Abstract
In a deep geological disposal facility for radioactive waste, precompacted bentonite is proposed as a sealing material for the isolation of boreholes, disposal galleries and deposition holes. The advective movement of repository gas in bentonite has been linked to the development of new porosity and propagation of dilatant pathways. For the first time we present a detailed analysis of stress field data during the generation and evolution of a gas network. A new experimental dataset, from a highly instrumented test, clearly shows the strong coupling between stress, gas pressure and flow in bentonite. Multiple discrete propagation events are observed, demonstrating spatial variability and time-dependency as permeability within the clay develops. Analysis of the stress data before, during and after gas entry indicates a heterogeneous stress field initially develops, resulting from the development of these pathways. The flow network is dynamic and continues to spatially evolve after gas entry, such that permeability under these conditions must be time-dependent in nature. Perturbation of the stress field is significant before all major gas outflow events, presumably resulting from the requirement to propagate an effective gas network before outflow is possible. In contrast, no major flow perturbations are detected which did not correlate with fluctuations in the stress field. The controls on the distribution and geometry of the resulting flow network are unclear, as well as its long-term evolution and stability. These will be beneficial in the assessment of gas pressure evolution as part of safety case development.
Highlights
The deep geological disposal of radioactive waste presents a number of significant engineering challenges, not least understanding the fate and impact of waste-package derived gas on the engineered barrier systems (EBS) and host rock, which form an integral part of a geological disposal facility (GDF)
This paper describes a highly instrumented and detailed test examining the interaction between gas pressure and stress during initial pathway generation and the development of permeability in an EBS consisting of saturated, precompacted bentonite
A free gas phase is likely to form in many radioactive waste repository concepts
Summary
The deep geological disposal of radioactive waste presents a number of significant engineering challenges, not least understanding the fate and impact of waste-package derived gas on the engineered barrier systems (EBS) and host rock, which form an integral part of a geological disposal facility (GDF). In scenarios where the rate of gas production exceeds the rate of gas diffusion through the EBS or host rock, a discrete gas phase will form (Weetjens and Sillen, 2006; Ortiz et al, 2002; Wikramaratna et al, 1993, Sellin and Leupin, 2013; SKB, 2006; Norris, 2015) Under these conditions, a free gas phase begins to accumulate until its pressure becomes sufficiently large for it to move, through advection, in the surrounding material (Sellin and Leupin, 2013; Graham et al, 2012; Harrington and Horseman, 1999, 2003; Horseman et al, 1999). Previous studies (Angeli et al, 2009; Skurtveit et al, 2012; Harrington et al, 2009, 2012a,b; Cuss et al, 2014a,b; Gerard et al, 2014; Rodwell, 2000) indicate that in the case of plastic clays (Whitlow, 2001) and in particular precompacted bentonite, advective gas flow is associated with the development of new pressure induced pathways leading to a complex coupling between gas pressure, stress state and volumetric strain (Gensterblum et al, 2015; Amann-Hildenbrand et al, 2015; Cuss et al, 2014a,b; Harrington et al, 2012a, 2012b; Graham et al, 2012; Horseman and Harrington, 1994; Horseman et al, 1999; Harrington and Horseman, 2003; Romero et al, 2012; Marschall et al, 2005; Wiseall et al, 2015)
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