![]() ![]() All compacts were subjected to a standard set of analyses that included the following: (1) detection of exposed fission products by Deconsolidation-Leach-Burn-Leach (DLBL), (2) measurement more » of gamma-emitting fission product inventory using the Irradiated Microsphere Gamma Analyzer (IMGA), and (3) microstructural examination by x-ray tomography and materialography. Three compacts (Compacts 6-1-1, 4-4-2, and 5-2-3) were examined in the as-irradiated condition and two compacts (Compacts 6-4-3 and 3-3-2) were subjected to safety testing at 1600☌, followed by post-safety testing examination similar to the PIE performed on the as-irradiated compacts. PIE has been completed on five compacts at the Oak Ridge National Laboratory (ORNL). Post-irradiation examination (PIE) is in progress on coated particle fuel compacts from the first Advanced Gas Reactor irradiation experiment (AGR-1). Furthermore, this work will enable more accurate fuel-performance simulations and will extend to new fuel types and operating conditions, all of which improve the fuel economics of nuclear energy and maintain high fuel reliability and safety. The model is validated by comparison to low-temperature experimental measurements on single-crystal hyperstoichiometric UO 2+x samples and high-temperature literature data. For each defect and fission product, scattering parameters are derived for application in both a Callaway model and the corresponding high-temperature model typically used in fuel-performance codes. Uranium defects reduce the thermal conductivity more than oxygen defects. High defect scattering is predicted for Xe atoms compared to that of La and Zr ions. ![]() Physical insights from the resonant phonon-spin-scattering mechanism due to spins on the magnetic uranium ions are introduced into the treatment of the MD results, with the corresponding relaxation time derived from existing experimental data. Potential parameters for UO 2+x and ZrO 2 are developed for the latter potential. These calculations employ a standard Buckingham-type interatomic potential and a potential that combines the many-body embedded-atom-method potential with Morse-Buckingham pair potentials. To generate a mechanistic thermal conductivity model, molecular dynamics (MD) simulations of UO 2 thermal conductivity including representative uranium and oxygen defects and fission products are carried out. This approach is able to represent the degradation of thermal conductivity due to each individual defect type, more » rather than the overall burn-up measure typically used, which is not an accurate representation of the chemical or microstructure state of the fuel that actually governs thermal conductivity and other properties. A primary source of uncertainty in these codes is thermal conductivity, and optimized fuel utilization may be possible if existing empirical models are replaced with models that incorporate explicit thermal-conductivity-degradation mechanisms during fuel burn up. The use of fuel performance codes by the industry to predict operational behavior is widespread. Uranium dioxide (UO 2) is the most commonly used fuel in light-water nuclear reactors and thermal conductivity controls the removal of heat produced by fission, thereby governing fuel temperature during normal and accident conditions.
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