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Determination of basic structure-property relations for processing and modeling of advanced nuclear fuels: Microstructural evolution and multi-scale characterization

Posted on:2011-08-17Degree:Ph.DType:Dissertation
University:Arizona State UniversityCandidate:Wheeler, KirkFull Text:PDF
GTID:1461390011971749Subject:Engineering
Abstract/Summary:PDF Full Text Request
This study was conducted to understand the mechanisms of microstructural evolution, deformation and fracture in nitride fuels by performing careful experimental evaluation in surrogate materials, particularly in the case of inert matrix fuels where less experimental characterization is available. Emphasis was placed on structure-property relations of both inert matrix materials as well as solid solutions of surrogate materials to simulate the behavior of inert matrix fuels and identify the mechanisms of deformation and fracture and their relationship to microstructure and its evolution.;Studies of the microstructure of zirconium nitride sintered under different conditions indicate that using an argon atmosphere leads to faster, more complete sintering and improved grain growth kinetics compared to those obtained when using a nitrogen environment due to an increased concentration of nitrogen vacancies. Samples with three distinct microstructures were selected for mechanical testing based on their applicability in a transmutation fuel.;Uniaxial compression tests indicate increasingly non-linear stress-strain behavior with increasing testing temperature suggesting improved damage tolerance at elevated temperatures, which may help reduce the severity of fuel cracking in service due to thermal gradients or pellet-cladding interactions. In most cases, the ultimate compression strength dropped ∼50% for tests at elevated temperatures with more than double the failure strain for 800°C tests and nearly five times the failure strain for 1200°C test. The high failure strain observed in 1200°C tests was due to high strain-rate (10 -2/s) dislocation creep attributed to the sliding of sub-granular dislocation cells.;Plasticity at room temperature was evaluated using nanoindentation performed on either monolithic or sintered zirconium nitride. Experiments showed that repeated polishing with colloidal silica increased the critical load required to initiate plastic deformation. The sudden avalanche of dislocations responsible for observed displacement jumps is linked to low dislocation density near the indenter tip.;The anisotropic elastic constants of sintered zirconium nitride were determined experimentally using the generalized Hooke's law and the Young's modulus measured using nanoindentation on grains with different crystallographic orientations. It was found that most of the elastic constants found in the literature do not represent the experimentally observed behavior of ZrN. As a result of this study, the material properties used in fuel performance models can be based on the properties of similar materials developed using processing conditions consistent with fuels technology...
Keywords/Search Tags:Fuels, Evolution, Using, Nitride, Materials
PDF Full Text Request
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