The high temperature reactor (HTR) requires a completely new fuel design as it operates at around 1000°C in normal conditions and can reach up to 1600°C in case of an accident. The fuel and its cladding consist fully of ceramic materials, which precludes the possibility of a core meltdown and thus ensures inherent safety. The integral part of all HTR core designs is the tristructural-isotropic (TRISO) particle, which encapsulates the fissionable materials in succeeding coatings of pyrolytic carbon and silicon carbide (SiC). An exceptional mechanical integrity of the silicon carbide layer in all conditions is required to ensure full fission product retention. Within this work simulated TRISO fuel has been fabricated by fluidized bed chemical vapour deposition and was annealed in protective atmosphere up to 2200°C for short durations. Subsequent mechanical tests showed only minor reductions in the fracture strength of the SiC up to 2000°C. Substantial weight loss and crystal growth were observed after annealing at 2100°C and above. Raman spectroscopy identified the formation of a multi-layered graphene film covering the SiC grains after annealing and scanning electron microscopy revealed significant porosity formation within the coating from 1800°C onwards. These observations were attributed towards an evaporation-precipitation mechanism of SiC at very elevated temperatures that only slightly diminishes the hardness, elastic modulus or fracture strength, but might still be problematic in respect to fission product retention of the SiC layer. The new technique of high temperature nanoindentation was applied to measure the elastic modulus and hardness of SiC in-situ up to 500°C in argon atmosphere. The elastic modulus was found to be only slightly reduced over the measurement range, while the hardness showed a significant drop. Investigations of the deformation zone beneath the indenter tip executed by transmission electron microscopy showed slip and deformation twinning. On specimens that had been subject to neutron irradiation an irradiation hardening effect was noted. The elastic modulus showed only a minor increase compared with the non-irradiated samples. Oxidation experiments were carried out in air up to 1500°C. Analysis of the oxidation layer showed the formation of amorphous silica and cristobalite for the highest temperatures.

For direct cycle gas cooled high temperature reactor designs, operating conditions may be limited as a result of excessive maintenance dose rates caused by the 110mAg source term on the turbine. The accurate prediction of silver fission and activation products’ release during normal operation is required to ensure regulatory compliance and economic viability of planned power plants. Fuel qualification programs should provide satisfactory results to ensure correct analyses, but will however not be available for many years. In the meantime data from the German fuel development program may be utilized. Traditionally diffusion models were used to derive transport parameters from limited irradiation testing of fuel materials and components. Best estimates for all applicable German fuel irradiation tests with defendable uncertainty ranges were never derived. However, diffusion theory and current parameters cannot account for all irradiation and heat-up test results, and for some tests, it appears unacceptably conservative. Other transport mechanisms have been suggested and alternative calculation models are being considered. In this thesis the relevant German material and irradiation tests were evaluated with the current PBMR metallic fission product release calculation model. Transport through all the fuel materials and components and from the sphere to the coolant gas was considered and best possible models and parameters were suggested. For the transport of silver through the SiC layer an alternative suggested model called the Molecular Vapour Transport Release (MVR) Model was evaluated against the traditional diffusion model. From this evaluation it was shown that classical diffusion modelling was still a viable model to predict silver transport in SiC. The MVR model was found to be a feasible model as well. However, due to the much larger verification and validation effort required, it was decided to use the diffusion model until such time that experimental results become available that might elucidate the exact physical transport model. The evaluation also showed that the diffusion model used must be quantified in a detailed evaluation of all applicable irradiation tests. A study of all German irradiation tests was previously performed and the applicable irradiation tests were identified. A detailed evaluation of these irradiation tests were performed with an updated diffusion model. New transport and material parameters were derived in this detailed evaluation and compared with existing values. An evaluation of some heat-up tests of irradiated fuel spheres was performed to assess the range for which the newly derived transport parameters are valid. The different models with their old and newly derived parameters were used to analyse sample PBMR cores. Recommendations were made to the suitability of the different models and parameters for future PBMR silver fission and activation product analyses. / Thesis (DPhil)--University of Pretoria, 2009. / Physics / unrestricted