Abstract Understanding the behaviour of gases in the context of radioactive waste disposal is a fundamental requirement in developing a safety case for the disposal of radioactive waste. Of particular importance are the long-term performance of bentonite buffers and cement-based backfill materials that may be used to encapsulate and surround the waste in a repository, and the behaviour of plastic clays, indurated mudrocks and crystalline formations that may be the host rocks for a repository. The EC Euratom programme funded project, FORGE, has provided new insights into the processes and mechanisms governing gas generation and migration with the aim of reducing uncertainty. This volume brings together papers on aspects of this topic arising from both the FORGE project and work undertaken elsewhere. This has been achieved by the acquisition of new experimental data coupled with modelling, through a series of laboratory and field-scale experiments performed at a number of underground research laboratories throughout Europe.

Abstract Understanding the behaviour of gases in the context of radioactive waste disposal is a fundamental requirement and was the focus of the FORGE ( F ate O f R epository G as E s) project. Of particular importance is the long-term performance of the bentonite buffers and the cement-based backfill materials that may be used to encapsulate and surround the waste in a repository for the geological disposal of radioactive waste and plastic clays, indurated mudrocks, and the crystalline formations that may be the host rocks of a repository. FORGE did not study salt host rocks or salt based backfill materials. FORGE has provided new insights into the processes and mechanisms governing gas generation and migration, with the aim of reducing uncertainty relating to the quantitative treatment of gas in performance assessment. This has been achieved by the acquisition of new experimental data coupled with modelling through a series of laboratory and field-scale experiments (performed at a number of underground research laboratories throughout Europe), and modelling. New methods were developed for upscaling from laboratory to field conditions, allowing the optimization of disposal concepts through detailed scenario analysis. Understanding a repository system to an adequate level of detail is required to demonstrate confidence in the assessment of site performance, recognizing that a robust treatment of uncertainty is desirable throughout this process.

Abstract In deep geological repositories for the disposal of radioactive waste, gas can be generated by different mechanisms including anaerobic corrosion, radiolysis and microbial degradation. If the gas generation rate is larger than the capacity for the diffusive transport of the dissolved gas, a free gas phase will be formed, eventually leading to gas-breakthrough events. Depending on the timing of gas breakthrough, dissolved radionuclides (RN) and contaminants could be driven out of the clay faster than the normally expected diffusive transport. A column experiment was designed in which a water-saturated clay core is placed directly on top of a thin Boom Clay core that has been previously saturated with a tracer solution (0.01 mol l −1 NaI). A helium gas pressure is applied and stepwise increased. Upon gas breakthrough, the water on top of the column is expelled and analysed for its iodide content. The measured concentration iodide is linked to the amount of NaI-saturated pore water that was displaced. Based on the results, it can be concluded that the transport of radionuclides and contaminants because of a gas-breakthrough event is possible but appears to be very limited. The volume of water displaced is very low (three orders of magnitude) compared to the volume of gas transported upon breakthrough.

Abstract Deep underground repositories for radioactive waste generally rely on a multibarrier system to isolate the waste from the biosphere. It consists of the natural geological barrier provided by the repository host rock and its surroundings, the waste container and an engineered barrier system (EBS): that is, the backfilling and sealing of shafts and galleries to block any preferential path for radioactive contaminants. Bentonite emplaced in compacted block form is the preferred option for the clay buffer for most waste management organizations. In assessing the performance of bentonite block masonries, conductive discrete interfaces inside the sealing elements (i.e. contacts between blocks) and to the host rock may act not only as mechanical weakness planes but also as preferential fluid pathways. We performed hydraulic tests on prefabricated bentonite–sand block assemblies (60:40). The results document that despite existing interfaces, the investigated bentonite block assembly behaves no different to that of the homogenous matrix during the saturation of the buffer. This has been confirmed by gas-injection tests on the former interface, as well by shear tests. The outstanding observation is that our results convincingly demonstrate that interfaces between bentonite bricks may ‘heal’ (not only seal), as was physically verified by confirmation of cohesion after presaturation.

Abstract A UK repository concept currently under consideration for the disposal of intermediate-level radioactive waste and some low-level waste not suitable for surface disposal involves using large quantities of cementitious materials for construction, grouting, waste containers, waste isolation matrix and buffer/backfill. CO 2 generated from the degradation of organic material in the waste will result in cement carbonation and associated mineralogical changes. Hydraulic and gas permeability tests were performed on Nirex Reference Vault Backfill (NRVB) cement at 40 °C and either 4 or 8 MPa. Carbonation reactions using CO 2 gas halved the permeability of the NRVB under simulated repository conditions. A greater decrease in permeability (by three orders of magnitude) was found during carbonation using dissolved CO 2 . Mineralogical changes were found to occur throughout the cement as a result of the reaction with CO 2 . However, a narrow zone along the leading edge of a migrating reaction front was associated with the greatest decrease in porosity. Fluid pressures increased slightly due to permeability reductions but fluid flow still continued (albeit at a lower rate) preventing the build-up of overly high pressures. Overall, the observed reductions in permeability could be beneficial in that they may help reduce the potential for fluid flow and radionuclide migration. However, continued carbonation could lead to potential issues with regards to gas pressure build-up.

