Author: Xue Wei Publisher: ISBN: Category : Languages : en Pages :
Book Description
With the widening and increasing use of nuclear energy, it is very important to design and build long-term deep geological repositories (DGRs) to manage radioactive waste. The disposal of nuclear waste in deep rock formations is currently being investigated in several countries (e.g., Canada, China, France, Germany, India, Japan and Switzerland). In Canada, a repository for low and intermediate level radioactive waste is being proposed in Ontario's sedimentary rock formations. During the post-closure phase of a repository, significant quantities of gas will be generated from several processes, such as corrosion of metal containers or microbial degradation of organic waste. The gas pressure could influence the engineered barrier system and host rock and might disturb the pressure-head gradients and groundwater flows near the repository. An increasing gas pressure could also cause damage to the host rock by inducing the development of micro-/macro-cracks. This will further cause perturbation to the hydrogeological properties of the host rock such as desiccation of the porous media, change in degree of saturation and hydraulic conductivity. In this regard, gas generation and migration may affect the stability or integrity of the integrate barriers and threaten the biosphere through the transmitting gaseous radionuclides as long-term contaminants. Thus, from the safety perspective of DGRs, gas generation and migration should be considered in their design and construction. The understanding and modelling of gas migration within the host rock (natural barrier) and the associated potential impacts on the integrity of the natural barrier are important for the safety assessment of a DGR. Therefore, the key objectives of this Ph.D. study include (i) the development of a simulator for coupled modelling of gas migration in the host rock of a DGR for nuclear waste; and (ii) the numerical investigation of gas migration in the host rock of a DGR for nuclear waste in Ontario by using the developed simulator. Firstly, a new thermo-hydro-mechanical-chemical (THMC) simulator (TOUGHREACT-COMSOL) has been developed to address these objectives. This simulator results from the coupling of the well-established numerical codes, TOUGHREACT and COMSOL. A series of mathematical models, which include an elastoplastic-damage model have been developed and then implemented into the simulator. Then, the predictive ability of the simulator is validated against laboratory and field tests on gas migration in host rocks. The validation results have shown that the developed simulator can predict well the gas migration in host rocks. This agreement between the predicted results and the experimental data indicates that the developed simulator can reasonably predict gas migration in DGR systems. The new simulator is used to predict gas migration and its effects in a potential DGR site in Ontario. Valuable results regarding gas migration in a potential DGR located in Ontario have been obtained. The research conducted in this Ph.D. study will provide a useful tool and information for the understanding and prediction of gas migration and its effect in a DGR, particularly in Ontario.
Author: Laura Gonzalez‐Blanco Publisher: ISBN: Category : Languages : en Pages : 318
Book Description
Deep geological disposal remains the preferred option at present for the management of long-living and heat-emitting radioactive waste, which consists of confining the waste during a very long period (several hundreds of thousands of years) by placing them in a deep geological formation. Therefore, the understanding of the long-term behaviour of formations is becoming a key issue to ensure the feasibility of the geological disposal facilities, particularly regarding the generation and migration of gases. The present PhD work aims at better understanding the complex hydro-mechanical response of different argillaceous formations to gas migration process. To this end, gas flow through Boom Clay (one of the potential candidate plastic Paleogene clay formations to host nuclear waste in Belgium) has been deeply investigated on the basis of laboratory experiments at different scales and their numerical modelling. This main study has been complemented by presenting tests on two indurated and deeper claystone Mesozoicformations, considered as candidate host rocks in the Swiss programme for deep geological disposal, namely Opalinus Clay and ‘Brauner Dogger'. The different materials have been firstly characterised to evaluate mechanical (compressibility on loading) and two phase flow properties (water retention and permeability). Gas injection tests under oedometer and isotropic conditions have been performed following different testing protocols, in which boundary conditions have been carefully controlled. Major relevance has been given to restore the in situ stress state and to ensure full saturation conditions before the gas tests. Special emphasis has been placed in measuring sample deformation along the gas injection and dissipation process. The anisotropy of Boom Clay has been studied by carrying out tests with bedding planes parallel and normal to flow. Air injections have been performed at three different controlled-volume rates. The dissipation stages after shut-off have been also analysed to study air intrinsic permeability changes. Microstructure of samples before and after air injection tests has been evaluated by different techniques: mercury intrusion porosimetry, field-emission scanning electron microscopy and micro-focus Xray computed tomography. Gas migration turned out to be a fully coupled hydro-mechanical process. Air injection at constant stress induced expansion of the samples during pressure front propagation and compression during air pressure dissipation. The deformational behaviour was dependent on the injection rate. At slower injection rates expansion occurred during the injection while at higher rates it was delayed in time. Air intrinsic permeability resulted higher than water permeability suggesting that air flow took place along preferential pathways. Evaluation of the microstructural changes induced by air migration revealed the opening of fissures and allowed quantifying their apertures and separation, as well as their volume and connectivity. Air intrinsic permeability was found to be dependent on the fissured volume. To complete and better understand the gas transport mechanisms, numerical simulations of the experimental results have been performed using a fully coupled hydro-mechanical finite element code, which incorporates an embedded fracture permeability model to account for the correct simulation of the gas flow along preferential pathways. Clay intrinsic permeability and its retention curve have been made depend on strains through fracture aperture changes. Numerical results not only accounted for the correct simulation of the recorded upstream pressures and outflow volumes and pressures, but also for the volume change behaviour. The experimental and numerical information provided a good insight into the mechanisms of gas transport in deep clay formations and highlighted the role played by the deformational response on the air transport properties of argillaceous rock formations.
