Nuclear fuel performance is highly affected by the behaviour of fission gases, particularly at elevated burnups, where large amounts of gas are produced and can potentially be released. The importance of fission gas release was the motivation for large efforts, both experimentally and theoretically, in order to increase our understanding of the different steps of the process, and to continuously improve our models. Extensions to higher burnups, together with the growing interest in novel types of fuels such as inert matrix fuels envisaged for the transmutation of minor actinides, make that one is still looking for a permanently better modelling, based on a physical understanding and description of all stages of the release mechanism. Computer simulations are nowadays envisaged in order to provide a better description and understanding of atomic-scale processes such as diffusion, but even in order to gain insight on specific processes that are inaccessible by experimental means, such as the fuel behaviour during thermal spikes. In the present work simulation techniques based on empirical potentials have been used, focusing in a first stage on pure uranium dioxide. The behaviour of point defects was at the core of this part, but also the estimation of elastic and melting properties. Then, in a second stage, the study has been extended to the behaviour of helium and xenon. For helium, the diffusion in different domains of stoichiometry was considered. The simulations enabled to determine the diffusion coefficient and the migration mechanism, using both molecular dynamics and static calculation techniques. Xenon behaviour has been investigated with the additional intention to model the behaviour of small intragranular bubbles, particularly their interaction with thermal spikes accompanying the recoil of fission fragments. For that purpose, a simplified description of these events has been proposed, which opens perspectives for further work. / Les performances du combustible nucléaire sont fortement affectées par le comportement des gaz de fission, et ce particulièrement lorsqu’un taux d’épuisement élevé est atteint, puisque d’importantes quantités de gaz sont alors produites et peuvent potentiellement être relâchées. Les enjeux, entre autre économiques, liés au relâchement de gaz de fission ont donné lieu à d’importants efforts, tant sur le plan expérimental que théorique, afin d’accroître notre compréhension des différentes étapes du processus, et d’améliorer sans cesse les mod`eles. Les extensions à des taux d’épuisements encore plus élevés ainsi que l’intérêt croissant pour de nouveaux types de combustible tels que les matrices inertes, envisages en vue de la transmutation des actinides mineures, font qu’à l’heure actuelle, le besoin permanent d’une meilleure modélisation, basée sur une compréhension et une description physique des différentes étapes du processus de relâchement de gaz de fission, est toujours de mise. Les simulations par ordinateur ont ainsi été considérée comme un nouvel angle de recherche sur les processus élémentaires se produisant à l’échelle atomique, à la fois afin d’obtenir une meilleure compréhension de processus tels que la diffusion atomique ; mais aussi afin d’avoir accès à certains processus qui ne sont pas observables par des voies expérimentales, tels que la le comportement du combustible lors de pointes thermiques. Dans ce travail, deux techniques, basées sur l’utilisation de potentiels interatomiques empiriques, ont permis d’étudier le dioxyde d’uranium, dans un premier temps en l’absence d’impuretés. Cette partie était principalement centrée sur le comportement des défauts ponctuels, mais a aussi concerné différentes propriétés élastiques, ainsi que le processus de fusion du composé. Ensuite l’étude a été étendue aux comportements de l’hélium de du xénon. Pour ce qui a trait à l’hélium, la diffusion dans différents domaines de stoechiométrie a été considérée. Les simulations ont permis de déterminer le coefficient de diffusion ainsi que le mécanisme de migration lui-même. Quant au xénon, outre les propriétés de diffusion, l’intention fut de se diriger vers la modélisation des petites bulles intragranulaires, et plus précisément vers leur interaction avec les pointes thermiques, créées lors du recul des fragments de fission. Une description simplifiée de ce processus a été proposée, qui offre de nouvelles perspectives dans ce domaine.