Abstract The gas-breakthrough pressure values in saturated FEBEX bentonite were determined for different dry densities and sample sizes. They increased clearly with dry density and were always higher than the swelling pressure of the bentonite. In high-density samples, gas flow tended to stop abruptly after breakthrough, whereas, in lower density samples, gas flow decreased gradually until a given pressure gradient was reached. The permeabilities computed after breakthrough (which usually did not stabilize) were inversely related to dry density. This would indicate that, despite the fact that flow took place through preferential pathways, the bentonite matrix and its swelling conditioned the ease of pathway formation. These paths sometimes closed quickly after breakthrough and others remained open, allowing a gradual decrease in gas flow. After resaturation of the bentonite, the same breakthrough pressures and permeabilities were found, pointing to the perfect healing of these preferential pathways. A sealed interface along the bentonite did not seem to affect the breakthrough pressure or permeability values.

Abstract Concrete is used as a barrier on surface or near-surface facilities for the final disposal of low- and intermediate-level radioactive waste, where gas can be generated and affect the hydraulic properties and the processes taking place in concrete. In this framework, gas-transport properties of concrete samples were investigated using two different laboratory test set-ups: a non-steady-state equipment working under low injection pressures; and a newly fine-tuned steady-state set-up working under different pressures. Permeability decreased with water content increase but was also greatly affected by the hydraulic history of concrete (i.e. if it had been previously dried or wetted). The intrinsic permeability determined with gas flow was about two orders of magnitude higher than that determined with liquid water (10 −16 v. 10 −18 m 2 ), probably due to the chemical reactions taking place during saturation (carbonation). The relative gas permeability of concrete increased sharply for water degrees of saturation smaller than 50%. The boundary conditions also affected the gas permeability, which seemed to be mostly conditioned by the back pressure and the confining pressure, on the whole decreasing as the effective pressure increased. It is considered that the Klinkenberg effect was not relevant in the range of pressures applied.

Abstract The excavation damaged zone (EDZ) around a radioactive waste repository will act as a potential escape route for gases produced from anoxic corrosion of waste containers and other metallic components within the underground facility. For assessment of the impact of the gases on the repository safety, the gas-flow behaviour of the EDZ has to be characterized, understood and predicted. This issue has been recently investigated for the Callovo-Oxfordian and Opalinus claystones at the GRS laboratory. Various kinds of gas-flow experiments were carried out by flushing nitrogen gas through cracked and resealed claystone samples under different hydromechanical conditions. Extensive results obtained include: (a) gas permeability of fractured claystones in relation to fracture closure under loads; (b) effects of gas humidity on the sealing and gas permeability of fractures; (c) gas-flow behaviour in water-saturated and resealed fractures characterized by three key parameters, namely gas-breakthrough pressure, permeability and shut-in pressure; (d) relationships of the gas parameters to the intrinsic permeability of the resealed claystone and the applied confining stress, as well as relationships between the gas parameters; and (e) the impact of gas-pressure rise on the reopening of the closed pathways and/or creation of new ones in resealed claystone.

Abstract Centre of Experimental Geotechnics (CEG) participated in the FORGE project within WP4, which was focused on disturbed host rock formations. The Czech deep radioactive repository concept envisages crystalline rock as the host rock environment; therefore the CEG concentrated on such formations. Large-scale gas injection tests were carried out in Josef Underground Laboratory as part of the project aiming to study the behaviour of both the excavation damaged/disturbed zone (EDZ) and the undisturbed rock environment. The project schedule involved the performance of a large number of tests at various pressure levels and flow rates and in different parts of the rock mass. The various phenomena were studied employing gas injection test techniques using both single- and double-packer equipment. The tests performed revealed a significant difference in terms of gas permeability between the EDZ and the undisturbed rock mass. An investigation of the influence of the effect of pressure on gas permeability was also performed in order to study the process of the opening and closing of fractures (pathway dilation).

