Rod bundle is a typical constitutive element of a very wide range of nuclear reactor designs. This thesis describes the investigation of such geometry with wall-resolved Large Eddy Simulation (LES). In order to alleviate the mesh constraint, imposed by the near wall resolution, the usage of embedded refinements and polyhedral meshes is analysed firstly with a inviscid laminar case (Taylor Green vortices) and secondly with a fully turbulent case (channel flow only with embedded refinement). The inviscid test case shows that the addition of embedded refinements decreases the conservation properties of the code. Indeed the accuracy decreases from second order in a structured conformal mesh, to something in between first and second order depending on the quality of the unstructured mesh. Better results are obtained when the interface between refined and coarse areas presents a more regular and structured pattern, reducing the generation of skewed and stretched cells. The channel flow simulation shows that the Reynolds stresses, of some embedded refined meshes, are affected by spurious oscillations. Surprisingly this effect is present in the unstructured meshes with the best orthogonal properties. Indeed analysis of Reynolds stress budgets shows that terms, where the gradient in the wall normal direction is dominant, have a largely oscillatory behaviour. The cause of the problem is attributed to the convective term and in particular in the method used for the gradient reconstruction. As a consequence of these contradictory signs between the inviscid and the fully turbulent cases, the rod bundle test case is analysed using a conventional body fitted multiblock mesh. Two different Reynolds numbers are investigated reporting Reynolds stresses and budgets. The flow is characterised by an energetic and almost periodic azimuthal flow pulsation in the gap region between adjacent sub-channels, which makes turbulent quantities largely different from those in plane channel and pipes and enhances mixing. Experiments found that a constant Strouhal number, with the variation of the Reynolds number, characterises the phenomenon. The frequency analysis finds that present simulations are distinguished by three dominant frequencies, the first in agreement with the experimental value and two higher ones, which might be due to the correlation of the azimuthal velocity in the streamwise direction. Several passive temperature fields are added at the simulations in order to study the effects of the variation of the Prandtl number and the change in boundary conditions (Neumann and Dirichlet). A simplified case where an imbalance of the scalar between adjacent sub-channels is also investigated in order to evaluate the variation of the heat fluxes with respect to the homogeneous case. An alternative solution, to reduce the mesh constraint imposed by the wall, is to hybridize LES with RANS. The main achievement of this work is to integrate the heat transfer modelling to the already existing model for the dynamic part. Further investigations of the blending function, used to merge the two velocity fields, are carried out in conjunction with a study of the model dependency on the mesh resolution. The validation is performed on a fully developed channel flow at different Reynolds numbers and with constant wall heat flux. On coarse meshes the model shows an improvement of the results for both thermal and hydraulic parts with respect to a standard LES. On refined meshes, suitable for wall-resolved LES, the model suffers from a problem of double counting of modelled Reynolds stresses and heat fluxes because the RANS contribution does not naturally disappear as the mesh resolution increases.

The present work was produced as part of the MUZIC consortium, a collaboration between a multi-university team from the UK and industrial partners working on the field of nuclear energy, fabrication of alloys and nuclear research. The aim of the project is to establish a multidiscipline mechanistic understanding of the corrosion and breakaway processes of zirconium alloys used as fuel cladding materials in the nuclear industry. A better understanding of the corrosion mechanism of zirconium alloys will not only aid the development of better performing alloys, but will also allow more accurate models to be developed to reliably predict the service life of existing alloys. This could lead to higher burn-up, increase of energy production and reduction of nuclear waste produced.This work seeks to provide a better understanding of the role of residual stresses in the oxide, which are produced during oxidation due to high Pilling-Bedworth ratio and their impact on oxide phase transformation and oxidation kinetics by employing high energy synchrotron X-ray diffraction techniques. This is achieved by observing how stresses change as oxide growth approaches and passes through transition of the corrosion kinetics, their evolution across the oxide thickness, in situ characterising stresses and phase growth early in oxidation process and how stress changes can affect corrosion properties.It was found that relatively high compressive stresses in the two oxide crystal structures are present. The stresses relax with time up to moments before transition where a possible threshold stress magnitude is reached to aid an extensive tetragonal to monoclinic phase transformation. This generalised tetragonal to monoclinic transformation is believed to produce highly stressed monoclinic crystal structure grains and cause defects in the oxide. The above observation is further supported by a decrease of the tetragonal zirconia content. This is the moment that the oxide looses its protective character and a transition of the corrosion kinetics occurs. By comparing different materials it was observed that the minimum magnitude of the tetragonal phase is lower in better performing alloys while the tetragonal content is some cases was relatively low. It is suggested that the amount of the tetragonal phase, in the oxide layer, is not as important as the rate of it transforming into monoclinic. The extent of tetragonal to monoclinic transformation, that introduces defects in the oxide, defines how protective an oxide layer is. The present work provides a contribution to the available knowledge of the importance of residual stresses in the oxide layer and metal substrate of zirconium alloys and how they can affect corrosion rates or act as a precursor to the corrosion transition.