The variability of the postsynaptic response following a presynaptic action potential arises from: i) the neurotransmitter release being probabilistic and ii) channels in the postsynaptic cell involved in the response to neurotransmitter release, having stochastic properties. Spike timing dependent plasticity (STDP) is a form of plasticity that exhibits LTP or LTD depending on the precise order and timing of the firing of the synaptic cells. STDP plays a role in fundamental tasks such as learning and memory, thus understanding and characterizing the effect variability in synaptic transmission has on STDP is essential. To that end a model incorporating both forms of variability was constructed. It was shown that ion channel stochasticity increased the magnitude of maximal potentiation, increased the window of potentiation and severely reduced the post-LTP associated LTD in the STDP curves. The variability due to short term plasticity decreased the magnitude of maximal potentiation.

The variability of the postsynaptic response following a presynaptic action potential arises from: i) the neurotransmitter release being probabilistic and ii) channels in the postsynaptic cell involved in the response to neurotransmitter release, having stochastic properties. Spike timing dependent plasticity (STDP) is a form of plasticity that exhibits LTP or LTD depending on the precise order and timing of the firing of the synaptic cells. STDP plays a role in fundamental tasks such as learning and memory, thus understanding and characterizing the effect variability in synaptic transmission has on STDP is essential. To that end a model incorporating both forms of variability was constructed. It was shown that ion channel stochasticity increased the magnitude of maximal potentiation, increased the window of potentiation and severely reduced the post-LTP associated LTD in the STDP curves. The variability due to short term plasticity decreased the magnitude of maximal potentiation.

Chromium (VI) is a pervasive groundwater contaminant that poses a considerable threat to human health. Remediation techniques have focused on the reduction of the highly mobile Cr(VI) to the sparingly soluble, and less toxic, Cr(III) species. Traditionally, remediation performance has been evaluated through the measurement of Cr(VI) concentrations; however, this method is both costly and time-consuming, and provides little information regarding the mechanism of Cr(VI) removal. More recently, Cr isotope analysis has been proposed as a tool for tracking Cr(VI) migration in groundwater. Redox processes have been shown to produce significant Cr isotope fractionation, where enrichment in the ⁵³Cr/⁵²Cr ratio in the remaining Cr(VI) pool is indicative of a mass-transfer process. This thesis describes laboratory batch and column experiments that evaluate the Cr isotope fractionation associated with the reduction of Cr(VI) by various materials and under various conditions. Laboratory batch experiments were conducted to characterize the isotope fractionation during Cr(VI) reduction by granular zero-valent iron (ZVI) and organic carbon (OC). A decrease in Cr(VI) concentrations was accompanied by an increase in δ⁵³Cr values for the ZVI experiments. Data were fitted to a Rayleigh-type curve, which produced a fractionation factor α = 0.9994, suggesting a sorption-dominated removal mechanism. Scanning electron microscopy (SEM), X-ray absorption near-edge structure (XANES) spectroscopy, and X-ray photoelectron spectroscopy (XPS) indicated the presence of Cr(III) on the solid material, suggesting that reduction of Cr(VI) occurred. A series of batch experiments determined that reaction rate, experimental design, and pre-treatment of the ZVI had little to no effect on the Cr isotope fractionation. The interpretation of isotope results for the organic carbon experiments was complicated by the presence of both Cr(VI) and Cr(III) co-existing in solution, suggesting that further testing is required. A laboratory column experiment was conducted to evaluate isotopic fractionation of Cr during Cr(VI) reduction by OC under saturated flow conditions. Although decreasing dissolved Cr(VI) concentrations also were accompanied by an increase in δ⁵³Cr values, the isotope ratio values did not fit a Rayleigh-type fractionation curve. Instead, the data followed a linear regression equation yielding α = 0.9979. Solid-phase analysis indicated the presence of Cr(III) on the surface of the OC. Both the results of the solid-phase Cr and isotope analyses suggest a combination of Cr(VI) reduction mechanisms, including reduction in solution, and sorption prior to reduction. The linear characteristic of the δ⁵³Cr data may reflect the contribution of transport on Cr isotope fractionation.

