Summary report on UO₂ thermal conductivity model refinement and assessment studies

Technical Report · Fri Feb 03 04:00:00 EST 2017

DOI:https://doi.org/10.2172/1342869· OSTI ID:1342869

Liu, Xiang-Yang ^[1]; Cooper, Michael William Donald ^[1]; Mcclellan, Kenneth James ^[1]; Lashley, Jason Charles ^[1]; Byler, Darrin David ^[1]; Bell, B. D.C. ^[2]; Grimes, R. W. ^[2]; Stanek, Christopher Richard ^[1]; Andersson, David Ragnar ^[1]

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1. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
2. Imperial College, London (United Kingdom)

Uranium dioxide (UO₂) is the most commonly used fuel in light water nuclear reactors and thermal conductivity controls the removal of heat produced by fission, therefore, governing fuel temperature during normal and accident conditions. The use of fuel performance codes by the industry to predict operational behavior is widespread. A primary source of uncertainty in these codes is thermal conductivity, and optimized fuel utilization may be possible if existing empirical models were replaced with models that incorporate explicit thermal conductivity degradation mechanisms during fuel burn-up. This approach is able to represent the degradation of thermal conductivity due to each individual defect type, 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. To generate a mechanistic thermal conductivity model, molecular dynamics (MD) simulations of UO₂ thermal conductivity including representative uranium and oxygen defects and fission products are carried out. 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. Potential parameters for UO_2+x and ZrO₂ are developed for the latter potential. Physical insights from the resonant phonon-spin scattering mechanism due to spins on the magnetic uranium ions have been introduced into the treatment of the MD results, with the corresponding relaxation time derived from existing experimental data. High defect scattering is predicted for Xe atoms compared to that of La and Zr ions. Uranium defects reduce the thermal conductivity more than oxygen defects. 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. The model is validated by comparison to low-temperature experimental measurements on single crystal hyper-stoichiometric UO_2+x samples and high-temperature literature data. Ongoing works include investigation of the effect of phase separation to UO₂+U₄O₉ on the low temperature thermal conductivity of UO_2+x, and modeling of thermal conductivity using the Green-Kubo method. Ultimately, this work will enable more accurate fuel performance simulations as well as extension to new fuel types and operating conditions, all of which improve the fuel economics of nuclear energy and maintain high fuel reliability and safety.

• Research Organization: Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
• Sponsoring Organization: USDOE Office of Nuclear Energy (NE)
• DOE Contract Number: AC52-06NA25396
• OSTI ID: 1342869
• Report Number(s): LA-UR--16-28212
• Country of Publication: United States
• Language: English

Related Subjects

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS
21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS