This thesis presents the design of a condition monitoring scheme for the neutral beam cryogenic pumping system deployed in the Joint European Torus. The performance of the scheme is demonstrated by analysing its response to a range of fault scenarios. Condition monitoring has been successfully used in a diverse range of industries, from rail transport, to commercial power generation, to semiconductor manufacturing, among others. The application of model based condition monitoring to fusion applications has, however, been very limited. Given the importance of improving the availability of fusion devices, it was hypothesised that model based condition monitoring techniques could be used to good effect for this application. This provided the motivation for this research, which had the ultimate objective of demonstrating the usefulness of model based condition monitoring for fusion devices. The cryogenic pumping system used in the neutral beam heating devices operated by the project sponsor, the Culham Centre for Fusion Energy, was selected as the target for a demonstration condition monitoring scheme. This choice of target system was made and justified by the author through an analysis of its role in the neutral beam devices. The relative merits of several model based approaches were investigated. An observer based residual generation scheme, utilising a Kalman filter bank and residual thresholding arrangement was determined to be most suitable. A novel, accurate non-linear simulation model of the cryogenic pumping system was developed to act as a surrogate plant during the research, to facilitate the design and test procedure. This model was validated using historical process data. Two system identification techniques were used to obtain a set of linear models of the system for use in the Kalman filter bank. The scheme was tested by using the non-linear model to simulate ten different faults, all with unique failure modes. Two residual thresholding arrangements were tested and their performance was analysed to find the arrangement with the best performance. It was found that both variations of the scheme could detect all ten faults. The scheme using dual thresholds to check both the direction and magnitude of the residual signals was, however, better at isolating specific faults. The non-linear simulation model developed during the research was proven to be a genuine representation of the plant, by validating its response using historical process data. As such, it could be used in the future as the basis for a model based control system design procedure. The effectiveness of the scheme at detecting a range of faults which can arise in neutral beam heating systems supports the case for the future use of model based condition monitoring in nuclear fusion research.

EDF Energy own and operate seven Advanced Gas cooled Reactor (AGR) nuclear power plants which are the only commercially operated high temperature nuclear reactors in the world. Being high temperature means that considerable numbers of components operate within the creep regime and as a result, creep-fatigue is a life limiting degradation mechanism for many reactor components. Nuclear safety is the overriding priority for the operator, EDF Energy. Therefore demonstrating the integrity of structural components is an important activity. This poses the greatest challenge for components within the reactor pressure vessel, such as boilers, insulation and support structures, because they can not be easily inspected or repaired due to accessibility. Therefore there is reliance upon theoretical structural integrity assessments to demonstrate components are safe to operate, using procedures such as the RS assessment procedure. This research reviews the requirements placed upon high temperature structural integrity assessments and the current solutions provided by the deterministic assessment procedures, using a systems engineering approach. This identified clear disparities, including: deterministic structural integrity output versus a probabilistic safety requirement; lack of communication about the extent and nature of uncertainty in deterministic assessment results; plant observations (survival, failure and inspections) cannot be reconciled with deterministic assessment results; deterministic assessments consider components in isolation and do not consider the functionality of a structural system. These disparities are of growing concern when considering the aged AGR plants, as component failures become more likely and lifetime extensions are sought. To address these disparities, a physics-of-failure reliability framework has been proposed as a solution for creep-fatigue assessments. An important aspect of this paradigm shift, from deterministic to probabilistic assessments, was in the management of uncertainty. Two classifications of uncertainty were defined to do this; aleatory uncertainty which is caused by variation within a structural system and epistemic uncertainty which arises from a lack of understanding about the structural system. These two definitions can be applied to all variables and uncertainties allowing them to be managed appropriately within the structural integrity reliability framework. Uncertainties which are involved in a structural integrity analysis, including material properties, operating conditions and geometric properties were explored, classified and quantified where possible. The use of plant data, including survival and failure data and inspection data was also considered within the reliability framework.

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Efficient heat transfer technologies are essential for magnetically confined fusion reactors; this applies to both the current generation of experimental reactors as well as future power plants. A number of High Heat Flux devices have therefore been developed specifically for this application. One of the most promising candidates is the HyperVapotron, a water cooled device which relies on internal fins and boiling heat transfer to maximise the heat transfer capability. Over the past 30 years, numerous variations of the HyperVapotron have been built and tested at fusion research centres around the globe resulting in devices that can now sustain heat fluxes in the region of 20 – 30MW/m2 in steady state. Unfortunately, there have been few attempts to model or understand the internal heat transfer mechanisms responsible for this exceptional performance with the result that design improvements are traditionally sought experimentally which is both inefficient and costly. This thesis seeks to develop an engineering model of the HyperVapotron device using commercial Computational Fluid Dynamics software. To establish the most appropriate modelling choices, in-depth studies were performed examining the turbulence models (within the Reynolds Averaged Navier Stokes framework), near wall methods, grid resolution and boiling submodels. Validation of the models is accomplished via comparison with experimental results as well as high order Implicit Large Eddy Simulation methods. It is shown that single phase cavity flows and their related heat transfer characteristics (time-averaged) can be accurately captured if the SST k-omega turbulence model is employed using a fine nearwall grid throughout the cavity (e.g. y+ < 1 throughout). Separately, multiphase solutions with tuned wall boiling models also showed reasonable agreement with experimental data for vertical boiling tubes. As more complex multiphase HyperVapotron models were constructed, it became clear that there is an intrinsic incompatibility between the fine grids required for the single phase heat transfer predictions and the coarser grids plus wall functions required by the boiling model. Ultimately, the full 3D solution was based on the coarser grids as the fall off in accuracy in single phase heat transfer only becomes significant for HyperVapotron designs with deeper cavities. Since it is also shown here that deeper cavities are generally less efficient, these grid induced errors become less relevant if the primary objective is to find optimised performance.Comparing the CFD solutions with HyperVapotron experimental data suggests that a RANS-based, multiphase model is indeed capable of predicting performance over a wide range of geometries and boundary conditions. Whilst a definitive set of design improvements is not defined here, it is expected that the methodologies and tools developed will enable designers of future High Heat Flux devices to perform significant virtual prototyping before embarking on the more costly build and test programmes.

