The overall purpose of this dissertation is to develop a multi-scale framework that can simulate radiation defect accumulation across a broad range of time and length scales in metals. In order to accurately describe defect accumulation in heterogeneous microstructures and under complex irradiation conditions, simulation methods are needed that can explicitly account for the effect of non-homogeneous microstructures on damage accumulation. In this dissertation, an advanced simulation tool called spatially resolved stochastic cluster dynamics (SRSCD) is developed for this purpose. The proposed approach relies on solving spatially resolved coupled rate equations of standard cluster dynamics methods in a kinetic Monte Carlo scheme. Large-scale simulations of radiation damage in polycrystalline materials are enabled through several improvements made to this method, including a pseudo-adaptive meshing scheme for cascade implantation and implementation of this method in a synchronous parallel kinetic Monte Carlo framework. The performance of the SRSCD framework developed in this dissertation is assessed by comparison to other simulation methods such as cluster dynamics and object kinetic Monte Carlo and experimental results including helium desorption from thin films and defect accumulation in neutron-irradiated bulk iron. The computational scaling of the parallel framework is also investigated for several test cases of irradiation conditions. SRSCD is next used to investigate radiation damage in three main types of microstructures, using α-iron as a test material: iron thin films, coarse-grained bulk iron, and nanocrystalline iron. SRSCD is used to investigate the mechanisms involved with defect accumulation in irradiated materials, such as effective diffusivity of helium in thin films and the effect of grain boundary sink strength on defect accumulation in nano-grained metals, and to predict defect populations in irradiated materials for comparison with experiments. Particular emphasis is placed on the role of microstructural features such as free surfaces and grain boundaries in influencing damage accumulation. Finally, the methodology developed in this dissertation is applied in the context of multiscale modeling and experimental design. To complete the multi-scale transition between defect-level behavior and macroscopic material property changes caused by irradiation, the relationship between mechanical loading and radiation damage is investigated. The impact of radiation damage on hardening of irradiated materials is investigated by using the results of SRSCD as inputs into polycrystalline crystal plasticity simulations. This is carried out in bulk iron by fitting hardening models to experimental data from neutron irradiation of iron and then used to predict hardening under irradiation conditions beyond what has already been accomplished in experimental studies. In addition, SRSCD is used to demonstrate the temperature shift required to achieve equivalent damage accumulation in irradiation conditions with significantly differing dose rates, such as in the case of using ion irradiation to simulate damage from neutron irradiation. In this dissertation, the development of SRSCD and its application in a multi-scale framework to predict macroscopic material property changes in metals represents a significant improvement over the state of the art due to improved simulations of defect accumulation and direct upscaling of results into polycrystalline plasticity models. The tools and understanding of defect behavior developed here will allow predictive modeling of metal degradation in reactor-relevant damage environments, including the defected microstructure and macroscopic material property changes due to irradiation.

Egyptian fruit bats were observed in Prague Zoo, where stable conditions are provided (year-round food supply, stable temperature and roosts). The colony has bimodal breeding cycle. Births are synchronized and occur on the turn of April and May and in September. The mating occurs throughout the year but there is an increase during lactation and in the period outside parturitions. There is no obvious pattern in cluster dynamics. Bats roost in four clusters (left, middle, upper right and lower right). The presence of clusters throughout the year is stable except upper right cluster. There are changes in the number of clusters and in the number of individuals inside each cluster. Fruit bats occupy approximately the same spots in clusters, however young individuals show the highest variability. An autogrooming is balanced throughout the year, there is no regular pattern in the observation period and even no increase after artificial twilight. From these activities a licking of wing membranes and fur are the most often. The mother cares for her young especially during lactation and weaning, while the care decreases with a development of the young. This is especially true for the licking of the wing membranes. An allogrooming is the most common between male and female during lactation and in the period...

