment to change profoundly over long timescales. Starting from the present status, the palaeogeographical and palaeohydrological evolution of the Campine area is described in the framework of the Quaternary geological history of the area. Finally, a first assessment of possible future conditions of the surface environment is given, based on the integration of the palaeorecord and several published modelling studies.

Abstract Fluid pressures were recorded over 6.5 years in borehole EST207 located at the Andra Meuse/Haute-Marne Underground Research Laboratory in Bure (France). The borehole is equipped with a multipacker system monitoring 11 intervals, including 8 in the Callovo-Oxfordian clay Formation, 2 in the Dogger Formation and 1 at the Oxfordian base. Pressure data were analysed for responses to Earth tides and barometric pressure in the 11intervals. Fourier analyses on the pressure data revealed some of the Earth tidal components. Use of ETERNA software determined a better estimate of Earth tidal wave parameters (amplitudes, phases and their standard deviations) and barometric efficiency. Estimates of the tidal parameters K1, O1, P1, S1 and M2, N2, S2, K2 were made before and after barometric correction. It is suggested that in the Callovo-Oxfordian clay, the greater the clay content of the formation, the greater the barometric efficiency values. A barometric and tidal responses classification using four groups as a function of the 11 borehole intervals is in good agreement with the geological configuration. This preliminary geological interpretation will allow both the estimation of poroelastic and hydrogeological parameters of the clay formation from Earth tidal and barometric responses without classical pump test experiments, and the monitoring of these properties over a long period.

Abstract A full-scale shaft seal was designed and installed in the 5 m-diameter access shaft at Atomic Energy of Canada Limited’s (AECL’s) Underground Research Laboratory at the point where the shaft intersects an ancient water-bearing, low-angle thrust fault at a depth of c. 275 m in granitic rock. The seal consists of a 6 m-thick bentonite-based component sandwiched between 3 m-thick, keyed upper and lower concrete components. This design was adopted in order to limit the mixing of saline groundwater from the deeper regime with the fresher, near-surface groundwater regime. Construction of the shaft seal was done as part of Canada’s Nuclear Legacy Liabilities Program. A jointly funded monitoring project, called the Enhanced Sealing Project (ESP), was developed by AECL (Canada) and jointly funded by NWMO (Canada), SKB (Sweden), Posiva Oy (Finland), and ANDRA (France), and since mid 2009 the thermal, hydraulic and mechanical evolution of the seal has been constantly monitored. The evolution of the type of seal being monitored in the ESP is of relevance to repository closure planning by demonstrating the functionality of shaft seals. Although constructed in a crystalline rock medium, the results of the ESP are relevant to the performance of seals in a variety of host rock types.

Abstract The evolution of the clay-based engineered barrier system (EBS) of geological repositories for radioactive waste has been the subject of many research programmes during the last decade. The early post-closure thermal behaviour is elucidated by the HE-E experiment, a 1:2 scale heating experiment (at the Mont Terri Rock Laboratory), which was implemented in the first semester of 2011, with the initiation of the heating phase in June 2011. A maximum temperature of 140 °C was reached in June 2012. After 15 months of heating, the temperature evolution in the EBS and the Opalinus Clay reflects the design calculations, and thermally induced porewater overpressures are being measured at a few metres’ distance in the Opalinus Clay. Seismic methods proved to be a sensitive tool for the continuous characterization of changes of EBS and Opalinus Clay properties. Design modelling and predictive modelling based on the as-built parameter dataset with established coupled codes (TOUGH, CODE_BRIGHT; using various geometries) are described. The results indicate that the models are generally in agreement with the observations and capable of capturing the evolution of the experiment.

Abstract This paper describes a large-scale experiment on gas transport and hydromechanical processes around underground structures as part of a long-term geoscientific research programme at the Mont Terri Underground Rock Laboratory in the Jura Mountains of Switzerland. A horizontal microtunnel with a diameter of 1 m and a length of 13 m was drilled in an overconsolidated claystone formation. After installing monitoring instruments in the open tunnel, the end of the tunnel was backfilled with sand (test section) and a large hydraulic packer was emplaced in the seal section. The packer was inflated and subsequently the test interval was saturated with a synthetic pore-water. Following saturation an extended programme of hydraulic testing was performed over a two year period. A series of gas injection tests was then performed over a period of approximately 1.5 years. Following this first series of gas injections, a long post-gas hydraulic test has been initiated. The paper presents data and interpretation of the gas injections and subsequent hydraulic testing. The ability of the excavation damage zone to transport gas at pressures below fracturing is demonstrated. The post-gas hydraulic performance is considered and related to the self-sealing of the damage zone observed during saturation and hydraulic testing.

