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NKS Programme Area:NKS-R
Research Area:Reactor Physics
Report Number:NKS-464
Report Title:Micromechanical modeling of single crystal and polycrystalline UO2 at elevated temperatures
Activity Acronym:POMMI
Authors:Matti Lindroos, Napat Vajragupta, Janne Heikinheimo, Diogo Ribeiro Costa, Abishek Biswas, Tom Andersson, Pär Olsson,
Abstract:Understanding cracking phenomena in polycrystalline UO2 fuel pellet requires detailed knowledge of the fuel's local microscopic stresses and operational conditions. High stress states emerge in the pellet due to a deep parabolic temperature gradient between the center and cylindrical edges of the pellet. In normal operating conditions, the outer regions of the fuel pellet are brittle and experience tensile stresses. On the other hand, the inner parts of the pellet experience compressive stresses, and the behavior is increasingly plastic. Macroscopic stress in the pellet can cause diverging local stresses and microcracking due to e.g. strain incompatibilities between the grains and dislocation pile-ups. Also, porosity, localization of pores in grains and at grain boundaries, their size, and shape substantially impact UO2 cracking behavior. This study proposes a micromechanical modeling workflow to study how pores affect the plasticity of polycrystalline UO2. SEM-EBSD was performed on standard UO2 pellets with dense microstructure and round pores at inter and intra-granular locations. The characterized microstructure was meshed and coupled with a dislocation-based crystal plasticity model to simulate the uniaxial compression behavior at 1600 K. To this end, we propose the constitutive model that potentially can describe the deformation of a single crystal and polycrystalline UO2. We have also developed the inverse optimization method to identify material parameters providing a unique solution at different length scales. With the element removal technique, we have assessed the influence of pores size, volume fraction, and their distribution in microstructures on the mechanical response of polycrystalline UO2.
Keywords:Crystal plasticity, Uranium dioxide, Mechanical behavior, Porosity, Strain localization, SEM-EBSD
Publication date:21 Nov 2022
Number of downloads:453
Download:pdf 111010214698247.pdf
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