In this thesis the effect of the Hall term in the generalised Ohm's law on Alfvén (shear) and fast wave propagation and dissipation in the ion cyclotron frequency range is investigated. The damping of an initially Gaussian field perturbation in a uniform Hall MHD plasma is treated analytically. Subsequently a 2D Lagrangian remap code (Lare2d) is used to study the damping and phase mixing of initially Gaussian field perturbations and a harmonic series of boundary-driven perturbations in a uniform field (in the presence of a transverse equilibrium density gradient). The same code is then used to study a range of initially shear and fast-wave perturbations in the vicinity of a magnetic X-type null point. The magnetic energy associated with an initially Gaussian field perturbation in a uniform resistive plasma is shown to decay algebraically at a rate that is unaffected by the Hall term to leading order in kδ where k is wavenumber and δ is ion skin depth. A similar decay law applies to whistler perturbations in the limit kδ>>>1. We demonstrate that in both geometries considered, the inclusion of the Hall term reduces the effectiveness of phase-mixing in plasma heating. The reduction in the damping rate in the uniform field (non-uniform density) cases, arising from dispersive effects, tends to zero in both the weak and strong phase mixing limits. In the Hall MHD X-point case, minimal reductions are seen for initially shear wave pulses, suggesting that little or no phase-mixing takes place. Nonlinear fast wave pulses which interact with the initial X-point destabilise the local field sufficiently to generate multiple null pairs; subsequent oscillatory current sheet behaviour appears unaffected by earlier differences between the MHD and Hall MHD cases.

Life-extension of EDF Energy's existing nuclear fleet is based on an assumption of continued safe operation. Potential fracture of graphite bricks in the nuclear reactor core of a power station represents an unknown variable in the equation. An understanding of the nature of this phenomenon and the impact on operation of the power station is desired. This work prepares the way for the future study of fracture in graphite bricks in a reactor core subject to dynamic excitation. Methodology to couple a multi-body finite element contact code to a crack propagation code is thus developed. Three important scientific contributions have been made: (i) An optimisation problem formulated on a smooth manifold to yield the rotation responsible for infinitesimal rigid body motion. This involves an iterative scheme in the form of Newton's method that takes into account the geometry of the underlying parameter space. There are no issues with singularities or additional computations in each iteration to scale the solution onto the manifold. (ii) An energy consistent crack initiation criterion for brittle material where nucleation is treated as a sudden and discrete rupture event at the macroscopic level. At the heart of the criterion is the finite difference form of the energy release rate; an expression for the characteristic length is derived and the change in total potential energy is obtained from an asymptotic argument involving the topological derivative. The criterion can predict crack onset at a sharp or blunt notch. Fracture toughness and material strength are the only input requirements. (iii) Algorithms related to the detection of sharp notches in a tetrahedral finite element mesh and a general computational procedure for evaluation of non-local crack initiation criteria. The only tool in the implementation of these algorithms is C++11. There is no need for a complex data structure storing all incidence information. Unordered associative containers in the standard library are exploited in the design of these rather efficient algorithms, which cover surface extraction and provide connectivity of the edges representing a sharp notch tip. A mesh re-generation routine for purposes of refinement at the sharp notch tips has also been developed.

To understand the physical mechanism underlying the cross-field transport of mass and energy in magnetoplasmas is a long-standing problem in fusion research. Woods (1987) has recently developed a second-order transport theory which has been used to explain a number of transport-related phenomena observed in tokamaks. Here, we apply second-order transport theory to the reverse field pinch (RFP) and a phenomenon observed in tokamaks known as 'snakes'. Expressions for the mass and energy confinement times in the RFP, τ_p and τ_e, are deduced and agreement with experimental results from HBTX is found. For typical operating conditions the times τ_p ~ 0.1ms and τ_e ~ 0.2ms are observed in HBTX. Second-order transport theory predicts τ_p ~ 0.4ms and τ_e ~ 0.4ms for this machine. Scaling laws for β_p versus η_e,β_p versus I_φ and τ_e versus I_φ are compared with measurements from HBTX and agree well with observation. Snakes are large density perturbations observed in JET after fuel pellet injection. Typical snakes in JET are remarkably stable and are found to have density decay times longer than predictions based on neoclassical theory (Stringer 1987). After their formation, snakes have even been observed to grow (Weller <i>et al.</i> 1987), which suggests the presence of an inward diffusion mechanism. There is also some evidence for a temperature depression in the snakes region. An explanation of the stability and energy balance in snakes based on second-order transport theory is proposed.

