b'Figure 3. Comparison of the stressthe radial porosity profile, showeddata from fresh and pre-strained fresh exponents measured on the irradiatedin Figure 1. Electron Backscatteredmaterial. The results highlighted that fuel with those measured on fresh UO 2 Diffraction (EBSD) orientation mapsthe creep deformation mechanism of with different microstructures. (left) The center region of the pellet at 300C,were collected using the EDAX Hikariirradiated material is very different the periphery at (middle and right,Super EBSD detector of the PFIB andfrom the fresh material and similar respectively) 300C and 500C analyzed to study grain subdivisionto the pre-strained material. At 300C across the pellet radius. Three majorthe stress exponent for both the zones were identified: (1) a centralpre-strained sample and the irradiated area till r/r00.55 characterized byUO 2would suggest that the materials The currentgrain subdivision by polygonizationare deforming by dislocation glide studies providedand porosity between 2% to 4%,in the material, while at 500C the a comprehensive(2) an intermediate region from r/ exponent is indicative of deforma-description of ther00.55 till r/r00.750.80 withtion by grain boundary sliding. The irradiation-inducedlow porosity and absence of grainresults highlight important effect of modifications occurringsubdivision and (3) the rim regionirradiation defects (i.e., dislocation) in high burnup UO 2 with the exponential increase ofin determining the deformation fuel and are inputsporosity and grain subdivision typicalbehavior of irradiated material. to any integral andof the high burnup structures (HBS)Finally, local thermal properties have semi-integral resultsformation, both in transition andbeen measured using the Thermal interpretation, as well asfully developed. Examples of the grainConductivity Microscope, a new source for meso-scalestructure are shown in Figure 2. Antechnique developed at INL which models to describeAlemnis standard assembly equippedprovides both thermal diffusivity and performance of highwith the high-temperature modulethermal conductivity measurements burnup fuels. was used for the elevated temperaturewith micrometric resolution. Figure 4 nanoindentation and nanoindentationshows the radial profile of the thermal creep experiments on the sample. Thediffusivity as a function of the fuel nanoindentation creep measurementsradius. The data, in combination with were performed at 300C and 500C.the microstructural characterization The results of the nanoindentationcould be used to validate microstruc-creep tests are shown in Figure 3,ture-based meso-scale models of where the stress exponents obtainedthermal properties, which are critical for the irradiated fuel are compared toto correctly predict fuel temperatures56 2021|AFC ACCOMPLISHMENTS'