No description available.

The nonlinear MHD ballooning model is exploited for two distinct studies: firstly, the interaction of multiple filamentary eruptions in magnetised plasmas in a slab geometry is investigated and secondly, this model is examined quantitatively against experimental observations of ELMs in MAST and JET-like geometries. The model consists of two differential equations which characterise the spatial and temporal evolution of the displacement: the first differential equation describes the displacement along the field line, the second differential equation is a two-dimensional nonlinear ballooning-like equation which is often second order in time, but can involve a fractional derivative in a tokamak geometry. Filaments always evolve independently in the linear regime and equally sized filaments evolve independently in the nonlinear regime. However, we find that filaments with varying heights interact with each other in the nonlinear regime: Smaller filaments are slowed down and eventually are completely suppressed by the larger filaments which grow faster due to the interaction. This mechanism is explained by the down-draft caused by the nonlinear drive of the larger filaments which pushes the smaller filaments downwards. To employ the second differential equation for a specific geometry one has to evaluate the coefficients of the equation which is non-trivial in a tokamak geometry as it involves field line averaging of slowly converging functions. The coefficients of a Type I ELMy equilibrium from MAST and a Type II ELMy JET-like equilibrium have been determined. In both cases the two coefficients of the nonlinear terms are negative which would imply imploding rather than exploding filaments. By changing the equilibrium the signs of these coefficients can be inverted. This suggests that either the nonlinear Ballooning model does not capture the behaviour of Type I and Type II ELMs, or that the calculation of the coefficients are too sensitive to a given equilibrium.

An industrial seal manufactured from James Walker Co. and made from an acrylonitrile-butadiene based synthetic elastomer, reinforced with carbon black and incorporating glass cloth, brass wire and a sulphur based curing system has undergone a variety of tests to identify its physical and mechanical properties, how these properties change under exposure to gamma radiation and the effect pressurised CO2 has on this radiation induced degradation. The seal was found to have a Δ(M%) in toluene of 46% ± 2 , Tg of -24.9 ℃ ± 0.7 and a Retention Factor of 0.413 ± 0.004. Δ(M%) decreased with radiation and Tg and RF increased suggesting crosslink formation. SEM also revealed the development of microcracks on the outside curvature of the seal. Following exposure to gamma radiation under 32 bar pressurised CO2 the same changes were seen in all properties. These results were compared against three commercially available nitrile rubber sheets sourced from Whitby and Chandler Ltd; at hardness grades BA40, BA50 and BA60. Analysis of peak heights found in IR spectra suggested that degradation in BA40 proceeded through the ionisation of the BDN monomer and crosslinking between cis C=C and C=CH2 followed at higher doses by ionisation of both BDN and ACN monomers and crosslinking between trans C=C and C≡N groups. In BA50 it appeared that degradation of both monomers occurred concurrently. Those samples with a higher BDN content also displayed an increased proclivity for crosslink clustering, reducing the detrimental impact of gamma radiation. The IR spectra for samples irradiated under 32 bar pressurised CO2 showed no significant change. Service life has been simulated on a custom built Test Rig. This has shown an exponentially decaying increase in static friction with time, an increase in force required to move the seals with exposure to CO2 and the deleterious effects of grease accumulation in flowrate sensing ports.

This study investigates erosion, deposition and material migration in the divertor of the fusion tokamak JET. Nuclear fusion provides a potential method for sustainable energy generation without large carbon emissions or long-term radioactive waste. Toroidal chambers with magnetic field coils known as tokamaks are used to contain the plasma of hydrogen fuel. The fuel ions can erode the plasma-facing materials, leading to degradation of plasma performance, limitation of vessel lifetimes and fuel retention. Plasma-material interaction is particularly significant in the divertor region of the tokamak. The carbon walls of JET-C have been replaced with the beryllium/tungsten walls of JET-ILW in anticipation of their use in the larger ITER tokamak. Determination and analysis of the different erosion/deposition characteristics provides vital information for the efficient, economic and safe operation of ITER. A combination of diagnostics and modelling techniques have been applied to produce a detailed study of the important processes and results. Rotating collectors provide time-resolved deposition measurements through varying the surfaces deposited on; quartz microbalances (QMBs) use piezoelectric crystals to measure changes in deposited mass. A simple, geometrical model is used to describe the rotating collector depositions over long timescales, incorporating experimental data from sources such as spectroscopy. More detailed, higher time resolution modelling of erosion, deposition and transport in the JET-ILW divertor is performed with a Monte Carlo code written for this study. The rotating collectors demonstrate a replacement of carbon by beryllium as the dominant impurity deposit in JET-ILW relative to JET-C and an associated reduction in deuterium retention. The total deposition rate on the JET-ILW collectors is reduced by an order of magnitude. In general, time-dependent modelled and experimental collector deposition profiles show good qualitative agreement. A lack of carbon deposition in the remote JET-C outer divertor for corner strike points is determined from the collector modelling and QMB measurements; similar behaviour is not observed for beryllium in JET-ILW. Additionally, there is a reversal of deposition asymmetry between the inner and outer divertor corners in JET-ILW. These different distributions of deposits are attributed to the different chemical properties of carbon and beryllium and their associated responses to elevated temperatures. Local beryllium surface coverages have a considerable impact on erosion and deposition behaviour in JET-ILW due to reduced impurity concentrations. Monte Carlo modelling is used to assess the impact of varying strike points, beryllium fluxes, beryllium coverages and plasma temperatures/densities. Further insight is gained through comparison of modelling and experimental results. Peaking of the beryllium influx is investigated using divertor spectroscopy and modelling, revealing the importance of the limiter phase and initial divertor phase for beryllium erosion, deposition and transport.

