Abstract
Gases can potentially generate in a deep geological repository (DGR) for the long-term containment of radioactive waste. Natural and engineered barriers provide containment of the waste by mitigating contaminant migration. However, if gas pressures exceed the mechanical strength of these barriers, preferential flow pathways for both the gases and the porewater could form, providing a source of potential exposure to people and the environment. Expansive soils, such as bentonite-based materials, are widely considered as sealing materials. Understanding the long-term performance of these seals as barriers against gas migration is an important component in the design and the long-term safety assessment of a DGR. This study proposes a hydro-mechanical mathematical model for migration of gas through a low-permeable swelling geomaterial based on the theoretical framework of poromechanics. Using the finite element method, the model is used to simulate 1D flow through a confined cylindrical sample of near-saturated low-permeable soil under a constant volume boundary stress condition. The study expands upon previous work by the authors by assessing the influence of heterogeneity, the Klinkenberg “slip flow” effect, and a swelling stress on flow behavior. Based on the results, this study provides fundamental insight into a number of factors that may influence two-phase flow.
Highlights
In Canada, nuclear waste has been generated and accumulated since the 1930s when the PortRadium radium mine began operating in the Northwest Territories [1]
It should be noted that, with the introduction of slip flow, there is some migration of poregas at low gas injection pressures; the predominant breakthrough of gas into the sample occurs once the air-entry value (AEV) has been reached
One notable observation in the evolution of poregas migration is that the Klinkenberg effect tends to saturate the gas in the bentonite specimen and does not aid in the formation of distinct preferential flow pathways
Summary
In Canada, nuclear waste has been generated and accumulated since the 1930s when the Port. Marschall et al [10] recognized that gas transport through porous media is controlled by a number of the media’s hydraulic and mechanical characteristics, such as the intrinsic permeability, porosity, and material strength They identified the importance of the hydro-mechanical state of the rock or the soil media (i.e., water saturation, porewater pressure, and stress state) and the gas pressure at focal points that could lead to microfracturing [10]. The study concluded that, in order to simulate dilatancy-controlled gas flow, additional mechanisms need to be considered within the model These include the use of advanced mechanism of mechanical deformation to be coupled to gas transport, consideration of heterogeneity within the soil sample to help induce preferential flow, inclusion of a swelling stress term to incorporate the swelling behavior of expansive soils, and the incorporation of a self-healing mechanism to represent observed phenomena of experimental studies [11]. This task, led by the British Geological Survey (BGS), further attempts to identify the physical HM mechanisms required to adequately model dilatancy-controlled gas migration
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