Spent Nuclear Fuel (SNF) is a by-product of existing nuclear reactors; SNF consists of long-lived radioactive actinides which have an average half-life of several thousand years (e.g. Plutonium-239 with a half-life of 24,000 years, and Americium-243 with a half-life of 7,360 years). Several multinational organizations are making an attempt to extract the energetic value out of these nuclear stockpiles in order to minimize the risk of nuclear proliferation and reduce waste volume. The Inert Matrix Fuel (IMF) concept is being considered as an option to reuse the radioactive actinides present in spent nuclear fuel by means of a transmutation process. Due to the volatile nature of these radioactive actinides, it is expected that the high-temperature conventional processing of IMFs will result in a significant loss of material. This study investigates microwave sintering of inert matrix material (excluding actinide fuel) as an alternative route to conventional processing. It was observed that microwave sintering showed a reduction of 300°C in temperature required for full densification when compared to conventional sintering. The reduction in sintering temperatures did not show any significant variation in the resulting properties (hardness and grain size). While these results satisfy the need for the application, it is important to understand why microwaves enhance the sintering phenomena. It is speculated (by many researchers) that the electric field associated with microwave energy is enhancing flux leading to accelerated densification during microwave sintering. This study has observed a decrease in the activation energy (for sintering 8YZ) with the increase in the magnitude of the applied electric field. / Ph. D.

In this thesis we develop inverse scattering algorithms towards the ultimate goal of online diagnostic methods. The aim is to detect structural changes inside power transformers and other major power grid components, like generators, shunt reactors etc. Power grid components, such as large power transformers, are not readily available from the manufacturers as standard designs. They are generally optimized for specific functions at a specific position in the power grid. Their replacement is very costly and takes a long time. Online methods for the diagnostics of adverse changes of the mechanical structure and the integrity of the dielectric insulation in power transformers and other power grid components, are therefore essential for the continuous operation of a power grid. Efficient online diagnostic methods can provide a real-time monitoring of mechanical structures and dielectric insulation in the active parts of power grid components. Microwave scattering is a candidate that may detect these early adverse changes of the mechanical structure or the dielectric insulation. Upon early detection, proper actions to avoid failure or, if necessary, to prepare for the timely replacement of the damaged component can be taken. The existing diagnostic methods lack the ability to provide online reliable information about adverse changes inside the active parts. More details about the existing diagnostic methods, both online and offline, and their limitations can be found in the licentiate thesis preceding the present PhD thesis. We use microwave scattering together with the inverse scattering algorithms, developed in the present work, to reconstruct the shapes of adverse mechanical structure changes. We model the propagation environment as a waveguide, in which measurement data can be obtained only at two ends (ports). Since we want to detect the onset of some deformation, that only slightly alters the scattering situation (weak scattering), we have linearized the inverse problem with good results. We have calculated the scattering parameters of the waveguide in the first-order perturbation, where they have linear dependencies on the continuous deformation function. A linearized inverse problem with a weak scattering assumption typically results in an ill-conditioned linear equation system. This is handled using Tikhonov regularization, with the L-curve method for tuning regularization parameters. We show that localized one-dimensional and two-dimensional shape deformations, for rectangular and coaxial waveguide models, are efficiently reconstructed using the inverse scattering algorithms developed from the first principles, i.e. Maxwell’s theory of electromagnetism. An excellent agreement is obtained between the reconstructed and actual deformation shapes for a number of studied cases. These results show that it is possible to use the inverse algorithms, developed in the present thesis, as a theoretical basis for the design of a future diagnostic device. As a part of the future work, it remains to experimentally verify the results obtained so far, as well as to further study the theoretical limitations posed by linearization (first-order perturbation theory) and by the assumption of the continuity of the metallic waveguide boundaries and their deformations. / <p>QC 20160119</p>

The work presented within this thesis details the experimental investigation of the surface waves supported on metasurfaces. Particular attention has been given to the reflection of these surface waves from planar discontinuities associated with these metasurfaces. Various experimental techniques have been developed throughout this work to characterise surface wave supporting metasurfaces. These include a new technique for measuring the dispersion of surface waves supported on metasurfaces, characterisation of the near-field associated with the surface waves, a device for launching planar phase front surface waves and finally a technique for measuring the surface wave reflection coefficient. The dispersion of surface waves on a square array of square cross-section metal pillars has been fully characterised and compared to FEM modelling. The results show that a family of surface waves may be supported by pillar or crossed slit structures rather than just holes even though there is now no lowest cut-off frequency. A family of TM surface modes have been shown to exist with dispersions which asymptote to frequencies defined by the pillar heights (slit depth) and the refractive index of the material filling the slits. Primarily this work focussed on the surface wave properties associated with a square array of square metal patches on a dielectric coated ground plane and a Sievenpiper `mushroom' metasurface. The amplitude reflection coefficient of these surface waves has been studied for three distinct systems: Firstly for surface waves incident upon the termination of a these metasurfaces to free space, secondly for surface waves incident upon the interface between a dielectric coated and uncoated metasurface and thirdly for surface waves incident on the boundary between two metaurfaces. The reflection coefficient of surface waves incident upon the termination of the metasurface to free space is found to increase significantly with the confinement of the surface mode. This confinement, and therefore the form of the reflection coefficient, is significantly different for the two metasurfaces considered due to their dispersions. This increase in the reflection coefficient is caused by both the momentum mismatch of the surface wave compared to the freely propagating modes and the different field distributions of the two modes. The reflection coefficient of surface waves incident upon the boundary between a coated and uncoated metasurface has been experimentally characterised for the metal patch array and Sievenpiper `mushroom' metasurfaces. It is shown that the addition of a thin, significantly subwavelength, dielectric overlayer onto the metasurface vastly perturbs the surface wave dispersion. The reflection coefficient of the surface waves is found to depend on the dispersion of the mode supported on the coated and uncoated metasurface and the overlayer thickness. Most noticeably the thickness of the overlayer, by comparison to the surface wave decay length, has a significant effect on scattering to free space associated with the surface wave reflection. The final system considered was designed to investigate the impedance approximation, often used to describe metasurfaces, and found it to be an incomplete description of the surface waves supported on the metasurfaces used within this study. In the impedance approximation the two surfaces considered are said to be `impedance matched` at certain frequencies. It is demonstrated that the failure of the impedance approximation to accurately describe this system is due to the behaviour of the electric field within the metasurfaces. These are not analytically described in the impedance approximation and are required for an accurate description of the surface waves supported on these metasurfaces.

