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Title: The high temperature mechanical properties of silicon carbide in TRISO particle fuel
Author: Rohbeck, Nadia
ISNI:       0000 0004 5993 8028
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2014
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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.
Supervisor: Xiao, Ping Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Silicon Carbide ; Nanoindentation ; TRISO Particle ; HTR fuel