Metakaolin is a pozzolanic material with the potential to reduce permeability and chloride ingress; however, quantification of the effects of metakaolin use on the cracking sensitivity of concrete mixtures is needed to ensure that these improvements in performance are not compromised. This study was conducted to investigate the early age cracking potential due to restraint stresses from incorporating metakaolin in concrete. Calorimetry testing showed that metakaolin was more sensitive to temperature than mixtures with only Portland cement. Results showed more shrinkage, less stress relaxation, and higher restraint stress from the inclusion of metakaolin, potentially increasing cracking sensitivity of mixtures. 1 This section was published in Construction and Building Materials[57]. Permission is included in Appendix A

Social and environmental justice for a growing and developing global population requires significant increases in energy use. A possible means of contributing to this energy increase is to incinerate plutonium from spent fuel of pressurised light water reactors (Pu(PWR)) in high-temperature reactors such as the Pebble Bed Modular Reactor Demonstration Power Plant 400 MWth (PBMR-DPP-400). Previous studies showed that at low temperatures a 3 g Pu(PWR) loading per fuel sphere or less had a positive uniform temperature reactivity coefficient (UTC) in a PBMR DPP-400. The licensing of this fuel design is consequently unlikely. In the present study it was shown by diffusion simulations of the neutronics, using VSOP-99/05, that there is a fuel design containing thorium and plutonium that achieves a negative maximum UTC. Further, a fuel design containing 12 g Pu(PWR) loading per fuel sphere achieved a negative maximum UTC as well as the other PBMR (Ltd.) safety limits of maximum power per fuel sphere, fast fluence and maximum temperatures. It is proposed that the low average thermal neutron flux, caused by reduced moderation and increased absorption of thermal neutrons due to the higher plutonium loading, is responsible for these effects. However, to fully understand the mechanisms involved a detailed quantitative analysis of the roll of each factor is required. A 12 g Pu(PWR) loading per fuel sphere analysis shows a burn-up of 180.7 GWd/tHM which is approximately double the proposed PBMR (Ltd.) low enriched uranium fuel burn-up. The spent fuel has only a decrease of 24.5 % in the Pu content which is sub-optimal with respect to proliferation and waste disposal objectives. Incinerating Pu(PWR) in the PBMR-DPP 400 MWth is potentially licensable and economically feasible and should be considered for application by industry. / MIng (Nuclear Engineering), North-West University, Potchefstroom Campus, 2012

Social and environmental justice for a growing and developing global population requires significant increases in energy use. A possible means of contributing to this energy increase is to incinerate plutonium from spent fuel of pressurised light water reactors (Pu(PWR)) in high-temperature reactors such as the Pebble Bed Modular Reactor Demonstration Power Plant 400 MWth (PBMR-DPP-400). Previous studies showed that at low temperatures a 3 g Pu(PWR) loading per fuel sphere or less had a positive uniform temperature reactivity coefficient (UTC) in a PBMR DPP-400. The licensing of this fuel design is consequently unlikely. In the present study it was shown by diffusion simulations of the neutronics, using VSOP-99/05, that there is a fuel design containing thorium and plutonium that achieves a negative maximum UTC. Further, a fuel design containing 12 g Pu(PWR) loading per fuel sphere achieved a negative maximum UTC as well as the other PBMR (Ltd.) safety limits of maximum power per fuel sphere, fast fluence and maximum temperatures. It is proposed that the low average thermal neutron flux, caused by reduced moderation and increased absorption of thermal neutrons due to the higher plutonium loading, is responsible for these effects. However, to fully understand the mechanisms involved a detailed quantitative analysis of the roll of each factor is required. A 12 g Pu(PWR) loading per fuel sphere analysis shows a burn-up of 180.7 GWd/tHM which is approximately double the proposed PBMR (Ltd.) low enriched uranium fuel burn-up. The spent fuel has only a decrease of 24.5 % in the Pu content which is sub-optimal with respect to proliferation and waste disposal objectives. Incinerating Pu(PWR) in the PBMR-DPP 400 MWth is potentially licensable and economically feasible and should be considered for application by industry. / MIng (Nuclear Engineering), North-West University, Potchefstroom Campus, 2012

In this thesis advanced fuel cycles for the incineration, i.e. deep–burn, of weapons–grade plutonium, reactor–grade plutonium from pressurised light water reactors and reactor–grade plutonium + the associated Minor Actinides in the 400 MWth Pebble Bed Modular Reactor Demonstration Power Plant was simulated with the VSOP 99/05 diffusion code. These results were also compared to the standard 9 g/fuel sphere U/Pu 9.6% enriched uranium fuel cycle. The addition of the Minor Actinides to the reactor–grade plutonium caused an unacceptable decrease in the burn–up and thus an unacceptable increase in the heavy metal (HM) content in the spent fuel, which is intended for direct disposal in a deep geological repository, without chemical reprocessing. All the Pu fuel cycles failed the adopted safety limits in that either the maximum fuel temperature of 1130°C, during normal operation, or the maximum power of 4.5 kW/sphere was exceeded. All the Pu cycles also produced positive Uniform Temperature Reactivity Coefficients, i.e. the coefficient where the temperature of the fuel and the graphite moderator in the fuel spheres are varied together. these positive temperature coefficients were experienced at low temperatures, typically below 700°C. This was due to the influence of the thermal fission resonance of 241Pu. The safety performance of the weapons–grade plutonium was the worst. The safety performance of the reactor–grade plutonium also deteriorated when the heavy metal loading was reduced from 3 g/sphere to 2 g or 1 g. In view of these safety problems, these Pu fuel cycles were judged to be not licensable in the PBMR DPP–400 reactor. Therefore a redesign of the fuel cycle for reactor–grade plutonium, the power conversion system and the reactor geometry was proposed in order to solve these problems. The main elements of these proposals are: v 1. The use of 3 g reactor–grade plutonium fuel spheres should be the point of departure. 232Th will then be added in order to restore negative Uniform Temperature Reactivity Coefficients. 2. The introduction of neutron poisons into the reflectors, in order to suppress the power density peaks and thus the temperature peaks. 3. In order to counter the reduction in burn–up by this introduction of neutron poisons, a thinning of the central reflector was proposed. / Thesis (PhD (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2012.