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Zirconium alloys are an important material in the nuclear industry, used as a fuel cladding material in water cooled reactors. However, aqueous corrosion of the alloy results in the production of hydrogen at the interface of the cladding with the water. Some of the hydrogen produced may diffuse through the passive oxide layer and be absorbed by the zirconium in solid solution. It has a deleterious effect on the mechanical properties of the cladding alloy, particularly if enough hydrogen is absorbed to cause precipitation of zirconium hydrides. There are multiple different hydride phases that may form, which have complex interactions with alloying additions, hydrogen concentration, stress and temperature. Of particular note is the phenomenon of Delayed Hydride Cracking (DHC), where the interaction of stress fields around crack-tips is believed to bring about the precipitation of hydrides, which promotes further growth of cracks. Overall, the embrittlement and cracking of the cladding by hydride related mechanisms represents a risk for reactor operators, which can be mitigated by a better understanding of the processes at work. This thesis presents an investigation into the mechanisms of hydride formation in zirconium alloys. In order to understand the mechanisms involved, a perspective of the system is has been sought using the ab-initio, quantum mechanical, atomistic simulation technique of Density Functional Theory (DFT). The first component of this study was an examination of the stability of different alloying additions in zirconium. The alloying additions of chromium, iron, nickel, niobium, tin, vanadium and yttrium have been examined, comparing the energy of them existing in a solid solution or in different inter-metallic structures. These intermetallic structures included multiple Laves phase structures, as well as a variety of other configurations. It was found that the thermodynamic driving forces in this system can be correlated with trends in atomic radii and the relative electronegativities of the different species. These same parameters also correlate with the formation energy differences between the different Laves phase polymorphs. Fe and Cr were found to prefer interstitial sites over substitutional locations in the Zr lattice. The Fe atoms had a similar energy of solution in both tetrahedral and octahedral sites, which may have implications for diffusion pathways. Formation energies of Fe, Ni and Sn based intermetallic compounds were found to be negative, and the Zr2Fe and Zr2Ni intermetallics were metastable. Most elements displayed negative energies of solution in beta-zirconium but positive energies in the alpha-phase, with the exception of Sn (which was negative for both) and Y (which was positive for both). Solutions formed from intermetallics showed a similar trend. Incorporation energies onto vacant sites in the Zr lattice were also investigated. It was found that all of the elements investigated showed a driving force to incorporate onto vacancies in alpha-Zr but some would not incorporate into beta-Zr. Different hydride structures were investigated, including the zeta, gamma, delta and epsilon-phases, and some speculative hydride structures. These were also compared with a large number of structures, which were generated with a random positioning of the H on appropriate sites in the Zr lattice. In alpha-Zr, inserting a H atom on the tetrahedral site had a more negative solution enthalpy than the octahedral site. This was also found to be true when the Zr was in a FCC lattice structure, and may have been related to the relative size of the sites. The gamma and epsilon-hydrides appear to be thermodynamically stable, while the delta hydride was very close in energy, but not quite favourable and thus, not quite stable. The zeta-phase, and speculative phases containing H atoms on octahedral sites were significantly less stable. The precipitation reaction was looked at in the context of variations in temperature and pressure. This was done by using phonon density of states data to calculated thermodynamic properties such as heat capacities and vibrational entropies. It was found that at any temperature, the concentration of the initial solid solution had to be at least 300 ppm before precipitation could become favourable. This implies local concentration of hydrogen atoms within a lattice must be significantly greater than the global concentration normally found in experimental results. Increases in temperature were found to drive the reaction towards solution, as would be expected from experimental results. The entropic components of the overall free energy are the main causes of this effect. Finally, the impact of pressure on hydride precipitation was examined, by applying a range of hydrostatic stresses to the alpha-zirconium lattice. Importantly, it was found that a tensile hydrostatic Zr lattice has a more favourable hydrogen solution enthalpy than a relaxed or compressed lattice. This implies that hydrogen may prefer to diffuse towards areas of the lattice which are under tension, in agreement with the Diffusion First Model of DHC. Different hydrides were placed under compressive and tensile stress, which demonstrated that the normally anisotropic stiffness of zirconium hydride lattice becomes isotropic for a stoichiometric ratio of around ZrH1.66.