Yield stress dependency on the evolution of bubble populations generated in consolidated soft sediments

Abstract

Retention of hydrogen bubbles within consolidated soft sediments represents an important safety consideration for the management of legacy nuclear wastes due to the potential for acute gas release. Gas retention sufficiently reduced the bulk density of intermediate yield stress (<800 Pa) sediments for the bed to become buoyant with respect to an aqueous supernatant, potentially inducing Rayleigh-Taylor instabilities. X-ray computed tomography revealed that beds of 7-234 Pa yield stress retained very similar, steady state size distributions of mature bubbles, limited to 9 mm equivalent spherical diameter, for long residence times. This implied a dominant gas release mechanism dictated by the pore to millimeter scale bubble population, not previously identified in such weak sediments and unrelated to the bubbles' buoyant force. At 1112 Pa yield stress, large bubbles of up to 20 mm diameter were observed to grow through induction of lateral cracks, facilitating gas transport to the bed periphery, thereby limiting the maximum void fraction, while nonhomogeneous gas generation promoted the formation of low density regions rich with microbubbles which similarly provide pathways for gas release.

Enhanced gas migration through permeable bubble networks within consolidated soft sediments

“…6,16 Fracture toughness typically diminishes with increased yield stress 17 promoting burrowing within highly consolidated sediments of kiloPascal scale yield stress. Furthermore, another x-ray tomography study by the current authors 11 observed that the largest, most mature bubbles remain static on time-scales of several hours during periods of appreciable chronic gas release from sediments of 7-1112 Pa, implying the existence of a gas release mechanism spanning a broad range of shear yield stress conditions which does not entail macroscopic migration of large individual bubbles. A succession of small partial rollover events 18 was briefly considered to explain the chronic gas release, whereby a region of sediment with a high voidage of sub-millimeter bubbles breaks free of the surrounding bulk material and rises through the bed, however no characteristic disturbances in topography or profile of the bed were apparent from tomographic imaging.…”

“…9 The presence of gas bubbles has also provoked research into their impact on the bulk strength and elasticity of marine sediments due to the significant implications for the offshore construction industry. 10 Legacy nuclear waste sludge deposits, for example, at Sellafield, UK 11 and Hanford, USA, 12,13 also experience in situ gas (mainly hydrogen) generation due to corrosion of metallic cladding materials and exposed fuel elements, combined with radiolysis of aqueous liquors by water soluble fission products. Further understanding of gas migration and chronic release is required to mitigate against acute gas release events 13,14 with the potential to present a flammability hazard which could risk a breach of containment.…”

“…Relatively modest yield stresses 15 are required to resist the buoyant stress of the <20 mm bubbles typical within consolidated soft sediments. 11,14 Sherwood and Eduardo Sáez 14 suggested that bubble ebullition, possibly liberating a buoyant flux, or cascade, of bubbles is most likely in the 3 < τ < 70 Pa yield stress range.…”

“…A succession of small partial rollover events 18 was briefly considered to explain the chronic gas release, whereby a region of sediment with a high voidage of sub-millimeter bubbles breaks free of the surrounding bulk material and rises through the bed, however no characteristic disturbances in topography or profile of the bed were apparent from tomographic imaging. 11 Thus, the established mechanisms for gas migration cannot account for chronic release from low-medium ( ≈10 Pa < τ < kPa) yield stress conditions, sufficient in strength to resist ebullition but insufficient in stress to support deep drainage channels or enable burrowing. It was postulated by Johnson et al 11 that permeable networks of partially coalesced sub-millimeter bubbles or diffusion pathways through fine fractures, 4 often below the detection limit of large scale clinical tomography, might offer a more realistic explanation for gas migration and chronic release.…”

“…11 Thus, the established mechanisms for gas migration cannot account for chronic release from low-medium ( ≈10 Pa < τ < kPa) yield stress conditions, sufficient in strength to resist ebullition but insufficient in stress to support deep drainage channels or enable burrowing. It was postulated by Johnson et al 11 that permeable networks of partially coalesced sub-millimeter bubbles or diffusion pathways through fine fractures, 4 often below the detection limit of large scale clinical tomography, might offer a more realistic explanation for gas migration and chronic release. Similarly, Wheeler 2 highlighted the possibility that retained free gas within unsaturated soils may be continuous at large void fractions where the saturation, given by the ratio of the pore water volume to the combined volumes of pore water and retained gas, falls below a value of around 0.85.…”