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

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 updated assessment of the Agulhas sole resource, Austroglossus pectoralis

This analysis updates that of Butterworth and Glazer (2014), which considered two hypotheses of decreasing catchability and of decreasing productivity to account for a recent large drop in CPUE. Two further years of data reflect some increase in CPUE. For the most pessimistic scenario (a decrease in productivity, which remains at its current reduced level into the future), projections are somewhat more positive than previously, with recent biomasses estimated higher and projected to decrease more slowly if the 2013 effort level is maintained

Engaging Undergraduates in Science Research: Not Just About Faculty Willingness.

Despite the many benefits of involving undergraduates in research and the growing number of undergraduate research programs, few scholars have investigated the factors that affect faculty members' decisions to involve undergraduates in their research projects. We investigated the individual factors and institutional contexts that predict faculty members' likelihood of engaging undergraduates in their research project(s). Using data from the Higher Education Research Institute's 2007-2008 Faculty Survey, we employ hierarchical generalized linear modeling to analyze data from 4,832 science, technology, engineering, and mathematics (STEM) faculty across 194 institutions to examine how organizational citizenship behavior theory and social exchange theory relate to mentoring students in research. Key findings show that faculty who work in the life sciences and those who receive government funding for their research are more likely to involve undergraduates in their research project(s). In addition, faculty at liberal arts or historically Black colleges are significantly more likely to involve undergraduate students in research. Implications for advancing undergraduate research opportunities are discussed

Large-scale trials of a real-time acoustic backscatter system for solids concentration measurement during nuclear waste cleanup

Real time in situ characterisation of solids concentration would aid operational understanding and improve efficiency in many industrial systems. This is especially true in the processing of legacy nuclear wastes where hazardous material is encountered. Acoustic methods have been previously demonstrated for the measurement of concentration in solid-liquid systems at a small scale. This study explores the use of the ultrasound array research platform (UARP) for backscatter measurements of concentration at a large scale in a dynamic settling system. The theory of acoustic measurement of solids concentration is described for both backscatter based attenuation and backscatter power methods. Acoustic based backscatter power and attenuation measurements are compared to laboratory analysed samples. Ultrasonic solids concentration analysis is shown to reveal flow dynamics within the settling tank

Diphacinone and coumatetralyl persistence in deer and implications for wildlife management

Eason, C.T., Murphy, E., Ross, J.G., Hix, S., Arthur, D., MacMorran, D., Broome, K., Fairweather, A

Reasoning with heuristics

Which rules should guide our reasoning? Human reasoners often use reasoning shortcuts, called heuristics, which function well in some contexts but lack the universality of reasoning rules like deductive implication or inference to the best explanation. Does it follow that human reasoning is hopelessly irrational? I argue: no. Heuristic reasoning often represents human reasoners reaching a local rational maximum, reasoning more accurately than if they try to implement more “ideal” rules of reasoning. I argue this is a genuine rational achievement. Our ideal rational advisors would advise us to reason with heuristic rules, not more complicated ideal rules. I argue we do not need a radical new account of epistemic norms to make sense of the success of heuristic reasoning

Gas Retention and Release from Nuclear Legacy Waste

The retention of gas within 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, including the magnox swarf storage silos and first generation magnox storage pond. A series of laboratory scale gas retention tests within magnesium hydroxide soft sediments have revealed a 4-51 Pa yield stress range where consolidated beds could retain sufficient gas to become buoyant with respect to a water supernatant. Density inversion could lead to a Rayleigh-Taylor style instability which could result in an upward transfer of radioactive material from the consolidated bed. The applicable yield stress range suggests that such rollover events may occur in weaker sediment than previously hypothesized, based on current understanding of the fluidization and stable channel mechanisms for gas release from weak and very strong sediments respectively. Xray computed tomography images of gas retained by 7 Pa yield stress soft sediment reveal both a stable foam layer at the top of the bed and regions dense with microbubbles which could provide pathways for gas transport through the bed. Extension of these pathways, hidden below the surface of the sediment, to the container walls and the foam layer could represent a novel mechanism for gas release from intermediate strength sediments of <100 Pa yield stress

Measuring particle concentration in multiphase pipe flow using acoustic backscatter: Generalization of the dual-frequency inversion method.

A technique that is an extension of an earlier approach for marine sediments is presented for determining the acoustic attenuation and backscattering coefficients of suspensions of particles of arbitrary materials of general engineering interest. It is necessary to know these coefficients (published values of which exist for quartz sand only) in order to implement an ultrasonic dual-frequency inversion method, in which the backscattered signals received by transducers operating at two frequencies in the megahertz range are used to determine the concentration profile in suspensions of solid particles in a carrier fluid. To demonstrate the application of this dual-frequency method to engineering flows, particle concentration profiles are calculated in turbulent, horizontal pipe flow. The observed trends in the measured attenuation and backscatter coefficients, which are compared to estimates based on the available quartz sand data, and the resulting concentration profiles, demonstrate that this method has potential for measuring the settling and segregation behavior of real suspensions and slurries in a range of applications, such as the nuclear and minerals processing industries, and is able to distinguish between homogeneous, heterogeneous, and bed-forming flow regimes