No description available.

Neoclassical tearing modes (NTMs) are plasma instabilities that can limit the performance of tokamaks and cause a termination of the plasma if allowed to grow. Systems to mitigate NTMs exist but have significant power requirements, which motivates further study of the mechanisms that lead to their growth in order to assist in the development of NTM avoidance strategies. NTMs typically require a seed magnetic island, above some threshold width, before they become unstable. The best available description of this threshold is the modified Rutherford equation (MRE) for NTM evolution; a combination of different models, which includes the effect of transport on NTM stability. Finite transport across magnetic field lines means that magnetic islands smaller than a critical width, w_c, do not completely flatten the pressure profiles and have a reduced bootstrap current perturbation, which leads to a threshold width, w_th. This thesis describes novel measurements of NTMs with mode structure m/n=2/1 on the MAST spherical tokamak (ST), which have allowed a direct evaluation of the effect of transport on island behaviour for the first time on an ST. Temperature profiles obtained with the upgraded Thomson scattering system on MAST have been used to constrain the solutions of a heat transport equation for a magnetic island, allowing the experimental determination of w_c, an important parameter in the MRE. The measured value of w_c = 0.7\pm 0.2cm obtained for an ensemble of MAST discharges is used in an analysis of the MRE for 2/1 NTM onset and saturation on MAST. By using a probabilistic method for parameter and error estimation, which takes account of the experimental uncertainty on measured equilibrium parameters, it is found that the temporal evolution of the island size is well described by marginally, classically unstable NTMs (that is, Delta'>0) with strongly destabilising bootstrap current and stabilising curvature terms. Finally, an analysis of two beta ramp-down discharges is presented, in which the measured w_c value explains the observed threshold width well.

This research is dedicated to develop a fully integrated system for heavy metals determination in water samples based on micro fluidic plasma atomizers. Several configurations of dielectric barrier discharge (DBD) atomizer are designed, fabricated and tested toward this target. Finally, a combination of annular and rectangular DBD atomizers has been utilized to develop a scheme for heavy metals determination. The present thesis has combined both theoretical and experimental investigations to fulfil the requirements. Several mathematical studies are implemented to explore the optimal design parameters for best system performance. On the other hand, expanded experimental explorations are conducted to assess the proposed operational approaches. The experiments were designed according to a central composite rotatable design; hence, an empirical model has been produced for each studied case. Moreover, several statistical approaches are adopted to analyse the system performance and to deduce the optimal operational parameters. The introduction of the examined analyte to the plasma atomizer has been achieved by applying chemical schemes, where the element in the sample has been derivitized by using different kinds of reducing agents to produce vapour species (e.g. hydrides) for a group of nine elements examined in this research individually and simultaneously. Moreover, other derivatization schemes based on photochemical vapour generation assisted by ultrasound irradiation are also investigated. Generally speaking, the detection limits achieved in this research for the examined set of elements (by applying hydroborate scheme) are found to be acceptable in accordance with the standard limits in drinking water. The results of copper compared with the data from other technologies in the literature, showed a competitive detection limit obtained from applying the developed scheme, with an advantage of conducting simultaneous, fully automated, insitu, online- real time analysis as well as a possibility of connecting the proposed device to control loops.

