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

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 non-homogeneous gas generation promoted the formation of low density regions rich with micro-bubbles which similarly provide pathways for gas release

An X-ray Tomography Study of Gas Retention in Nuclear Legacy Waste

The retention and release of flammable gases from corroded Magnox sludge waste at Sellafield, UK and secondary reprocessing waste at Hanford, USA has significant economic and safety implications for decommissioning various nuclear legacy buildings. Magnesium hydroxide is the primary precipitation product from the corrosion of first generation nuclear fuel in the UK, with hydrogen gas produced as a reaction by-product. Depending on the bed microstructure, wettability and shear yield stress behaviour, some consolidated sediments of these corrosion products are able to trap a substantial volume of gas, sufficient in some instances to become buoyant with respect to a water supernatant, resulting in an undesirable upward transfer of radioactive material from the consolidated bed. These phenomena are investigated using the decomposition of hydrogen peroxide to produce oxygen bubbles within magnesium hydroxide soft sediments at laboratory scale. X-ray tomography analysis showed that high strength sediments of 1112 Pa shear yield stress supported much larger bubbles up to 20 mm equivalent spherical diameter than beds in the 7-234 Pa range, which demonstrated almost identical bubble size distributions across the range. The largest retained bubbles became progressively more distorted with increased sediment strength until the lateral cracks consistent with tensile fracture became apparent in the 1112 Pa bed. These cracks significantly limited the capacity for bed swell as gas diffusion along the cracks to the container walls provided a continuous escape route. The capacity for gas retention was also substantially reduced when gas generation was not homogeneous through the bed as localised gas generation promoted the formation of low density pathways, rich with micro-bubbles, which enable gas transport through the bed

Flow dynamics and mixing processes in hydraulic jump arrays: Implications for channel-lobe transition zones

A detailed field investigation of a saline gravity current in the southwest Black Sea has enabled the first complete analysis of three-dimensional flow structure and dynamics of a series of linked hydraulic jumps in stratified, density-driven, flows. These field observations were collected using an acoustic Doppler current profiler mounted on an autonomous underwater vehicle, and reveal that internal mixing processes in hydraulic jumps, including flow expansion and recirculation, provide a previously unrecognised mechanism for grain-size sorting and segregation in stratified density-driven flows. Field observations suggest a newly identified type of hydraulic jump, that is a stratified low Froude number (< 1.5–2) subaqueous hydraulic jump, with an enhanced ability to transport sediment downstream of the jump, in comparison to hydraulic jumps in other subaerial and submarine flows. These novel field data underpin a new process-based conceptual model of channel lobe transition zones (CLTZs) that explains the scattered offset nature of scours within such settings, the temporal variations in infill and erosion between adjacent scours, how bed shear stresses are maintained across the CLTZ, and why the locus of deposition is so far downstream of the scour zone

Development of novel multiplex microsatellite polymerase chain reactions to enable high-throughput population genetic studies of Schistosoma haematobium

© 2015 Webster et al. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. The attached file is the published version of the article

First direct measurements of hydraulic jumps in an active submarine density current

For almost half a century, it has been suspected that hydraulic jumps, which consist of a sudden decrease in downstream velocity and increase in flow thickness, are an important feature of submarine density currents such as turbidity currents and debris flows. Hydraulic jumps are implicated in major seafloor processes, including changes from channel erosion to fan deposition, flow transformations from debris flow to turbidity current, and large-scale seafloor scouring. We provide the first direct evidence of hydraulic jumps in a submarine density current and show that the observed hydraulic jumps are in phase with seafloor scours. Our measurements reveal strong vertical velocities across the jumps and smaller than predicted decreases in downstream velocity. Thus, we demonstrate that hydraulic jumps need not cause instantaneous and catastrophic deposition from the flow as previously suspected. Furthermore, our unique data set highlights problems in using depth-averaged velocities to calculate densimetric Froude numbers for gravity currents