Cette thèse s'intéresse au vieillissement des métaux au niveau microstructural. On étudie en particulier les défauts (amas de lacunes, interstitiels ou solutés) via un modèle de dynamique d'amas (DA), qui permet de prédire l'évolution des concentrations de défauts sur des temps longs (plusieurs dizaines d'années). Ce modèle est décrit par un système d'équations différentielles ordinaires (EDOs) de très grande taille, pouvant excéder la centaine de milliards d'équations. Les méthodes numériques classiques de simulation d'EDOs ne sont alors pas efficaces pour de tels systèmes. On montre dans un premier temps que la DA est bien posée et qu'elle vérifie certaines bonnes propriétés physiques comme la conservation de la quantité de matière et la positivité de la solution. On s'intéresse également à une approximation de la DA, qui prend la forme d'une équation aux dérivées partielles, de type Fokker--Planck. On caractérise en particulier l'erreur d'approximation entre la DA et cette approximation. Dans un second temps, on introduit un algorithme de simulation de la DA. Cet algorithme est basé sur un splitting de la dynamique ainsi que sur une interprétation probabiliste des équations de la DA (sous la forme d'un processus de saut) ou de son approximation de Fokker--Planck (sous la forme d'un processus de Langevin). Le but est de réduire le nombre d'équations à résoudre et d'accélérer par conséquent les simulations. On utilise enfin cet algorithme de simulation à différents modèles physiques. On confirme l'intérêt de ce nouvel algorithme pour des modèles complexes. On montre également que cet algorithme permet d'enrichir le modèle de dynamique d'amas à moindre coût / We study ageing of materials at a microstructural level. In particular, defects such as vacancies, interstitials and solute atoms are described by a model called Cluster Dynamics (CD), which characterize the evolution of the concentrations of such defects, on period of times as long as decades. CD is a set of ordinary differential equations (ODEs), which might contain up to hundred of billions of equations. Therefore, classical methods used for solving system of ODEs are not suited in term of efficiency. We first show that CD is well-posed and that physical properties such as the conservation of matter and the preservation of the sign of the solution are verified. We also study an approximation of CD, namely the Fokker--Planck approximation, which is a partial differential equation. We quantify the error between CD and its approximation. We then introduce an algorithm for simulating CD. The algorithm is based on a splitting of the dynamics and couples a deterministic and a stochastic approach of CD. The stochastic approach interprets directly CD as a jump process or its approximation as a Langevin process. The aim is to reduce the number of equations to solve, hence reducing the computation time. We finally apply this algorithm to physical models. The interest of this approach is validated on complex models. Moreover, we show that CD can be efficiently improved thanks to the versatility of the algorithm

La dynamique d'amas (DA) est une méthode de simulation en champ moyen permettant la prédiction de l'évolution des matériaux sous irradiation. Dans cette thèse, on s'intéresse aux forces de puits, qui sont les paramètres utilisés en DA pour représenter la capacité des puits (dislocations, cavités...) à absorber les défauts ponctuels (lacunes, interstitiels). Pour calculer les forces de puits, on réalise des simulations Monte Carlo cinétique sur objets (OKMC) dans lesquelles l'énergie des défauts ponctuels au point stable et au point col est décrite à l'aide de dipôles élastiques. Ces quantités sont préalablement calculées pour un cristal d'aluminium en utilisant la théorie de la fonctionnelle de la densité. Une première partie de ce travail vise à évaluer les forces des principaux puits présents dans les microstructures sous irradiation (cavité, dislocation droite et boucle de dislocation) dans des configurations simples. L'étude met en évidence l'importance des interactions élastiques, et permet d'identifier l'anisotropie des dipôles élastiques au point col comme un paramètre très important qui modifie à la fois les trajectoires des défauts ponctuels et les valeurs de forces de puits. On s'intéresse dans une deuxième partie à l'importance de la position relative des puits sur leur capacité à absorber les défauts ponctuels. Des microstructures contenant un grand nombre de boucles de dislocation sont générées par des simulations OKMC et l'absorption des défauts ponctuels par ces microstructures est mesurée. Il est montré que le voisinage d'un puits modifie sensiblement sa capacité à absorber les défauts ponctuels et ce comportement est rationalisé en utilisant le volume de Voronoï associé à chaque puits. Cette étude conduit à proposer une nouvelle expression de force de puits, ainsi qu'un nouveau formalisme de DA dans lequel les amas sont caractérisés par leur taille et par le volume de leur cellule de Voronoï. Il est montré que ce formalisme permet d'améliorer fortement la prédiction de l'évolution des distributions de tailles d’amas lors d'une irradiation. / Cluster Dynamics (CD) is a mean field simulation method which makes it possible to predict the materials evolution under irradiation. In this work, we focus on sink strengths, which are the parameters used in CD to represent the capacity of sinks (dislocations, cavities…) to absorb point defects (vacancies, self-interstitials). To calculate the sink strengths, object kinetic Monte Carlo (OKMC) simulations are performed. The energy of point defects at stable and saddle points is described through their elastic dipoles. These elements are computed in an aluminum crystal using density functional theory. In a first part, the sink strengths of the main objects found in irradiated microstructures (dislocations, cavities and dislocation loops) are calculated on simple configurations. This study reveals the importance of elastic interactions, and enables us to identify the saddle point anisotropy of point defects as an important parameter, as it modifies both the point defects trajectories and the sink strength values. Then, we focus on the role of the relative position of sinks in their capacity to absorb point defects. Microstructures containing a large number of dislocation loops are generated by OKMC simulations and the absorption of point defects by those microstructures is measured. It is shown that the neighborhood of a sink modifies noticeably its capacity to absorb the point defects and this behavior is rationalized through the Voronoi volume associated with each sink. This study leads to the proposal of a new sink strength expression, and of a new formalism for CD in which clusters are described by their size and their Voronoi volume. The results show that this formalism makes it possible to strongly improve the prediction of the evolution of cluster size distributions during irradiation.