Abstract In 2011 the last phase in the installation of the PRACLAY In-Situ Experiment in the underground research facility HADES (Mol, Belgium) was completed. The main goal of the experiment is to perform a large-scale in-situ Heater Test. The Heater Test will examine the effect of the thermal load generated by heat-emitting waste on Boom Clay, currently considered as a potential host rock in the Belgian R&D programme for geological disposal. In 2007 the PRACLAY gallery was constructed to host the Heater Test. In 2010 a bentonite-based hydraulic seal was installed in this gallery, isolating the heated part from the non-heated part of the PRACLAY gallery. The primary objective of the seal is to provide undrained hydraulic boundary conditions for the Heater Test. As the performance of the seal is crucial to the Heater Test, it has been instrumented accordingly. The seal also provides an opportunity to gather additional information on the in-situ behaviour of bentonite-based repository structures. Finally the placement of a heating system and water-saturated sand in the heated section of the PRACLAY gallery completed the experiment installation. The water-saturated backfill sand has to assure undrained hydraulic boundary conditions at the interface between the clay and the gallery lining.

Abstract The hydro-chemical (CH) interaction between swelling Eurobitum bituminized radioactive waste (BW) and Boom Clay was investigated to assess the feasibility of geological disposal for the long-term management of this waste. First, the long-term behaviour of BW in contact with water was studied. A CH formulation of chemically and hydraulically coupled flow processes in porous materials containing salt crystals is discussed. The formulation incorporates the strong dependence of the osmotic efficiency of the bitumen membrane on porosity and assumes the existence of high salt concentration gradients that are maintained for a long time and that influence the density and motion of the fluid. The impacts of temporal and spatial variations of key transport parameters (i.e. osmotic efficiency (σ), intrinsic permeability (k), diffusion, etc.) were investigated. Porosity was considered the basic variable. For BW porosity varies in time because of the water uptake and subsequent processes (i.e. dissolution of salt crystals, swelling of hydrating layers, compression of highly leached layers). New expressions of σ and k describing the dependence of these parameters on porosity are proposed. Several cases were analysed. The numerical analysis was proven to be able to furnish a satisfactory representation of the main observed patterns of the behaviour in terms of osmotic-induced swelling, leached mass of NaNO 3 and progression of the hydration front when heterogeneous porosity and crystal distributions have been assumed. Second, the long-term behaviour of real Eurobitum drums in disposal conditions, and in particular its interaction with the surrounding clay, was investigated. Results of a CH analysis are presented.

Abstract Bentonite buffer is a part of the engineered barrier systems (EBS) for sub-surface disposal. The EBS is required to meet two quality standards at completion of construction. One is very low permeability of less than 5×10 −13 m s −1 and the other is very low diffusibility of less than 1×10 −12 m 2 s −1 . The bentonite buffer is required for the first quality standard of low permeability and must be constructed with high density in order to make the buffer a low-permeability layer. To confirm that the bentonite buffer is actually constructed with high density, we have carried out a full-scale mock-up test for part of the Demonstration Test of Underground Cavern-Type Disposal Facilities commissioned by the Japanese government in fiscal year 2005. This paper covers the test results from the construction of the bentonite buffer.

Abstract In the 2009 Andra repository concept, Intermediate Level Long Lived Wastes are stored in several 100-m-long storage cells. Concrete over-packs are stacked in piles before being emplaced by row. The large sections opened in the slab floor to enable emplacement by the conveyor could have a major effect on the temperature distribution in the storage cell. The aim of the study is to assess the ability of the ventilation to regulate temperatures along the cell. We focus on the period before closure, when the ventilation rate is at a minimum. The issue has been addressed by undertaking numerical simulations. A specific modelling approach based on head-loss correlations has been used to calculate the distributions of temperature and air velocity at a decimetre scale along the storage cell. The heat transfer and the air flow problems have been solved on a 3D mesh representing 225 rows of waste packages, the concrete walls and the surrounding rock. Different scenarios have been considered about the air flow rate, the heat release and the closure of the conveyor sections at several locations. Results have been analysed in terms of flow patterns, temperature distributions and thermal gradients in air and in concrete.

Abstract For more than 35 years, SCK•CEN has been investigating the possibility of high-level radioactive waste disposal in the Boom Clay in northeastern Belgium. This research, defined in the long-term management programme for high and medium long-lived waste of ONDRAF/NIRAS, includes the regional hydrogeological modelling of the aquifer systems surrounding the Boom Clay. In this paper, the most recent update of the Deep Aquifer Pumping model (DAP) is described, which is capable of quantifying the regional groundwater flow in the complex confined aquifers lying below the Boom Clay in NE Belgium. The DAP model was successfully calibrated using an automated inverse optimization algorithm and is able to reproduce satisfactorily the general trends in the observed groundwater heads. This model can be used as a tool for planning future characterization efforts and reducing the predictive model uncertainty.

Abstract The primary purpose of the present work was to qualitatively evaluate the relevance of microbial activity for the long-term performance of a deep geological repository for high-level radioactive waste and spent nuclear fuel utilizing clay and to identify which safety-relevant processes and properties can be potentially influenced by this activity. Deterioration of clay properties accompanying destabilization and destruction of clay mineral structure as a result of microbial actions can be considered as the primary microbial impact on clay. The present analysis identified eight clay properties essential for maintaining safety functions of containment and retardation of the disposal system – swelling pressure, specific surface area, cation exchange capacity, anion sorption capacity, porosity, permeability, fluid pressure and plasticity – which can potentially be influenced by microbial processes in clay-based materials and claystone within a repository. Radioactive waste canisters and over-packs made from cast metal or steel represent a further component of the engineered barrier system which can be strongly affected by microbial activity in the clay buffer or in the adjacent host rock. This work should provide a basis for a follow-up quantitative estimation of the maximum possible effects of microbial processes on the barrier system of a deep geological repository in clay.