Disposal of the UK's legacy nuclear waste is the biggest challenge facing the industry at present. There is currently no long term storage facility in the UK and the inventory is continually growing. This project investigates the role that digital geoscientific data collection, analysis and modelling techniques play in the search for, and development of, a Geological Disposal Facility (GDF), critically analyses classical techniques and new, digital methodologies to assess what their impact would be on any site investigation. The Borrowdale Volcanic Group outcrop in Cumbria, NW England was chosen as it provides an analogue to a higher-strength crystalline basement setting for a GDF. Terrestrial lidar and photogrammetric surveys were conducted at four locations around the study area. These provided information on the fracture geostatistics which are the main fluid migration pathways in the subsurface in the BVG. The mechanics of deformation are identified by analysing the clustering of data points via digital stereonet analysis. The analysis shows the rocks sampled are highly fractured and their orientations and dips reflected the extensional tectonism experienced in the area. These are in the form of adjacent sets trending broadly NNE-SSW and NNW-SSE at very high angler dips (~70 degrees). A new workflow developed for this work demonstrates how a potential site's fracture statistics, and indeed the 3D geology, should be investigated as part of future GDF site investigations. Areas of complex geology such as the BVG present many difficulties in interpretation and analysis due to the poorly constrained polyphase nature of the deformation. These complexities make characterisation and modelling highly problematic, and as such, areas of simpler geology should be investigated first. Assessments which were based on early geological studies using traditional field data collection techniques underestimated the impact of heterogeneity on fluid flow migration modelling within the subsurface. This suggests that, should a GDF should be developed in such a geological setting, huge difficulties may be encountered. These will be associated with the development of performance assessments and safety cases which are typically based on geological models that should use such complex data. In addition to this, datasets collected using digital methods are a powerful visualisation tools for communication of complex geology, that can be utilised in stakeholder engagement activities that will form a key part of any GDF development process.