Cette thèse est consacrée à l’étude des bulles d’hélium dans le silicium. Dans un premier temps, des simulations atomistiques de type dynamique moléculaire (DM) ont été réalisées afin d’étudier les propriétés physiques d’une bulle nanométrique : densité et pression d’hélium en fonction du diamètre de la bulle et de la quantité d’hélium initialement introduite dans la bulle. Nos calculs ont montré que la déformation plastique de la matrice ne permettait pas de dépasser une valeur de densité d’hélium nettement inférieure aux résultats expérimentaux. En modélisant un système plus réaliste, un excellent accord avec les mesures expérimentales a été retrouvé. Nous avons aussi montré que le choix du potentiel interatomique était capital afin de modéliser des bulles sous fortes pressions. En outre, nos résultats ont montré que la loi de Laplace-Young n’était pas adaptée pour prédire la pression dans les bulles d’hélium dans du silicium. Enfin, l’étude de l’état de l’hélium dans la bulle indique que l’hélium peut être sous forme solide à 300 K, du fait des pressions élevées. Dans un second temps, une approche de type dynamique d’amas (DA) a été employée afin de comprendre les premiers stades d’évolution des bulles. Nous avons construit un modèle applicable à la DA à l’aide d’un grand nombre de calculs DM. Une étude paramétrique a ensuite été réalisée. Ceci nous a permis d’étudier les mécanismes de croissance des bulles, en prenant en compte l’influence de l’hélium et la présence ou non d’une source continue de défauts. / This thesis is devoted to the study of helium bubbles in silicon. First, molecular dynamics (MD) simulations were performed in order to study the physical properties of a nanobubble: helium density and pressure as a function of the bubble diameter and of the initial helium quantity introduced in the bubble. A maximum value of the helium density in the bubble is reached due to the plastic deformation of the matrix; this value is significantly lower than the experimental results. By modelling a more realistic system, an excellent agreement with the experimental measurements is found. Moreover, the choice of the interatomic potential is essential to model the properties of bubbles under high pressure. Further, the Laplace-Young law is not appropriate to predict the pressure in helium bubbles in silicon. In addition, the investigation of the helium configuration in the bubble indicates that helium can be in the solid state at 300 K. In a second part, a cluster dynamics (CD) approach was employed to understand the early stages of bubble evolution. A helium in silicon interaction model to be used in CD was built by means of a large number of MD calculations. A parametric study was then performed taking into account the influence of helium and the presence or not of a continuous source of defects and allowing for the study of the mechanisms of bubble growth.