Abstract Besides other host rocks, rock salt formations are considered for the long-term storage of radioactive waste to exclude any threat to the biosphere. This means that the host-rock’s integrity has to be guaranteed during construction, operation and in the post-closure phase of a repository. Gas-transport properties are key issues in the long-term assessment of the storage of high-level radioactive or toxic waste in salt formations. In this context, the impacts of disturbance induced by the excavation of the underground facilities and long-term effects during recompaction of the excavation disturbed zone (EDZ) are important items but in the past significant progress has been made. However, significant gas quantities may be generated in the long term (e.g. due to anaerobic corrosion, if humidity is present), resulting in a time-dependent pressure build-up that may affect the barrier integrity if the acting minimal stress is exceeded. The objective of this paper is to review the current understanding of the gas-transport properties in a salt environment at increased gas pressures associated with a radioactive waste repository. This knowledge has mainly been developed by laboratory work and fieldwork over the last decade.

Abstract This paper presents the results of a benchmark study that compares a number of numerical models applied to a specific problem in the context of hydrogen flow and transport in a nuclear waste repository. The processes modelled are two-phase (water and hydrogen) immiscible compressible two-component transient flow in a heterogeneous porous medium under isothermal conditions. The three-dimensional (3D) model represents a module of a repository for high-level waste in a clay host rock. An upscaling technique and a vertex-centred finite-volume method are employed to yield very accurate solutions. Since the full range of results required in the benchmark is too large to be displayed in this paper, we focus on the evolution of the pressures, the saturations, the fluxes and the comparison of the numerical results with the other participants. A homemade C++ upscaling code and the parallel multiphase flow simulator DuMu X have been adopted for this study.

Abstract The HG-A in situ test , located at the Mont Terri Underground Rock Laboratory (URL), was analysed as part of the FORGE project. This test investigated the behaviour of the excavation damage zone (EDZ) around a backfilled microtunnel by a series of water- and gas-injection tests. Prior to testing, the backfilled microtunnel was sealed with a hydraulic megapacker system. A key aspect was the investigation of crack opening and closure along the EDZ in response to water and gas injections in the context of radioactive waste disposal. In the model, the intrinsic permeability of the EDZ was assumed to depend on deformation, and additional simplifying assumptions were considered: axisymmetry about the tunnel axis; no gravity; soil slices orthogonal to the tunnel axis move independently and in plane strain; liquid and gas flows along the EDZ parallel to the tunnel axis; and undrained saturated conditions for the Opalinus Clay. As a result, the field equations were reduced to differential equations for liquid and gas pressures defined in a one-dimensional (1D) domain representing the EDZ. The main trends of the pressure evolution observed in the test section were reproduced. A 2D axisymmetric model confirmed the validity of the simplifying assumptions, except for small zones of the EDZ near the megapacker ends.

Abstract The aim of this work is to provide an improved mathematical and physical description of two-phase flow in tight porous media in order to model hydrogen migration within a geological repository for radioactive waste as required to further assess the behaviour in time and space of such facilities. First, we introduce a general physical framework describing multicomponent and two-phase (liquid and gas) flow in porous media with capillary pressure curve and mass exchange between phases. Assuming thermodynamic equilibrium, mass exchange between phases is governed by thermodynamic principles which we briefly describe. This physical modelling is able to describe single-phase flow (only gas mixture or liquid solution) as well as two-phase flow (both liquid and gas phases are present). On the basis of this treatment we obtain a generalized formulation of Henry and Raoult–Kelvin laws. We show that, for a hydrogen–water mixture in repository conditions of pressure and suction, these relationships constitute an adequate mass equilibrium description. Second, we consider the mathematical formulation of the problem, which is mainly determined by the choice of primary variables. Contrary to existing approaches, we are interested in avoiding the need to switch primary variables during calculations. Indeed, such a switch is often proposed as a solution to treat phase appearance or disappearance. We will show that, in our physical framework, taking into account the capillary pressure and mass exchanges between phases, it is possible to work with universal primary variables to correctly describe gas phase appearance or disappearance. Simple numerical simulations are presented for validation as well as an application of this model to the FORGE benchmark on the cell scale together with a sensitivity analysis guided by physical considerations.

Abstract This paper presents an investigation of the transport and fate of hydrogen gas through compacted bentonite buffer. Various geochemical reactions that may occur in the multiphase and multicomponent system of the unsaturated bentonite buffer are considered. A reactive gas transport model, developed within an established coupled thermal, hydraulic, chemical and mechanical (THCM) framework, is presented. The reactive transport module of the model considers the transport of multicomponent chemicals both in liquid and gas phases, together with an advanced geochemical reaction model. The results of a series of numerical simulations of the reactive transport of hydrogen in unsaturated bentonite are presented in which hydrogen gas, because of the corrosion of a steel canister, has been injected at a realistic rate into a partially saturated bentonite buffer. Gas pressure development and the fate of the hydrogen gas with respect to the geochemical reactions are studied. The results show the high buffering capacity of unsaturated bentonite, when considering a steel canister, over a period of 10 000 years. The presence of accessory minerals is shown to have an important role in mitigating excess hydrogen ions, thus increasing the dissolution capacity of the system to gas. The development of various forms of aqueous complexations between the inorganic components and the hydrogen ions were also found to be important in buffering the excess hydrogen that evolved. Based on the results obtained, it is postulated that the presence of various chemical components in the clay buffer may influence the transport and fate of the hydrogen gas.