Fusion is seen by many as the ultimate energy source, capable of providing safe, clean and sustainable energy. Research has been carried out into fusion since the 1920s and substantial progress has been made. While the ultimate goal of providing energy from fusion remains elusive there is a clear understanding of the tasks that must be achieved to make fusion energy a reality. Deuterium and tritium (DT) offer a high probability of fusion when compared to any other combination of isotopes. Consequently DT fusion is the focus of all large scale fusion research programmes. As there is no natural source of tritium, fusion reactors are being designed with tritium breeder blankets to ensure self-sufficiency. The research contained within this thesis contributes to the ongoing development of breeder blankets for fusion reactors in terms of reducing their cost and improving their performance. The thesis follows a general theme of varying material composition to better utilise the local neutron spectra within fusion breeder blankets. The novel contributions of this thesis are as follows: The first contribution of this thesis is a technique that improves the accuracy of simulations involving time varying tritium production [171]. The technique identifies a minimum spacial resolution that should be used when performing burn-up studies in solid-type breeder blankets. Previously the tritium production with respect to time has been overestimated due to lack of spatial segmentation within the breeder blanket. Following on from this a parameter study was carried out to ascertain a more optimal composition for breeder blankets. The results of this study allows blanket designers to minimise the cost of the blanket while increasing the heat generated or maximising the tritium produced. The composition of breeder blankets operating with a DD neutron source was also optimised. This allows breeder blankets to create tritium from DD plasmas more efficiently. The use of DD plasmas to generate tritium is a proposed method of negating the need for an external supply of tritium to start up reactors. The sustainability of fusion is investigated and a method of reducing the use of beryllium within breeder blankets is presented. Blankets utilising this method were also shown to generate more heat, produce more tritium and showed lower peak heating. Varying the isotopic composition of materials was considered as a method to reduce helium production and improve the material properties without additional activity [132]. For the first, time cost benefit analysis of isotopically tailored materials in fusion reactors has been carried out and various methods to offset the enrichment cost have been identified. Multilayer blankets are investigated as a method of increasing the tritium production and are shown to achieve tritium production levels that are unobtainable for blankets with a uniform composition. Higher tritium production from blankets is particularly necessary in reactor designs that involve a reduction in the blanket volume. The composition of structural materials used within breeder blankets was also investigated. New material compositions are considered that offer reduced helium production. Additionally, materials that potentially offer improved material properties (e.g. fracture toughness and yield stress) are shown to be achievable. This is achieved by the addition of enriched Ni or Mo; the enrichment of the natural element allows it to be reintroduced without significant increase in activity. The final chapter summarises the research carried out, makes recommendations with regards to the design of future breeder blankets and presents further research opportunities.

Ice pigging is a novel technology using thick ice slurry (a two-phase mixture of ice crystals and freezing point depressant solution) to clean the internal surfaces of pipes or ducts; this mixture displays semi-solid characteristics. When pumped through a pipe, the slurry adopts plug flow, forming an 'ice pig'; slip occurs at the interface with the pipe walls generating high shear stresses; thus able to mobilise and remove sediment residing at the pipe wall. Ice pigs are able to navigate demanding topologies such as vertical falls, diameter changes, orifice plates, heat exchangers, and intrusive instrumentation; they provide a method of removing fouling without the need for dis-assembly, reducing valuable down-time, labour intensive pipe work dismantling, and subsequent manual cleaning. Many decades of nuclear activity here in the UK have produced unique and difficult challenges that require solving at Sellafield, the UK's nuclear waste reprocessing site. The drive to produce plutonium for atomic weapons during the 1950's, with very little foresight towards how the wastes and facilities would be dealt with, has brought about significant challenges. As these facilities are nearing the end of their design lives, the time has come to assess methods of treating these wastes and decommissioning the facilities in a safe, controlled, and cost-effective manner. Ice pigging is one of many technologies being assessed for such a task; this thesis details specific areas of application where experimental work has been conducted. Experimental work conducted in this thesis has: developed a method of characterising the ice pig's sediment removal performance compared to simple water flushing, assessed the ice pig's ability to remove representative sediments, assessed the ice pig's suitability for removing sediment from heat exchangers to restore thermal performance, and analysed the rate of percolation of the driving fluid through the ice pig body, such that the suitability of the ice pig for separating fluids can be established.

The work in this thesis focused on several problems relating to the growth and fouling of crystal mass in industrial environments, due to leakage of salt solutions or process liquor. The work has direct application to the nuclear industry, where the size and morphology of material deposits heavily impact on their associated criticality risk. An absence of clear methods and techniques to either predict or non-invasively monitor the growth of these crystalline deposits proves problematic for industrial specialists. Therefore the main part of the thesis focused on the development and implementation of models such that the growth behaviour of crystalline formations could be evaluated and quantified for varying physical parameters. This was accomplished through both the adaptation of previous geological models, and the development of a coupled multi-physics model such that fluid flow, heat transfer and crystallisation mechanisms could be considered. The models were validated against an experimental dataset provided by the National Nuclear Laboratory, and results were shown to be in good agreement. Through parametric studies it was determined that the characteristic shape of the formation was heavily determined by the initial solution concentration, flow velocity, temperature and the rate of evaporation. A method for the non-invasive monitoring of the deposits was also investigated through the solution of a geometric inverse problem governed by the transient two-dimensional heat equation. A meshless numerical method, namely the method of fundamental solutions was used as a direct solver in a complicated highly non-linear constrained minimisation. The model was shown to perform well when reconstructing simple shapes with highly contaminated input data. Additionally, complex shapes were also captured with a reasonable degree of accuracy and stability.