Les radiations sont omniprésentes. Elles ont de nombreuses applications dans des domaines variés: en médecine, elles permettent de réaliser des diagnostiques et de guérir des patients; en communication, tous les systèmes modernes utilisent des formes de rayonnements électromagnétiques; et en science, les chercheurs les utilisent pour découvrir la composition et la structure des matériaux, pour n'en nommer que quelques-unes. Concrètement, la radiation est un processus au cours duquel des particules ou des ondes voyagent à travers différents types de matériaux. La radiation peut être très énergétique, et aller jusqu'à casser les atomes de la matière ordinaire. Dans ce cas, on parlera de radiation ionisante. Il est communément admis que la radiation ionisante peut être bien plus nocif pour les êtres vivants que la radiation non ionisante. Dans cette dissertation, nous traiterons de la radiation ionisante. La radioactivité est le processus d'émission des radiations ionisantes. Elle existe sous forme naturelle, et est présente dans les sols, dans l'air et notre planète entière est bombardée en permanence de rayonnements cosmiques énergétiques. Depuis le début du XXe siècle, les chercheurs sont capables de créer artificiellement de la matière radioactive. Cette découverte a offert de multiples avancées technologiques, mais a eu également de lourdes conséquences pour l'humanité comme l'ont démontrés les évènements de Tchernobyl et de Fukushima ou d'autres accidents dans le monde médical. Cette dangerosité a conduit à l'élaboration d'un système de radioprotection. Dans la pratique, la radioprotection est principalement mise en œuvre en utilisant la méthode ALARA. Cette méthodologie consiste à justifier, optimiser et limiter les doses reçues. Elle est utilisée conjointement avec les limites légales. Le facteur d'optimisation est contraint par le fait que l'exposition volontaire d'un travailleur aux radiations lors d'une opération doit être plus bénéfique que si aucune intervention humaine n'était conduite dans une situation donnée. Dans le monde industriel et scientifique, il existe des infrastructures qui émettent des rayonnements ionisants. La plupart d'entre elles nécessitent des opérations de maintenance. Dans l'esprit du principe ALARA, ces interventions doivent être optimisées pour réduire l'exposition des travailleurs aux rayonnements ionisants. Cette optimisation ne peut pas être réalisée de manière automatique car la faisabilité des interventions nécessite dans tous les cas une évaluation humaine. La planification des interventions peut cependant être facilitée par des moyens techniques et scientifiques comme par exemple un outil informatique. Dans le contexte décrit ci-dessus, cette thèse regroupe des considérations techniques et scientifiques, et présente la méthodologie utilisée pour développer des outils logiciels pour la mise en œuvre de la radioprotection. / Radiation is omnipresent. It has many interesting applications: in medicine, where it allows curing and diagnosing patients; in communication, where modern communication systems make use of electromagnetic radiation; and in science, where it is used to discover the structure of materials; to name a few. Physically, radiation is a process in which particles or waves travel through any kind of material, usually air. Radiation can be very energetic, in which case it can break the atoms of ordinary matter (ionization). If this is the case, radiation is called ionizing. It is known that ionizing radiation can be far more harmful to living beings than non-ionizing radiation. In this dissertation, we are concerned with ionizing radiation. Naturally occurring ionizing radiation in the form of radioactivity is a most natural phenomenon. Almost everything is radioactive: there is radiation emerging from the soil, it is in the air, and the whole planet is constantly undergoing streams of energetic cosmic radiation. Since the beginning of the twentieth century, we are also able to artificially create radio-active matter. This has opened a lot of interesting technological opportunities, but has also given a tremendous responsibility to humanity, as the nuclear accidents in Chernobyl and Fukushima, and various accidents in the medical world have made clear. This has led to the elaboration of a radiological protection system. In practice, the radiological protection system is mostly implemented using a methodology that is indicated with the acronym ALARA: As Low As Reasonably Achievable. This methodology consists of justifying, optimizing and limiting the radiation dose received. This methodology is applied in conjunction with the legal limits. The word "reasonably" means that the optimization of radiation exposure has to be seen in context. The optimization is constrained by the fact that the positive effects of an operation might surpass the negative effects caused by the radiation. Several industrial and scientific procedures give rise to facilities with ionizing radiation. Most technical and scientific facilities also need maintenance operations. In the spirit of ALARA, these interventions need to be optimized in terms of the exposure of the maintenace workers to ionizing radiation. This optimization cannot be automated since the feasibility of the intervention tasks requires human assessment. The intervention planning could however be facilitated by technical-scientific means, e.g. software tools. In the context sketched above, this thesis provides technical-scientific considerations and the development of technical-scientific methodologies and software tools for the implementation of radiation protection.In particular, this thesis addresses the need for an interactive visual intervention planning tool in the context of high energy particle accelerator facilities.

In the UK, the debate surrounding energy production lies at the forefront of the political agenda, with growing emphasis on achieving an increasingly sustainable energy mix into the future. The nuclear option is especially debatable - issues such as waste management and decommissioning receive much attention. In addition, the many stakeholders interested in nuclear power display very divergent views on its sustainability. Since the turn of the century, nuclear power has received much attention globally, with many nations’ governments taking consideration of the potential benefits of new nuclear adoption. Conversely, the Fukushima nuclear disaster has led to new nuclear resistance in other nations, such as Germany, where plans have been made to stop nuclear power generation completely. This research aims to help inform the debate on nuclear power and the future UK electricity mix. A multi-criteria decision support framework (developed by the SPRIng Project) has been used for these purposes, taking into account technical, economic, environmental and social criteria.The methodology used in this work has involved: stakeholder consultation; use of future electricity scenarios; sustainability assessment of current and future electricity options (Pressurised Water Reactor, European Pressurised Reactor, European Fast Rector, coal, gas, solar and wind power, and coal carbon capture and storage [CCS] power); assessment of future electricity scenarios based on both sustainability impacts and stakeholder (expert and public) preferences for the sustainability indicators and electricity technologies. The sustainability assessment of future nuclear power options and coal CCS power have been carried out here for the first time in a UK-specific context.Based on the public and expert opinions on the importance of different sustainability indicators, results of the scenario analysis suggest that the scenario with a high penetration of low-carbon technologies (nuclear [60%] and offshore wind power [40%]) is the most sustainable. For the sample considered in this study, this finding is not sensitive to different stakeholder and public opinions on the importance of the sustainability indicators. However, when the stakeholder preferences for individual technologies are considered, scenarios with high penetration of renewables (26-40% solar and 20-48% wind) become the preferred options. This is due to the favourable stakeholder opinion on solar and wind power. In that case, the scenario with high penetration of nuclear is never the preferred option due to the low to moderate stakeholder preference for nuclear power.Therefore, the results from this research suggest that the ‘sustainability’ of different electricity options and scenarios is highly dependent on stakeholder preferences and priorities. Thus, for successful future deployment of these options and implementation of energy policy measures, transparency of information on the impacts of electricity options is key in ensuring that stakeholder opinions are founded in the actual rather than the perceived impacts of these options.