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Metamaterials are a class of artificial material, known to produce electromagnetic (EM) responses not found in nature due to their engineered subwavelength structure. In this thesis very thin subwavelength meshes are utilised to form layered metamaterials. The EM characteristics of the transmission and reflection response from these materials, including the polarisation converting behaviour, are explored to further understanding and develop structures to exploit and control the propagation of microwave radiation. Original experimental studies are presented across two sections; the first examines the response of stacks assembled from metallic meshes and dielectric plates; the second explores a rotated layered structure formed of square symmetric elements in a square subwavelength array that demonstrates chirality through evanescent coupling of the near fields. When metallic meshes are excited with EM radiation below the cut off frequency, only evanescently decaying fields are supported in the holes. By combining these subwavelength metallic meshes with dielectric plates in different arrangements, remarkably wide bands of high transmission and low reflection may be observed. The non-interacting resonant modes allow the response to be tuned through a suitable choice of the metallic mesh geometry and the properties of the dielectric. Further the low frequency band edge and the bandwidth are not dependent on the number of unit cells in the stack; but are dependent on the properties of the unit cell. The second section demonstrates ``evanescent handedness'' proposed as a new type of chirality. Two subwavelength square arrays of square elements are rotated with respect to one another. When the rotated arrays are positioned far from one another in the propagation direction, each acts as an effective medium layer. However when placed in close proximity the structure is shown to rotate the plane of polarisation of the incident radiation. All these mesh based structures share the property of producing an EM response that is tunable by design, allowing a structure to be tailored to a specific application.

Anaerobic digestion of waste activated sludge can be improved if hydrolysis of particulate substrates is enhanced and available substrate is made more accessible by both breakup of the sludge matrix floc and rupture of the cell wall. Microwave (MW) pretreatment was suggested and studied as a way to improve digestion efficiency. The work done focuses on the effects of MW pretreatment on the characteristics of the sludge, due to thermal and athermal effects. It also evaluates the effects some process variables in the activated sludge process have on the pretreatment efficiency as well as the effect operating conditions in the downstream anaerobic digestion process have on the biodegradability efficiency of those sludges. Effects of athermal and thermal MW radiation were measured by use of a customized MW oven capable of providing MW radiation with uncoupled thermal and athermal effects. Athermal radiation was capable of increasing substrate present in the soluble phase of sludge, and had a positive effect in the digestion of athermal samples. The increases in biogas production and substrate solubilisation were smaller in magnitude than the increases measured for MW thermal tests. Further refining of the tests with athermal and thermal sludge, involved separation by size class of the solubilized substrate by means of ultrafiltration (UF), and revealed that changes in particle size distribution were significant not only for MW thermal tests, but also for athermal tests, with a particular emphasis in proteins in athermal tests. These changes had an effect on the biodegradability of the sludges by class size, with thermally pretreated sludge producing more biogas for smaller particles size classes but also exhibiting more inhibition. Tests were made with several combinations of sludge with different ages and subject to different MW pretreatment temperatures. The work showed that sludge age or solids retention time (SRT) has a significant effect on the pretreatment efficiency with maximum biogas improvements measured at different MW pretreatment temperatures depending on the SRT of the sludge tested, and with different behaviour for mesophilic and thermophilic digestion. Mesophilic tests showed greater improvements in terms of digestion effiency on average, but thermophilic tests showed more uniform performance, with a higher baseline efficiency. The presence of an optimum of MW pretreatment temperature and sludge SRT for maximal biogas production is more defined for mesophilic conditions than for thermophilic conditions. Semi-continuous studies were conducted with several combinations of single and two stage mesophilic and thermophilic digestors treating MW pretreated sludge and non-pretreated sludge. Staging and thermophilic digestion allowed the maintenance of a stable digestion process with high biogas productions and high solids removal efficiencies with production of sludge with good bacteriological characteristics for an very low residence time (5 d).

Thesis (M.S. in Systems Engineering (Electronic Warfare))--Naval Postgraduate School, September 1990. / Thesis Advisor(s): Atwater, Harry A. Second Reader: Janaswamy, Rama. "September 1990." Description based on title screen as viewed on December 29, 2009. DTIC Identifier(s): Suspended striplines, microstrip lines, equivalent circuits, program listings, theses. Author(s) subject terms: Suspended-microstrip line, step discontinuity, equivalent circuit model, step-change. Includes bibliographical references (p. 60). Also available in print.

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