In this thesis five different protection harnesses for hunting dogs against canidae attacks were assessed. Hunting dogs can be attacked and severely injured or killed by wolves (Canis lupus) when released during hunting. So far there is no effective protection method. Similar problems are reported with African wild dogs (Lycaon pictus) and dholes (Cuon alpinus) with other domestic animals. In this study the experimental harnesses were presented on a dummy to lure the animals to attack them. The harnesses with physical (screws or spikes on the back) and ultrasound (immediate bite controlled and 19 second continuous ultrasound) deterrents were only assessed during wolf attacks, whereas the harness with electric shocks was also tested on the other two species. Neither physical nor ultrasound deterrents showed a large enough aversive response in the wolves. Electric shocks, given to the animals when biting the dummy, triggered an immediate release of the dummy in all three species. Long term effects differed between species and individuals. The exposed wolf did not touch the dummy again after a second exposure, whereas all except one African wild dog bit the dummy again in consecutive trials. Some individuals returned to bite a second time even in the same trial. An assessment of the long term effect on dholes was not possible, as the individuals were undistinguishable. Based on the data obtained in this study a harness with electric deterrent seems the most promising.

Neural prostheses are being developed to provide motor capabilities to patients who suffer from motor-debilitating diseases and conditions. These brain-computer interfaces (BCI) will be controlled by activity from the brain and bypass damaged parts of the spinal cord or peripheral nervous system to re-establish volitional control of motor output. Spike sorting is a technologically expensive component of the signal processing chain required to interpret population spike activity acquired in a BCI. No systematic analysis of the need for spike sorting has been carried out and little is known about the effects of spike sorting error on the ability of a BCI to decode intended motor commands. We developed a theoretical framework and a modelling environment to examine the effects of spike processing on the information available to a BCI decoder. Shannon information theory was applied to simulated neural data. Results demonstrated that reported amounts of spike sorting error reduce mutual information (MI) significantly in single-unit spike trains. These results prompted investigation into how much information is available in a cluster of pooled signals. Indirect information analysis revealed the conditions under which pooled multi-unit signals can maintain the MI that is available in the corresponding sorted signals and how the information loss grows with dissimilarity of MI among the pooled responses. To reveal the differences in non-sorted spike activity within the context of a BCI, we simulated responses of 4 neurons with the commonly observed and exploited cosine-tuning property and with varying levels of sorting error. Tolerances of angular tuning differences and spike sorting error were given for MI loss due to pooling under various conditions, such as cases of inter- and/or intra-electrode differences and combinations of various mean firing rates and tuning depths. These analyses revealed the degree to which mutual information loss due to pooling spike activity depended upon differences in tuning between pooled neurons and the amount of spike error introduced by sorting. The theoretical framework and computational tools presented in this dissertation will BCI system designers to make decisions with an understanding of the tradeoffs between a system with and without spike sorting. / Dissertation

Die vorliegende Arbeit beschäftigt sich mit der Analyse des Volleyballangriffsschlages von der Position vier unter Wettkampfbedingungen. Der Angriffsschlag von dieser Position ist oft die spielentscheidende Einflußgröße, da die meisten Punkte von dieser Position aus erzielt werden. Es handelt sich um einen komplexen Bewegungsablauf und es gibt eine Vielzahl an Untersuchungen, die sich mit der bewegungsanalytischen Untersuchung des Angriffsschlages befassen. Eine Herausforderung der generellen Problemstellung liegt darin, den Bewegungsablauf adäquat zu parametrisieren. Verschiedene Studien beschäftigten sich beispielsweise ausschließllich mit der Armbewegung während der Schlagphase oder mit der Beinbewegung während der Absprungphase. Die Dissertation ist darauf ausgerichtet eine breite Datenbasis für die Analyse von Volleyballangriffsschlägen zu schaffen. Der innovative Charakter der Arbeit liegt dabei in drei wesentlichen Punkten: - Definition leistungsrelevanter Parameter - Analyse von Angriffsschlägen unter Wettkampfbedingungen . große Stichprobe Insbesondere die Analyse von Angriffsschlägen während internationaler Wettkämpfe stellt dabei eine Herausforderung dar und hebt diese Arbeit von anderen Studien in der Literatur ab. Einerseits bietet dieser Ansatz die Möglichkeit "reale" Bewegungsabläufe im Spiel zu betrachten, andererseits verringert sich dadurch die Standardisierbarkeit der Umgebungsbedingungen. Die methodische Innovation liegt darin, zu untersuchen, welche Bewegungsabläufe unter echten Wettkampfbedingungen ausgeführt werden. Die wissenschaftliche Innovation liegt in der Identifikation und Definition leistungsrelevanter Parameter, die den Bewegungsablauf quantifizieren können. Damit kann ein Einblick gewonnen werden, was unter Spielbedingungen einen erfolgreichen Angriffsschlag ausmacht.