Geological disposal of radioactive wastes currently stored at Earth's surface is now the favoured management pathway for these materials. Typically, intermediate level wastes (ILW) are grouted and emplaced in a geological disposal facility (GDF) which will be backfilled, possibly with cementitious materials. Post-closure leaching of the cementitious materials in a GDF is expected to create hyperalkaline conditions in and around the repository, resulting in mineral alteration and crystallisation, both within the engineered barrier and host rock; creating a persistent chemically disturbed zone (CDZ). Iron derived from within the host rock as a result of alkaline breakdown of Fe-bearing silicate minerals (e.g. biotite, chlorite); corrosion products formed within the repository; or iron contained within the waste; will form secondary iron (oxyhydr)oxide minerals. The formation and re-crystallisation of these reactive mineral phases may sequester radionuclides through a host of processes: surface-mediated reduction to less soluble forms; adsorption onto, and/or incorporation into stable secondary or tertiary iron oxide phases. Therefore iron (oxyhydr)oxides will be key to the fate of radionuclides potentially released from within radioactive wastes disposed of in a GDF.In this study, the fate of U(VI) and Tc(VII) was considered during crystallisation of ferrihydrite to more stable iron oxide phases (e.g. hematite and magnetite) and, in three synthetic cement leachates (pH 13.1, 12.5, 10.5) designed to reflect the early-, middle- and late-stage evolution of the CDZ. XRD and SEM/TEM have been used to characterise the mineralogy during crystallisation. Partitioning of U(VI) and Tc(VII) between the solid and solution has been followed throughout, with chemical extractions used to determine the distribution of the radionuclides adsorbed to, and incorporated within the solid. Synchrotron-based XAS techniques have been utilised to probe the oxidation state and molecular scale bonding environment of the radionuclides associated with the solids. The data suggest that: U(VI) is incorporated into the hematite structure in place of Fe(III), in a distorted octahedral environment with elongation of the uranyl bond; Tc(VII) is reduced to Tc(IV) and incorporated into the octahedral site within the magnetite structure in place of Fe(III), and is retained in the same environment even after extensive oxidation of the magnetite to maghemite; and that U(VI) may also be incorporated as U(V) or U(VI) into the magnetite structure, with similar recalcitrant behaviour during oxidation. These results highlight the importance of mineral reactions within the CDZ as potentially significant pathways for immobilising radionuclides released from a GDF.

Adverse changes in the Earth's climate are thought to be due to the output of carbon dioxide from power stations. This has led to the development of many new materials to remove CO2 from these gas streams. Polymers of intrinsic microporosity (PIMs) are a novel class of polymers that are rigid with sites of contortion. These properties result in inefficient packing and so lead to large pore volumes and high surface areas. The inclusion of Tröger’s base, a contortion site made up of two nitrogen atoms, is thought to lead to increased uptake of CO2. The combination of electrostatic interactions with strong van der Waals forces should interact favourable with the quadrupole moment of CO2.Here a molecular simulation study of a selection of these polymers is presented. The study begins by developing a quick screening method on single polymer chains. This shows that the high surface area and adsorption affinity are a result of the contorted nature of PIMs along with the inclusion of groups such as Tröger’s base.The creation of atomistic models that reproduce the space packing ability of these polymers is also explored. Methods developed for PIMs in literature are investigated along with a new method developed during this study. GCMC simulations are then used to investigate the adsorption of CO2. In this study it is seen that that these polymers possess a well percolated network of both ultramicropores and supermicropores with a significant fraction of these pores being close to the kinetic diameter of CO 2. It is posited that these pores may be the result of the inclusion of Tröger’s base. It is also shown that this produces a particularly favourable site for adsorption. The phenomenon of swelling as a result of CO2 adsorption is also investigated using a variety of methods that make use of the output from the GCMC simulations. It was found that swelling is negligible for pressures of up to 1 bar. This result is important as swelling in the polymer can lead to a reduction in selectivity and an increase in permeability, which can affect the overall material’s performance.