Testing and Characterization of Creep Deformation in UO2 Single Crystals at 1200 °C

Thermo-mechanical behavior of oxide fuels is critical to understand Pellet-Cladding Mechanical Interactions (PCMI) that can lead to fuel fracture, which affects fuel performance significantly. In addition, the effects of fuel creep on PCMI can also play an important role during accident conditions and they need to be understood to formulate, calibrate and validate predictive, physics-based models of fuel behavior during PCMI behavior that account for multiscale phenomena. In this regard, high temperature mechanical behavior, including elastic anisotropy, dislocation driven plasticity, and creep deformation mechanisms can also play a significant role on the mechanical behavior of fuels. This work investigates high temperature creep behavior using ex-situ mechanical testing of uranium oxide samples, sintered under oxidative atmospheres to obtain large grained (multicrystalline) pellets having different oxygen stoichiometries. Green compacts of uranium oxide powders were sintered using an argon-20% oxygen atmosphere at 1700 °C. The resulting pellets were polished and characterized using Electron Backscatter Diffraction (EBSD), which revealed that the pellets had grains large enough that single crystals with various orientations could be extracted from them. These single crystals were cut from the pellets in the form of rectangular parallelepipeds, and the final crystallographic direction of the samples was also measured using EBSD. Uniaxial compression testing was performed at high temperatures under controlled atmospheres to control stoichiometry, i.e., argon-4% hydrogen was used to maintain samples as close as possible to an oxide to metal ratio (O/M) equal to 2, while argon mixed with oxygen was used to obtain O/M > 2. Tests were performed at 1200 °C using a dead load to keep the stress approximately constant. Step tests, whereby additional loads were added to increase the stress during the test, were also performed, as a way to obtain creep strain rate data that could be used to deduce a stress exponent for the samples for the two stoichiometries studied.  The measurements provided data to assist in the correlation of stoichiometry, crystallography, and mechanical behavior of oxide fuels. Samples recovered after the test were characterized using Scanning Electron Microscopy (SEM) and (EBSD), as well as electron channeling contrast imaging (ECCI). The slip traces observed with SEM, the lattice rotations measured with EBSD and the presence of dislocations shown by ECCI were correlated to the stress-strain behavior and the stoichiometry to gain insight into intragranular creep behavior of UO₂. The results were used to inform a simple viscoplastic model to describe creep at the slip system level for UO₂ that is consistent with the all the observations and measurements made.