<p>Labour mobility can in theory be an efficient channel for knowledge transfer between cluster firms, thus contributing to growth and competitiveness. In the thesis labour mobility in two Swedish ICT clusters is studied. The purpose of the thesis is to develop an understanding of processes of labour mobility in clusters and to investigate whether mobility can be regarded as a cluster advantage. Both interview data and extensive registry data are used in order to analyse processes of mobility at three levels: individual, firm and cluster level.</p><p>The results show that labour mobility can to some extent be considered a cluster advantage for Swedish ICT firms, since cluster firms are likely to experience a higher level of labour mobility. It is also shown how mobility to and from the clusters contributes to the upgrading of formal competencies within cluster firms. However, the firms themselves are shown to rather focus on staff retention than turnover. </p><p>To some degree, labour mobility in the Swedish clusters in focus is presumably constrained by the formal institutional framework, as well as by informal rules and agreements between cluster firms. It is argued nonetheless that the sheer potential for mobility and the viability of informal hiring practices in clusters may be viewed as cluster advantages, besides the actual extent of labour mobility.</p>

Labour mobility can in theory be an efficient channel for knowledge transfer between cluster firms, thus contributing to growth and competitiveness. In the thesis labour mobility in two Swedish ICT clusters is studied. The purpose of the thesis is to develop an understanding of processes of labour mobility in clusters and to investigate whether mobility can be regarded as a cluster advantage. Both interview data and extensive registry data are used in order to analyse processes of mobility at three levels: individual, firm and cluster level. The results show that labour mobility can to some extent be considered a cluster advantage for Swedish ICT firms, since cluster firms are likely to experience a higher level of labour mobility. It is also shown how mobility to and from the clusters contributes to the upgrading of formal competencies within cluster firms. However, the firms themselves are shown to rather focus on staff retention than turnover. To some degree, labour mobility in the Swedish clusters in focus is presumably constrained by the formal institutional framework, as well as by informal rules and agreements between cluster firms. It is argued nonetheless that the sheer potential for mobility and the viability of informal hiring practices in clusters may be viewed as cluster advantages, besides the actual extent of labour mobility.

Le dioxyde d'uranium UO2 est le combustible nucléaire standard des réacteurs à eau pressurisée. Durant le fonctionnement du réacteur, la fission de l'uranium produit une grande variété de produits de fission, parmi lesquelles des gaz de fission, principalement le xénon et le krypton. En raison de leurs propriétés, ces gaz peuvent fortement impacter le comportement du combustible, et c’est pourquoi la maitrise de leur comportement est un enjeu industriel majeur et que de nombreux efforts de modélisation y sont consacrés depuis plusieurs dizaines d’années.Cette étude se base sur l’idée que la capacité prédictive des modèles de gaz est limitée par une description insuffisante des défauts ponctuels et de leurs interactions avec les atomes de gaz. Dans ce contexte, on applique à l’UO2 la Dynamique d’Amas, technique largement utilisée notamment pour décrire l’évolution de la concentration des défauts ponctuels et agrégés dans les métaux sous irradiation. Ce travail met plus particulièrement l’accent sur l’interprétation d’expériences de diffusion de gaz rares implantés dans l’UO2, en faisant appel au maximum à des résultats de modélisation atomistique pour évaluer les paramètres du modèle. / During in-pile irradiation of nuclear fuels a lot of rare gases are produced, mainly xenon and krypton. The behaviour of these highly insoluble fission gases may lead to an additional load of the cladding, which may have detrimental safety consequences. For these reasons, fission gas behaviour (diffusion and clustering) has been extensively studied for years.In this work, we present an application of Cluster Dynamics to address the behaviour of fission gases in UO2 which simultaneously describes changes in rare gas atom and point defect concentrations in addition to the bubble size distribution. This technique, applied to Kr implanted and annealed samples, yields a precise interpretation of the release curves and helps justifying the estimation of the Kr diffusion coefficient, which is a data very difficult to obtain due to the insolubility of the gas.