Abstract To investigate gas-migration processes in saturated low-permeability argillaceous rocks, gas-injection tests under different injection pressures were carried out at different scales: on core samples at the laboratory scale; in the packed-off section of boreholes at the borehole scale (HG-B); and in the sealed microtunnel at the tunnel scale (HG-A) – a 1:2 scale experiment at the Mont Terri Rock Laboratory, Switzerland. All three tests at the Mont Terri Rock Laboratory involved Opalinus Clay. A fully coupled hydromechanical model has been developed that takes account of elastic and plastic anisotropies, anisotropic two-phase flow based on the van Genuchten function, and permeability changes when evaluating the experimental data. Two different flow regimes were studied: two-phase flow under low gas-injection pressure and dilatancy-controlled gas flow under high gas-injection pressure above the confining pressure in the laboratory experiment or the minimal principal stress in situ . When dealing with the dilatancy-controlled gas-flow regime, special consideration was made by applying two permeability approaches in which (i) the permeability change was pore-gas-pressure dependent and (ii) where the permeability change was deformation dependent. Using the parameter values determined by laboratory data, the in situ borehole tests obtained under well-defined hydromechanical conditions could be analysed accordingly. The gas-flow regime in large-scale experiments, as in the case of HG-A, is mainly governed by experimental circumstances: in this case, the excavation-induced fractures around an opening with a permeability four order of magnitude higher than that in the undisturbed rock mass.

Abstract The ‘subsurface disposal’ concept has been proposed for relatively higher-activity low-level waste (LLW) in Japan. This concept includes a low-permeability layer (LPL) made of bentonite material and a low-diffusion layer (LDL) made of dense cementitious material. The influence of gas generation and migration on the mechanical stability of the engineered barrier system (EBS) is one of the issues for long-term performance assessment of the disposal facility. In this study, coupled hydromechanical modelling and analyses are carried out in order to evaluate the mechanical stability of the system. Two gas generation rate cases are simulated: (1) a reference case; and (2) a conservative case. It is found from the analyses that the tensile stress developed in the cementitious components due to accumulated gas pressure is lower than the tensile strength of the materials, and that stress developed in the LPL remains compressive apart from at the interface between the LPL and the LDL, which suggests that opening could occur at the interface. These results indicate that the gas pressure would not mechanically damage the EBS of the subsurface disposal even if a relatively high gas generation rate were assumed.

Abstract The Large Scale Gas Injection Test (Lasgit) is a field-scale experiment designed to study the impact of gas build-up and subsequent migration through an engineered barrier system (EBS). Lasgit has a substantial experimental dataset containing in excess of 26 million datum points. The dataset is anticipated to contain a wealth of information, ranging from long-term trends and system behaviours to small-scale or ‘second-order’ features. In order to interrogate the Lasgit dataset, a bespoke computational toolkit, designed to expose and quantify difficult to observe phenomena in large, non-uniform datasets, has been developed and applied. Presented results focus on the investigation and interpretation of second-order events occurring in close proximity (temporally and spatially) to a known macro-scale gas flow event that occurred during the second gas injection test. The similarity of the investigated event to dilatant flow observed in laboratory experiments is noted, as is the evidence for localized flow pathways in the bentonite EBS. The sensitivity of the toolkit’s ability to highlight second-order events is also evaluated.

Abstract A safety case for a geological disposal facility (GDF) is a set of claims concerning the environmental safety of the disposal of radioactive waste in a GDF, substantiated by a structured collection of arguments and evidence. Such a safety case needs to address environmental safety at the time of disposal and in the long term, after wastes have been emplaced and the facility has been closed. Waste-derived gas generation and its subsequent migration is an important component of a safety case, and was the focus of research in the integrated, multidisciplinary, European Commission ‘ F ate O f R epository G as E s’ project (FORGE) – a pan-European project with links to international radioactive waste management organizations, regulators and academia, specifically designed to tackle the key research issues associated with the generation and movement of repository gases. FORGE was targeted to address gas issues through a series of laboratory- and field-scale experiments, including the development of new methods for upscaling allowing the optimization of concepts through detailed scenario analysis. This paper summarizes the overall understanding on the implications of gas generation and migration for the safety functions of the engineered barrier system (EBS) and the host rock, as informed by the output of the FORGE project. Achievements and recommendations from the FORGE project are noted, emphasizing that FORGE has provided confidence in the basic physical understanding and modelling approaches relevant to GDF-derived gas that can be applied for different concepts – such understanding, and the derived component and system models, can be used in a safety case to address regulatory guidance.