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At the post-closure stage of a geological disposal facility for higher activity radioactive waste several species of gas are likely to be generated in the near-field environment. These could alter the sealing and chemical properties of the bentonite buffer and the local geochemical environment significantly. The authors' attempt to simulate multico...
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... this case the value is equal to 0.40. The diffusion coefficient data used in the multicomponent diffusive flow simulation has been presented in Table 1. ...
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... (H 2 ) and carbon dioxide (CO 2 ) are considered in this simulation as two representative gases. The model domain is In the second simulation, only the binary diffusion coefficients of gases through air have been used (data: first three rows of Table 1). The conditions of the simulation have been presented in Fig. 2. At the right face of model domain, x = 0.3 m, concentrations of oxygen, hydrogen and carbon dioxide have been fixed at 50.0 mol m À3 ; the left edge is impermeable. ...
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During the long-term operation of a deep geological repository, infiltration of pore fluids with different chemical compositions can influence the swelling behavior of the compacted bentonite. In this paper, using a self-developed swelling deformation apparatus, one-dimensional free swelling tests were conducted on densely compacted GMZ01 bentonite...
Citations
... Diffusion is driven by concentration gradients which depend on the partial gas pressure and properties of the gas species, but it also depends on the mixture composition. In multicomponent gas mixtures, concentration gradients of single components due to a non-constant gas composition can also drive a diffusive transport in the gaseous phase (Masum et al. 2012). If the gas production rate exceeds the capability of gas dissolution and diffusion to remove gas from the source, a distinct gas phase can form (Bourgeat et al. 2013;Ibtihel and Jaffré 2014) and gas transport takes place via viscocapillary two-phase flow in addition to the diffusion. ...
The present work deals with diffusion of gases in fully saturated porous media. We test and validate the gas transport mechanism of dissolution and diffusion, implemented in the TH²M process class in the open-source finite-element software OpenGeoSys. We discuss the importance of gas diffusion for the integrity of the multi-barrier system. Furthermore, we present a multi-component mass balance equation implementation in Python, which serves as a reference for the two-component TH²M implementation and allows for a discussion of multi-component gas diffusion in liquids. We verify and validate the numerical implementations as follows: First, we come up with a set of numerical benchmarks in which solutions obtained by the two-component TH²M and multi-component implementations are compared. Thus, we show under which conditions predictions made by the TH²M model can be used for multi-component gas systems. Finally, the work is validated using a through diffusion experiment performed at Belgium’s Nuclear Research Centre SCK CEN and a sensitivity analysis is conducted based on the featured experiment. The results of this work illustrate that predictions by both the two- and four-component models match the laboratory findings very well. Therefore, we conclude that also the two-component implementation can reflect the multi-component processes well under the given constraints such as full saturation.
... In recent studies, Cui et al. [6] conducted a series of gas migration and breakthrough experiments on GMZ bentonites and analysed the results by employing a novel diffusion and solubility-controlled (DSC) flow concept [4]. The concept was developed and implemented under a coupled reactive gas transport model [25,34]. The aim of the study [4] was to investigate the extent to which diffusion and solubility driven flow could explain gas migration behaviours in saturated, rigidly confined bentonites. ...
Gas migration behaviour in saturated, compacted bentonite, especially under rigid-boundary conditions, is controversial. Gas breakthrough phenomena, observed under higher pressure gradient conditions in laboratory experiments, are described in literatures by adopting visco-capillary or dilatancy-controlled flow concept. Since, under rigid-boundary conditions, volumetric expansion is restricted and/or water dissipation is not detected, these concepts cannot be implemented satisfactorily. Instead, a diffusion and solubility-controlled (DSC) flow concept was previously found to be adequate for describing the behaviours at lower temperatures (20 °C). The DSC concept describes gas breakthrough as a function of gas solubility. Breakthrough occurs when concentration of dissolved gas reaches or surpasses the solubility limit in the entire specimen. In this work, the DSC flow concept is applied to validate gas migration and breakthrough experiments conducted at higher temperatures, e.g. 40 and 60 °C. Good agreements are observed between the experimental and predicted results, suggesting that the DSC flow concept can be applied to describe gas migration behaviour satisfactorily in rigidly confined saturated bentonites (under constant volume conditions) for various temperature regimes. Results also show that helium dissolution and diffusion processes in saturated bentonite are sensitive to test temperature and pressure conditions. The processes become more stable with increasing gas injection pressure and ambient temperature.
... In a nuclear waste repository, development of gas pressures at the waste canister-buffer interface is related to the gas generation rates and gas conductivity of the buffer. Masum et al. [28] collected a range of realistic and conservative gas generation rates, from literatures, which were then included in a large number of practical repository simulation scenarios. A critical gas pressure that can cause pneumatic fracture in the buffer (e.g. ...
... The model has been used extensively to study gas transport processes in various soils and rocks, e.g. swelling clays, sandstones, coal etc. [3,28,45]. Here, in the model, an alternative to visco-capillary two-phase flow or dilatancy controlled flow termed as, diffusion and solubilitycontrolled flow, is adopted to explain the possible gas transport processes occurred during the experiments. The aim is to examine the extent to which this alternative mechanism can describe the helium transport and breakthrough in the saturated compacted bentonites under rigid boundary conditions. ...