Author: Elias Ernest Dagher Publisher: ISBN: Category : Languages : en Pages :
Book Description
This work provides a comprehensive assessment into the processes governing two-phase flow in a swelling geomaterial under critical gas pressures whereby the material strength of the soil may be exceeded and for which dilation and dilation-controlled gas flow is expected to occur. The author first provides background on the importance of understanding such processes in the safe geological disposal of radioactive waste. This is followed by a review of experimental studies to describe mechanisms of gas migration processes through natural (host rock formations) and engineered barrier materials, and an evaluation of existing studies that have attempted to numerically model multi-phase flow in expansive soils. Finally, the author provides a literary synthesis of the hydraulic and mechanical behaviour of bentonite clays observed in laboratory experiments under saturated and unsaturated conditions. The novel contribution of this work is then presented through a series of articles. The author first develops a fully-coupled hydro-mechanical (HM) mathematical model for advective-diffusive visco-capillary controlled two-phase flow through a geomaterial and applies it to simulate a 1-dimensional (1D) flow problem using the Finite Element Method (FEM) commercial code COMSOL Multiphysics®. The model results are compared to experimental data and although several key-features of the experimental results were realized, additional flow mechanisms would be necessary to achieve complete gas breakthrough of the sample. In the second article, a verification study is performed, whereby analytical solutions for a 1D steady-state and 1D transient gas flow problem under constant volume boundary conditions were derived. Successful verification of the numerical model was completed by comparing the pore-gas pressure evolution and stress evolution to that of the results of the analytical solution, which showed near-perfect agreement. Building upon the authors original mathematical model, an investigation of enhanced processes and characteristics which may be contributing to dilation-controlled gas flow were conducted. These processes included the introduction of material heterogeneity, consideration of the Klinkenberg "slip flow" effect, and the presence of a swelling strain, and were applied to simulate the same 1D flow problem. The results showed significant improvement over the previous work, including observing complete breakthrough and matching experimental stress evolution. However, the results showed a high degree of gas saturation within the sample and a plug outflow behavior, which were not characteristic of dilation-controlled gas flow. To further improve the mathematical model, the author conducted a detailed investigation of the highly coupled relationship between mechanical deformation and flow. This included assessing the effect of plasticity, damage, and non-localization to dilation-controlled gas flow, while incorporating the knowledge gained from previous studies. These advanced mathematical models were applied to numerically simulate 1D flow and a 3D spherical flow under constant boundary conditions. Some results demonstrated very good agreement with experimental data and provide further understanding of the processes involved. The collection of this research provides much needed insight into the possible mechanisms controlling two-phase flow and the capability of continuum models to do so. The intent of this research is to expand the literature further and provide a few steps closer in the development of a robust numerical model that can be used to support long-term safety assessments for radioactive waste disposal, by correctly capturing major features of two-phase flow.
Author: Publisher: ISBN: Category : Languages : en Pages : 30
Book Description
Hydrologic impacts of corrosive gas release from a hypothetical L/ILW nuclear waste repository at Oberbauenstock are explored by means of numerical simulation. A schematic two dimensional vertical section through the mountain is modeled with the simulator TOUGH, which describes two-phase flow of water and gas in porous and fractured media. Two reference cases are considered which represent the formations as a porous and as a fractured-porous (dual permeability) medium, respectively. Both cases predict similar and rather modest pressure increases, from ambient 10 bars to near 25 bars at the repository level. These results are to be considered preliminary because important parameters affecting two-phase flow, such as relative permeabilities of a fractured medium, are not well known at present. 24 refs., 15 figs., 5 tabs.
Author: Xue Wei
Author: Laura Gonzalez‐Blanco
Author: Elias Ernest Dagher
Author: M. Kolar
Author: L. Ortiz
Author: 
Author: A. Poller
Author: Norbert Jockwer
Author: William Stephen Head
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