Abstract The experimental determination of transport properties of low permeability clay rocks, especially of relative permeabilities and capillary pressure curves for water and gas, is a very challenging issue, in particular at high water saturation (very low gas permeability resulting in long equilibration times) and for gaseous hydrogen (due to the high pressures involved and the resulting explosion risk). Navier-Stokes equations are solved inside a porous medium on the pore scale, so as to derive the absolute and relative (two-phase-flow) permeabilities. For this purpose microtomography data of Opalinus clay samples acquired in the Mont Terri Ventilation Experiment are used to visualize the pore space in 3D at a micrometric scale (porosity size >0.7 µm). The corresponding percolating porosity is mainly composed of micrometric cracks parallel to the bedding and attributed to shrinkage. Two-phase flow is calculated in the percolating cracks by an immiscible Lattice Boltzmann (LBM) code. In addition, some validation results of the LBM model for three-phase systems (liquid-gas-solid) are presented.

Abstract Deep argillaceous rocks are reducing environments. When exposed to air, reduced minerals of these rocks react with oxygen, modifying the surrounding chemical conditions. Thus, oxidation is an issue in studies about the confining properties of such rocks in the framework of geological disposal projects for radioactive waste. Previous studies in several underground research laboratories (URLs) in argillaceous rocks have shown that oxidation reactions mainly occurred in the excavation-induced fracture network surrounding the drifts. In the Callovian–Oxfordian argillaceous rock, at −490 m in drifts from the Meuse/Haute-Marne URL, oxidized features were systematically looked for in 115 borehole cores. The concerned drifts were of various ages, from a few days to 6.5 years. After 5 months, oxidized features were encountered in numerous excavation-induced extensional fractures. In excavation-induced shear fractures, oxidized features were observed in a few borehole cores after 2 years, and they became frequent after 6 years. In all cases, the oxidized features observed were found on the fracture walls or were connected to them, and were less than 1.8 m from the drift walls. These observations about the oxidation front and its evolution over time provide insights regarding the properties of excavation-induced fractures with respect to oxygen transfer.

Abstract Recently, the hydraulic/mechanical/chemical (HMC) analytical method has been studied with the aim of evaluating the long-term performance of Trans-Uranium (TRU) geological repositories. In this particular research, the hydraulic/mechanical modelling of bentonite materials for HMC analyses has been studied, with bentonite materials considered as the engineered barrier. It is said that the bentonite material in the TRU disposal facilities is altered chemically by cement leachate. Moreover, there is a concern that chemical alteration changes the expansion characteristics and water permeability of the bentonite material. In this paper, using past test results, the influence of montmorillonite content and the exchange of sodium and calcium ions on mechanical and hydraulics behaviour is examined. Hydraulic/mechanical mathematical modelling is also presented, describing the mechanical behaviour of bentonite from an initially unsaturated state while also taking into account the hydraulic and mechanical characteristics of bentonite mentioned above. We also simulate the saturation process from an initially unsaturated state in a TRU disposal facility, which is modelled one-dimensionally.

Abstract The interaction of steel with bentonite used as buffer material in high-level waste repositories may result in changes to the properties of the buffer. One of the repository designs (KBS-3H) developed by Posiva and SKB foresees the horizontal emplacement of so-called supercontainers, consisting of copper canisters surrounded by compacted bentonite and an outer perforated steel shell. The corrosion of the steel shell and the interaction of iron with the clay may impair the long-term safety functions of the buffer. The corrosion and iron–clay interaction processes within the KBS-3H concept were assessed with a kinetically based reactive transport model and a comprehensive thermodynamic database. The large uncertainty related to precipitation rates of corrosion products and iron silicates was considered by defining a series of test cases. The results generally indicate a limited effect on the stability of montmorillonite, thus affecting only a few centimetres next to the iron source. Upon complete corrosion only insignificant changes are predicted. These results are explained by (i) the diffusional constraint of mass transfer, (ii) low solubility of corrosion products and (iii) slow transformation kinetics of montmorillonite. Model results further suggest that the largest impact arises from ‘indirect’ processes, such as microbial sulphate reduction, which may lead to a strong increase in pH.

Abstract The Prototype Repository is an on-going full-scale trial of a final disposal of spent nuclear fuel designed in close agreement with the Swedish KBS-3V design. It provides an excellent opportunity to test our modelling capabilities and our ability to predict evolution of conditions in the planned repository for nuclear waste in Sweden. Here we describe our current efforts to simulate the experiment and compare results with available measured data. To reduce the computational demand, a modelling strategy was adopted in which large-scale low-resolution models (including the entire experiment and a large volume of surrounding rock) were used to determine boundary conditions for local-scale high-resolution models (including one deposition hole). The results obtained so far generally show reasonably good agreement with measurements available from the field experiment.