... Theoretical background of the gas transport module, verifications, and numerous validation exercises of the COM-PASS model are presented elsewhere [2,26,28,37]. The formulations relevant to this study is presented below. ...
To ensure the long-term safety and performance efficiency of a deep geological repository for disposal of high-level radioactive waste, understanding and assessment of gas migration behaviour in its engineered barrier system are of significant importance. In this study, a coupled gas transport model is utilised to simulate and to analyse a series of gas injection/breakthrough experiments on saturated bentonite under rigid boundary or constant volume conditions. To explain the laboratory gas migration and breakthrough results, a diffusion and solubility-controlled gas transport mechanism, instead of controversial visco-capillary flow or dilatancy-controlled flow mechanisms, is implemented in the model. The aim is to examine the extent to which this mechanism can describe helium migration and breakthrough behaviours in rigidly confined, saturated bentonite specimens. The predicted results are found to be in good agreement, both qualitatively and quantitatively, with the observed experimental results indicating the adequacy of this mechanism to describe the transport processes. The model represents the gas breakthrough phenomenon as a function of gas solubility. When the dissolved concentration of the injected gas reaches the maximum soluble concentration in the entire porewater domain, gas breakthrough occurs. Since, the system reaches a steady state and no further gas can be dissolved in the rigidly confined specimens, any injected/dissolved gas must be equated by the amount dissipated to comply with the principle of mass conservation. The maximum solution concentrations of helium are predicted to be 2.01 × 10–5, 7.75 × 10–5, 1.07 × 10–4 mol/L for GMZ bentonite specimens with dry densities of 1.3, 1.5 and 1.7 g/cm³, respectively. The analysis of the injection pressure effects on gas migration behaviour revealed that, if sufficient time is permitted, gas breakthrough may occur at pressures lower than the laboratory observed injection pressures.
... The model investigates heat, liquid, moisture, gas and chemical flows, microbial, geochemical and biogeochemical reaction processes, and mechanical deformation under a coupled framework. The details of the model including theoretical and numerical formulation, verifications, validations and numerous applications are provided elsewhere (Thomas and He 1998, Masum et al. 2012, Masum 2012, Masum and Thomas 2018a. In the following sections, the theoretical basis of the model, pertinent to this study, is presented. ...
Significant amounts of heavy metals in the directly discharged wastewater released from the newly built tannery industrial site in Dhaka, Bangladesh are reported. Despite their detrimental impacts on public health and natural ecosystem, no environmental impact study is yet conducted. Therefore, information on safe discharge rates are not available. In this study, the extent of pollution around the industrial site is investigated for four representative trace metals. Temporal and spatial distributions of chromium (Cr), lead (Pb), cadmium (Cd) and arsenic (As) have been predicted using a numerical model with the aim to estimate safe discharge of these metal contaminants. From multiple simulation runs it has been estimated that a discharge of0.026 m ³ of wastewater per day can lead to high levels of Cr and Pb accumulation, exceeding the regulatory standard limits, in the study area. Whilst As and Cd concentrations remain below the advised limits in most cases at this rate. However, an order of magnitude reduction in the total discharge rate, i.e.,0.0026 m ³ per day, results into the metal accumulation below the recommended guidelines in all cases. Elevated concentration of Pb is found to be limited to the top 0.5 m of the soil as compared to Cr, As and Cd, which exhibit larger spread along the depth of the soil. The relative dominance of the metal contamination follows the sequence: Pb>Cr>As>Cd as sorbed concentration in soil aggregates and Cr>Pb>As>Cd as aqueous concentration in soil porewater. Further investigations that are essential for a comprehensive environmental impact assessment have been highlighted.
... Such an early stage (pre-saturation) model is useful as it is during this time that heat output is highest, and also thermal conductivity is the lowest, therefore design limits based on temperature will require such models, so as not to be overly conservative. Furthermore, the recent development of THM models to include consideration of chemical species (e.g., Cleall et al. 2007;Gens 2010) and the impact of advective processes on the geochemical conditions close to the canister Cleall et al. 2013;Masum et al. 2012) and the potential for enhanced corrosion and gas production highlights the need for confidence in the correct representation of moisture and gas movement in the short-term near field behaviour of repository systems. It is noted that the model requires a large number of material parameters to be determined. ...
An investigation of the three-dimensional coupled thermohydromechanical behaviour of a prototype repository in fractured granitic rock is presented. The pre-placement behaviour of the repository is first considered, making use of a full three-dimensional simulation of the problem. An effective continuum approach, augmented with discrete features with a high hydraulic conductivity, is employed. The method adopted is found to be able to simulate accurately the highly anisotropic flow regime observed at the pre-placement phase. The major features of the full repository experiment under applied heating conditions were then successfully simulated. The range of buffer hydration rates, the thermal response of the repository, and the associated mechanical response were successfully simulated. Approaches to capture both the transient microstructural behaviour of the compacted bentonite (MX-80 type) and a MX-80 pellet material are incorporated. The repository behaviour was shown to be strongly influenced by complex coupled processes, including interactions between the system components. The adoption of a three-dimensional modelling approach proved to be essential to realistically simulate the behaviour of a repository incorporating anisotropic flow behaviour. Finally, potential impacts of the processes considered on performance of the barrier system and in safety assessment are considered.