In this thesis advanced fuel cycles for the incineration, i.e. deep–burn, of weapons–grade plutonium, reactor–grade plutonium from pressurised light water reactors and reactor–grade plutonium + the associated Minor Actinides in the 400 MWth Pebble Bed Modular Reactor Demonstration Power Plant was simulated with the VSOP 99/05 diffusion code. These results were also compared to the standard 9 g/fuel sphere U/Pu 9.6% enriched uranium fuel cycle. The addition of the Minor Actinides to the reactor–grade plutonium caused an unacceptable decrease in the burn–up and thus an unacceptable increase in the heavy metal (HM) content in the spent fuel, which is intended for direct disposal in a deep geological repository, without chemical reprocessing. All the Pu fuel cycles failed the adopted safety limits in that either the maximum fuel temperature of 1130°C, during normal operation, or the maximum power of 4.5 kW/sphere was exceeded. All the Pu cycles also produced positive Uniform Temperature Reactivity Coefficients, i.e. the coefficient where the temperature of the fuel and the graphite moderator in the fuel spheres are varied together. these positive temperature coefficients were experienced at low temperatures, typically below 700°C. This was due to the influence of the thermal fission resonance of 241Pu. The safety performance of the weapons–grade plutonium was the worst. The safety performance of the reactor–grade plutonium also deteriorated when the heavy metal loading was reduced from 3 g/sphere to 2 g or 1 g. In view of these safety problems, these Pu fuel cycles were judged to be not licensable in the PBMR DPP–400 reactor. Therefore a redesign of the fuel cycle for reactor–grade plutonium, the power conversion system and the reactor geometry was proposed in order to solve these problems. The main elements of these proposals are: v 1. The use of 3 g reactor–grade plutonium fuel spheres should be the point of departure. 232Th will then be added in order to restore negative Uniform Temperature Reactivity Coefficients. 2. The introduction of neutron poisons into the reflectors, in order to suppress the power density peaks and thus the temperature peaks. 3. In order to counter the reduction in burn–up by this introduction of neutron poisons, a thinning of the central reflector was proposed. / Thesis (PhD (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2012.

Afin d’accroitre le rendement des turbomachines les motoristes aéronautiques visent à développer de nouveaux matériaux réfractaires permettant d’augmenter les températures de service des parties les plus chaudes. Les travaux présentés dans ce mémoire s’inscrivent dans ce contexte et ont pour cadre le projet européen FP7-HYSOP dans lequel les alliages Nbss-Nb5Si3 sont à l’étude. Alors que d’un point de vue mécanique leurs propriétés sont compatibles avec leur industrialisation, leur résistance à l’oxydation dès les températures intermédiaires (800°C) constitue l’obstacle majeur à leur application. Deux voies sont suivies dans ce travail pour améliorer ce comportement. La première vise à évaluer l’effet d’un affinement de microstructure en synthétisant l’alliage Nbss-Nb5Si3 soit par la voie fusion, soit par métallurgie des poudres. Il a été montré que les microstructures les plus fines permettent de diminuer les cinétiques d’oxydations à 1100°C, et de s’affranchir des phénomènes d’oxydation catastrophique dont souffrent les alliages à microstructure grossière à 815°C. Des modèles diffusionnels gouvernant l’oxydation de ces matériaux ont pu être proposés. La seconde partie se focalise sur le développement de revêtements à base de siliciures, par la technique de cémentation activée en caisse, dont le caractère protecteur est assuré principalement par la formation d’une couche de silice (SiO2). Une série de tests d’oxydation/corrosion, représentative des conditions extrêmes régnant en sortie de chambre de combustion (isotherme ou cyclique, sous air, air + H2O, mélange silicaté CMAS), a permis d’évaluer et de hiérarchiser les performances des différents systèmes revêtus / Progress in the field of gas-Turbine engines for aircrafts is controlled by the availability of new structural materials able to withstand higher temperatures than nickel based superalloys. The present PhD work was conducted in this context, within the framework of the European FP7-HYSOP project, in which Nbss-Nb5Si3 alloys are studied. From a mechanical point of view, the physicochemical properties (room temperature fracture toughness and creep rate) are compatible with the targeted temperature (1300°C). However, starting from the intermediate temperatures (800°C), the oxidation resistance of those alloys is the major obstacle to their use. In this work, two ways are investigated to improve this behavior: The first one aims at studying the effect of a refinement of microstructure synthesizing the Nbss-Nb5Si3 alloy both by fusion method and powder metallurgy route. It was shown that refining the microstructure led to decrease the oxidation kinetics at 1100°C and suppressed the catastrophic breakaway oxidation (pesting) typical of the Nb-Si alloys with coarse microstructure at 815°C. Furthermore, diffusion models were proposed to describe oxidation kinetics both at 815°C and 1100°C. The second part of this study is devoted to the development of silica forming protective coatings. Diffusion silicide coatings were manufactured by the halide activated pack-Cementation method. The various conditions of oxidation/corrosion tests (isothermal or cyclic, in air, air containing water vapor, CMAS silicate melt) allowed assessing and ranking the performance of coated systems