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 degrees 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 Am 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 degrees 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 UO2. The stress exponent measured on the conventional sintered UO2 at 500 degrees C was 7, which suggests that the deformation of UO2 at lower temperatures might be hindered by the energetic barriers to dislocation nucleation. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

The current challenges with uranium dioxide fuel degradation, specifically fuel swelling, cracking, and fission gas release, reduce the operational lifespan of commercial reactors. To prolong the lifespan of UO2 fuel, it is important to understand the mechanical properties of the high burnup structure that forms during the fuel cycle. Testing techniques like nanoindentation can be used to measure properties such as hardness, elastic modulus, and creep in UO2 samples with different microstructures. Test results show that different microstructures exhibit varying creep mechanisms, with nanocrystalline UO2 showing limited to no creep and samples with higher dislocation defect densities having lower creep stress exponents.