... Part of this work was initially presented in a report for the Nuclear Decommissioning Authority (Vardon et al., 2010). An initial model concerning reactive gas flow and behaviour, which complements this work, has since been produced (Masum et al., 2012). ...
The inventory of high-level radioactive waste in the UK is likely to be disposed of in a geological disposal facility, which will generate gases due to processes such as anoxic metal corrosion and water radiolysis. Such gases have the potential to migrate through the repository system and may be detrimental to the engineered barrier system by damaging the physical fabric of the buffer material through the build-up of pressure. An investigation into the migration of gases and the ranges of gas pressures expected in such a repository is presented. A model of gas transport is presented to simulate the transport of gases in conjunction with coupled thermo-hydro-chemical-mechanical processes. This model includes a boundary condition linking the corrosion behaviour to gas generation and water supply. A realistic range of gas generation rates has been established from available data alongside realistic ranges of material properties of a bentonite buffer. Numerical simulations were then undertaken for a range of realistic scenarios considering the corrosion processes and buffer properties. It was found that in all realistic cases, including realistic maxima and minima considered, the risk of pneumatic fracture was negligible.
... Discussion of gas begins with an overview of gas research in support of the UK geological disposal programme by Steve Williams of the NDA (Williams, 2012). There follow contributions which describe: gas migration experiments in bentonite and their implications for numerical modelling by Caroline Graham (BGS) and coworkers ; the rate and speciation of volatile carbon-14 and tritium releases from irradiated graphite by Graham Baston (AMEC) and co-workers (Baston et al., 2012d); gas flow in Callovo-Oxfordian clay including results from laboratory and field-scale measurements by Jon Harrington (BGS) and coworkers (Harrington et al., 2012a); the comparison of alternative approaches to modelling gas migration through a higher strength rock by Andrew Hoch and Martin James (AMEC) (Hoch and James, 2012); evidence for gas-induced pathways in clay using a nanoparticle injection technique by Jon Harrington (BGS) and coworkers (Harrington et al., 2012b); multicomponent gas flow through compacted clay buffer in a higher activity radioactive waste geological disposal facility by Shakil Masum (Cardiff University) and co-workers (Masum et al., 2012); the migration and fate of 14 CH 4 in subsoil: tracer experiments to inform model development by George Shaw (The University of Nottingham) and co-workers (Atkinson et al., 2012); a data analysis toolkit for long-term, largesc ale experiments by Daniel Benn ett (Geoenvironmental Research Centre) and coworkers (Bennett et al., 2012); and understanding the behaviour of gas in a geological disposal facility by George Towler (Quintessa Ltd) and coworkers (Towler et al., 2012). ...
The Royal Society of Chemistry, the Geological Society, the Institution of Civil Engineers, the Nuclear Institute, the Institution of Chemical Engineers, the Royal Academy of Engineering and the Mineralogical Society organized a wide-ranging conference themed around the geological disposal of radioactive waste from 18th–20th October 2011 at the Burleigh Court Conference Centre, Loughborough University, Loughborough, UK. One of its principal objectives was to showcase and publish research relevant to radioactive waste storage and disposal. The event was supported by the Nuclear Decommissioning Authority (NDA) and the Energy Generation and Supply Knowledge Transfer Network (KTN) for the Nuclear Sector, and kindly sponsored by AMEC and the National Nuclear Laboratory (NNL).
The conference brought together scientists, engineers and other interested specialists to discuss the chemical, biological, geological, hydrological, materials, engineering and other scientific and technological challenges associated with the long-term management of radioactive waste in the UK. The scientific committee peer-reviewed all abstracts received prior to the conference, and determined those best for oral and poster presentation. Papers presented in this issue of Mineralogical Magazine were subsequently subjected to a detailed and independent peer-review process.
The organizing committee consisted of Nick Evans (chair), Loughborough University; Katherine Morris, The University of Manchester; Neil Smart and Gordon Turner, Nuclear Decommissioning Authority; Nic Bilham, Geological Society; Glen Owen, Institution of Civil Engineers; Claire Gallery-Strong, Nuclear Institute; Sam Shaw, Mineralogical Society; Steve Lancaster, Royal Society of Chemistry; Richard Ploszek, Royal Academy of Engineering; and Richard Shaw, British Geological Survey. The organizing committee would like to place on record its thanks to the sponsors (see back cover) and exhibitors.
The conference was organized in thematic sessions and this volume is presented in the same manner. Each of the sections begins with a peer-reviewed paper from the NDA outlining its view on the state-of-the-art in …
This paper presents some aspects of the computational modelling of coupled processes related to geoenergy problems. Experiences gained from model development, validation and application in the areas of i) ground source heat and ii) carbon dioxide sequestration in coal are presented. A field scale validation exercise of ground source heat application is presented. Aspects related to the coupled physical, chemical and mechanical modelling of carbon sequestration in coal are presented.
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.