Le grandissement sous flux des tubes d’assemblages REP en alliages de zirconium est dû au fluage axial et au phénomène de croissance libre qui est associé à l’apparition des boucles <c> à forte dose d’irradiation. Ce travail de thèse vise à étudier le couplage entre ces deux phénomènes à travers l’analyse par Microscopie Electronique en Transmission de l’effet d’application d’une contrainte macroscopique sur la microstructure des boucles <c>. Des campagnes d’irradiation aux ions Zr+ (600 keV) ont été menées sur deux alliages de zirconium recristallisés : Zircaloy-4 et M5®. Grâce à un dispositif de mise en contrainte sous flux d’ions, différents niveaux de contrainte de traction ou de compression ont été appliqués. Les examens microscopiques ont montré que, conformément au mécanisme SIPA, la densité des boucles <c> diminue dans les grains d’axe <c> proche de la direction de traction ou éloigné de la direction de compression. Toutefois, l’analyse d’un grand nombre de grains a révélé une dispersion grain à grain. Cette dispersion, qui trouverait son origine dans les hétérogénéités intergranulaires, amoindrit l’amplitude de l’effet de la contrainte. Parallèlement à cette étude expérimentale, un modèle basé sur la méthode de dynamique d’amas a permis de décrire l’évolution de la microstructure sous irradiation du zirconium et du Zircaloy-4 et de rendre compte de l’effet de la contrainte. A l’échelle macroscopique, un modèle physique a été développé en vue de prédire le comportement en croissance et en fluage sous irradiation des tubes en alliages de zirconium. / The growth of zirconium alloy tubes of PWR fuel assemblies is the result of two phenomena: axial irradiation creep and stress “free” growth which is correlated to the formation of c-loops at high irradiation doses. This PhD work aims at investigating the coupling between these two phenomena through a fine Transmission Electron Microscopy analysis of the effect of a macroscopic applied stress on the c-loop microstructure. 600 keV Zr+ ion irradiations were performed at 300°C on two recrystallized zirconium alloys: Zircaloy-4 and M5®. Thanks to a device specifically designed, different tensile or compressive stress levels were applied under ion irradiation. The microstructural observations have shown that the c-loop density reduces in grains oriented with the c-axis close to the direction of the applied tensile stress or far from the direction of the applied compressive stress, which is in good agreement with the SIPA mechanism. Nevertheless, the examination of a large number of grains has revealed dispersion from grain to grain. This dispersion, which could be explained by the intergranular heterogeneities, reduces the magnitude of the stress effect on c-loop microstructure. In parallel to this experimental study, a cluster dynamics model has been able to describe the evolution under irradiation of zirconium and Zircaloy-4 microstructure and to assess the effect of stress on c-loop microstructure. On the macroscopic scale, a physical model was also developed to predict the irradiation growth and creep behaviour of zirconium alloy tubes.

In 1991, one year after the Namibian independence, the Ministry of Education and Culture was organised in six departments and six regional directorates. The regional directorates were established as early as in 1991. The establishment of the regional directorates was the first step towards decentralisation of education management, which took a form of de-concentration. During the late 1990s, the regional directorates were subdivided into thirteen regional education directorates to be in compliance with the central government policy of decentralising functions from the head offices of various ministries to the regional administrations in the thirteen regions. During 2000, the Namibian Ministry of Education introduced cluster-based school management as a decentralisation reform, granting authority and responsibility for managing school supervision and in-service training for school managers and teachers to clusters, to be implemented in all the thirteen education regions. Cluster–based school management reform has been adopted as a strategy for improving school supervision and teaching in Namibia. However, there is little empirical evidence on the effects of school clustering on the quality of teaching in Namibia. This study examined the implementation of cluster-based school management reform in the Namibian primary schools. The specific focus of the study was to assess: (1) the implementation of cluster-based school management reform in the Namibian primary schools; and (2) the relationship between cluster-based school management reform and improving teaching at classroom level. The data for this study were collected through: (1) survey research in thirty-seven primary schools in five regions: Caprivi; Erongo; Hardap; Karas and Kunene, and (2), case studies, based on interviews; focused group discussions; informal conversations; observations; and document analyses in the three primary school clusters in two of the five education regions. The study’s main findings are that the implementation of cluster-based school management reform has been constrained by resource scarcity and reluctance to share resources; potential threat to the authority of school inspectors and school principals; and incongruence between the ideologies existed prior to the introduction of the reform and the democratic ideology. The other main finding from this study is that there is insufficient evidence to show that the teaching methods of teachers who have received support from the school management reform are notably different from those who have not. This study demonstrates a number of obvious missing links between cluster-based school management and improving teaching, because the reform lacks: (1) clarity, guidelines and resources to support and monitor teaching in schools and at classroom level; (2) clarity on the roles and responsibilities of key implementers in improving teachers’ teaching practices; (3) capability to transform school traditions and culture into a culture which transforms teaching in schools; and (4) clarity on how teacher involvement can be utilised to improve teaching in schools. This study concludes that there is no evidence from this study that cluster-based school management reform relates to improving teaching. / Thesis (PhD)--University of Pretoria, 2009. / Education Management and Policy Studies / unrestricted