Elevated Temperature Nanoindentation Creep Study of Plastically Deformed and Spark Plasma Sintered UO₂

Current challenges with uranium dioxide fuel degradation include fuel swelling, cracking and fission gas release, all of which reduce the operational life span of the fuel in commercial reactors. One important factor in prolonging the life span of uranium dioxide (UO₂) fuel is developing a good understanding of the mechanical properties of the high burnup structure that forms at the periphery of the fuel pellet during the fuel cycle. Nanoindentation based testing techniques can probe the mechanical properties of small volumes of material and measure properties including hardness, elastic modulus, and creep. In this work, elevated temperature nanoindentation and nanoindentation creep testing is performed on UO₂ samples with different microstructures, including nanocrystalline (NC) grains, as well as microcrystalline samples with different grain sizes and creep prestrains introduced at high temperatures. Tests allowed measuring hardness, elastic modulus and creep stress exponents up to 500°C. The test results indicate that for temperatures and stresses used here, NC UO₂ had limited to no creep, with stress exponents greater than 10. For UO₂ samples with creep prestrain and spark plasma sintered (SPS) UO₂ with a 1.8 μm grain size the dominant creep mechanism at the high stresses and relatively low temperatures used is dislocation glide: the creep stress exponents of 3-4 at 500°C also matched well with recent literature values for macro scale compression creep tests. In addition, it was observed that the UO₂ samples containing higher dislocation defect densities had lower creep stress exponents than conventional sintered UO_2. The stress exponent measured on the conventional sintered UO₂ at 500°C was ~ 7, which suggests that the deformation of UO₂ at lower temperatures might be hindered by the energetic barriers to dislocation nucleation.