Phonon transport in large scale carbon-based disordered materials: Implementation of an efficient order-N and real-space Kubo methodology

We have developed an efficient order-N real-space Kubo approach for the calculation of the phonon conductivity which outperforms state-of-the-art alternative implementations based on the Green's function formalism. The method treats efficiently the time-dependent propagation of phonon wave packets in real space, and this dynamics is related to the calculation of the thermal conductance. Without loss of generality, we validate the accuracy of the method by comparing the calculated phonon mean free paths in disordered carbon nanotubes (isotope impurities) with other approaches, and further illustrate its upscalability by exploring the thermal conductance features in large width edge-disordered graphene nanoribbons (up to ~20 nm), which is out of the reach of more conventional techniques. We show that edge-disorder is the most important scattering mechanism for phonons in graphene nanoribbons with realistic sizes and thermal conductance can be reduced by a factor of ~10.Comment: Accepted for publication in Physical Review B - Rapid Communication

Utilisation of underwater acoustic backscatter systems to characterise nuclear waste suspensions remotely

This paper reports application of ABS (Acoustic Backscatter Systems) to address nuclear waste management within the UK. ABS offers a route towards an online monitoring system to characterise wastes safely and remotely in real-time, during pipeline transportation; resulting in reduced hazard reduction timescales and taxpayer cost savings. Here, an ultrasonic velocimetry profiler (UVP) was used to analyse glass dispersions of varying concentrations, to assess online waste monitoring applicability. Acoustic backscatter profiles were collected to establish attenuation coefficients for two glass sizes (∼40 and 80 µm) with 2 and 4 MHz probes. The 4 MHz probes were the most highly attenuating while transducer active radius had a negligible effect on probe sensitivity with either glass. A calibration procedure was used to measure sediment attenuation coefficients, which were compared to model estimates and experimental literature. For most systems, measured coefficients were close to estimated values, highlighting improved calibration accuracy, due to the mixing tank used. However, values for the smaller glass and 2 MHz probes overestimated model predictions, due to additional viscous attenuation. The measured value for the larger glass with 4 MHz probes was underestimated, this was caused by high attenuation reaching the instrument noise-floor, limiting the region available for analysis

The influence of relative fluid depth on initial bedform dynamics in closed, horizontal pipe flow

Measurements of time-dependent bedforms produced by the deposition of solid plastic particles in two-phase liquid-solid flows were performed using a novel ultrasonic echo method and via video image analysis in a 100-liter, closed-pipe slurry flow loop. Results are presented for the settled bed thicknesses over a range of nominal flow rates and initial bed depths and are combined into several phase diagrams based on various combinations of parameters, with the bedforms categorized into five types. The novel observation is made that the type of bedform that arises depends on both the flow rate and the initial relative bed or fluid depth, with both ripples and dunes being observed in the same system and in a single experiment. In addition, the critical Shields number at incipient particle motion is measured to be θsc = 0.094 ± 0.043, hysteretic behavior is observed, and the evolution and scaling of each time-dependent type of bedform is analyzed in detail and compared against several expressions for initial and equilibrium dimensions from the literature. A number of universal scalings for bedforms in any type of conduit are proposed with a view ultimately to unifying the observations of bedforms in pipes with those in channels and natural flows

The effect of pre-activation and milling on improving natural clinoptilolite for ion exchange of cesium and strontium

Natural clinoptilolite, of relatively low-grade, was investigated for its capability to remove cesium and strontium ions from water and simulated seawater. To improve its capacity, the material was pre-activated with concentrated NaCl and HCl solutions. Additionally, it was milled to a number of < 300 μm size fractions, to expose exchange sites. Electron microscopy was used to characterise the naturally occurring impurities, where regions of high iron and potassium content was shown to correlate to lower levels of cesium adsorption. Adsorption kinetics for natural and activated resins with 5, 300 and 1500 ppm salt solutions were fitted with the Pseudo-Second Order (PSO) rate model. Activation led to clear increases in initial adsorption rate for both Cs+ and Sr2+, but only enhanced the overall rate constant for Cs+, due to the weaker interaction of the Sr2+. Equilibrium isotherms were compared with Langmuir and Freundlich monolayer models, where the adsorption capacity (Qc) for Cs+ was 67 mg/g which increased by over 100% with NaCl activation to 140 mg/g. Values for Sr2+ were significantly lower at 35 mg/g, with a considerably smaller enhancement with activation to 52 mg/g. Milling of the natural clinoptilolite was found to increase Cs+ uptake to similar levels as activation, in a linear correlation with specific surface area; although, improvements for Sr2+ were again lower, due to its weaker interaction with surface sites. In simulated seawater solutions, all materials gave considerably reduced performance due to K+ ion competition, with Sr2+ uptake decreased more extensively compared to Cs+. Overall, this work highlights that pre-activation and milling of clinoptilolite can be used to significantly enhance the grade of the ore for nuclear effluent treatment in low-salinity conditions

The effect of cationic surfactants on improving natural clinoptilolite for the flotation of cesium

Flotation using cationic surfactants has been investigated as a rapid separation technique to dewater clinoptilolite ion exchange resins, for the decontamination of radioactive cesium ions (Cs+) from nuclear waste effluent. Initial kinetic and equilibrium adsorption studies of cesium, suggested the large surface area to volume ratio of the fine zeolite contributed to fast adsorption kinetics and high capacities (qc = 158.3 mg/g). Adsorption of ethylhexadecyldimethylammonium bromide (EHDa-Br) and cetylpyridinium chloride (CPC) surfactant collectors onto both clean and 5 ppm Cs+ contaminated clinoptilolite was then measured, where distribution coefficients (Kd) as high as 10,000 mL/g were evident with moderate concentrations CPC. Measurements of particle sizes confirmed that adsorption of surfactant monolayers did not lead to significant aggregation of the clinoptilolite, while 4, highlighting the great viability of flotation to separate and concentrate the contaminated powder in the froth phase

In situ characterization of mixing and sedimentation dynamics in an impinging jet ballast tank via acoustic backscatter

Impinging jets are utilized in numerous applications, including nuclear waste treatment, for both the erosion of sediment beds and maintaining particulates in suspension. Pulse-echo ultrasonic methods offer great potential for the in situ monitoring of critical mixing and settling dynamics, in concentrated dispersions. A non-active scaled version of a Highly Active Storage Tank at Sellafield, UK, was profiled with an acoustic backscatter system under various jet firing conditions. An advanced analysis technique enabled the direct quantification of dispersion concentration changes from the converted backscatter attenuation. Hence, the erosion and mixing capability of the jets, and settling kinetics were characterized. It was found that jet operation alone provided inadequate localized mixing of eroded sediment. An additional air-lift process operation was required to hinder the rapid re-settling of dispersed particulates

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