This research focuses on empirical observations of horizontal series arc faults. These faults differ from ground faults, for series faults encompass the electromagnetic transient effects of arc formations in series with sustained current flow when there is a break in the circuit. This may happen intentionally (as in a breaker) or unintentionally (as in a loose, damaged, or severed cable). This paper studies some of those transient effects during arc ignition, propagation, and cessation. Emphasis is on dc systems, for series faults present some of the more challenging safety concerns relating to widespread dc micro-grid acceptance and proliferation. However, arc behavior is also compared to that of ac systems under “quasi-equivalent”, passive circuit parameters. Variables of study primarily include arc voltage, current, and their relationship to electrode spacing under dynamic conditions. Results indicate that interruptions in dc current, while appearing more chaotic from a localized standpoint, do not produce the fast-acting transients associated with ac disturbances. Additionally, if dc arcs propagate over a slowly increasing distance of separation, they can be modeled as quasi-static in nature. An equation model is developed and curve-fitting parameters match well with historically tabulated constants. / text

This work introduces a combination of image processing and analysis methods that perform feature extraction, shape analysis and tomographic reconstruction of Electron Microscopy images. These have been implemented on images of the AIDS virus interacting with neutralizing molecules. The AIDS virus spike is the primary target of drug design as it is directly involved in infecting host cells. First, a fully automated technique is introduced that can extract sub-volumes of the AIDS virus spike and be used to build a statistical model without the need for any user supervision. Such an automatic feature extraction method can significantly enhance the overall process of shape analysis of the AIDS virus spike imaged through the electron microscope. Accurate models of the virus spike will help in the development of better drug design strategies. Secondly, a tomographic reconstruction method implemented using a shape based regularization technique is introduced. Spatial models of known features in the structure being reconstructed are integrated into the reconstruction process as regularizers. This regularization scheme is driven locally through shape information obtained from segmentation and compared with a known spatial model. This method shows reduced blurring, and an improvement in the resolution of the reconstructed volume was also measured. It performs better than popular current techniques and can be extended to other tomographic modalities. Improved Electron Tomography reconstructions will provide better structure elucidation and improved feature visualization, which can aid in solving key biological issues. / text

The work presented in this dissertation focuses on the use of computational and mathematical models to investigate how mammalian brains construct and maintain stable representations of space and location. Recordings of the activities of cells in the hippocampus and entorhinal cortex have provided strong, direct evidence that these cells and brain areas are involved in generating internal representations of the location of an animal in space. The emphasis of the first two portions of the dissertation are on understanding the factors that influence the scale and stability of these representations, both of which are important for accurate spatial navigation. In addition, it is argued in both cases that many of the computations observed in these systems emerge at least in part as a consequence of a particular type of network structure, where excitatory neurons are driven by external sources, and then mutually inhibit each other via interactions mediated by inhibitory cells. The first contribution of this thesis, which is described in chapter 2, is an investigation into the origin of the change in the scale of spatial representations across the dorsoventral axis of the hippocampus. Here it will be argued that this change in scale is due to increased processing of nonspatial information, rather than a dorsoventral change in the scale of the spatially-modulated inputs to this structure. Chapter 3 explores the factors influencing the dynamical stability of class of pattern-forming networks known as continuous attractor networks, which have been used to model various components of the spatial navigation systems, including head direction cells, place cells, and grid cells. Here it will be shown that network architecture, the amount of input drive, and the timescales at which cells interact all influence the stability of the patterns formed by these networks. Finally, in chapter 4, a new technique for analyzing neural data is introduced. This technique is a spike train similarity measure designed to compare spike trains on the basis of shared inhibition and bursts.