Report on simulation of fission gas and fission product diffusion in UO₂

Technical Report · Fri Jul 22 00:00:00 EDT 2016

DOI:https://doi.org/10.2172/1291258· OSTI ID:1291258

Andersson, Anders David ^[1]; Perriot, Romain Thibault ^[1]; Pastore, Giovanni ^[2]; Tonks, Michael R. ^[2]; Cooper, Michael William ^[1]; Liu, Xiang-Yang ^[1]; Goyal, Anuj ^[3]; Uberuaga, Blas P. ^[1]; Stanek, Christopher Richard ^[1]

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1. Los Alamos National Lab. (LANL), Los Alamos, NM (United States). Materials Science and Technology Division
2. Idaho National Lab. (INL), Idaho Falls, ID (United States). Fuel Modeling and Simulation Dept.
3. Univ. of Florida, Gainesville, FL (United States). Dept. of Materials Science and Engineering

In UO₂ nuclear fuel, the retention and release of fission gas atoms such as xenon (Xe) are important for nuclear fuel performance by, for example, reducing the fuel thermal conductivity, causing fuel swelling that leads to mechanical interaction with the clad, increasing the plenum pressure and reducing the fuel–clad gap thermal conductivity. We use multi-­scale simulations to determine fission gas diffusion mechanisms as well as the corresponding rates in UO₂ under both intrinsic and irradiation conditions. In addition to Xe and Kr, the fission products Zr, Ru, Ce, Y, La, Sr and Ba have been investigated. Density functional theory (DFT) calculations are used to study formation, binding and migration energies of small clusters of Xe atoms and vacancies. Empirical potential calculations enable us to determine the corresponding entropies and attempt frequencies for migration as well as investigate the properties of large clusters or small fission gas bubbles. A continuum reaction-­diffusion model is developed for Xe and point defects based on the mechanisms and rates obtained from atomistic simulations. Effective fission gas diffusivities are then obtained by solving this set of equations for different chemical and irradiation conditions using the MARMOT phase field code. The predictions are compared to available experimental data. The importance of the large Xe_U3O cluster (a Xe atom in a uranium + oxygen vacancy trap site with two bound uranium vacancies) is emphasized, which is a consequence of its high mobility and high binding energy. We find that the Xe_U3O cluster gives Xe diffusion coefficients that are higher for intrinsic conditions than under irradiation over a wide range of temperatures. Under irradiation the fast-­moving Xe_U3O cluster recombines quickly with irradiation-induced interstitial U ions, while this mechanism is less important for intrinsic conditions. The net result is higher concentration of the Xe_U3O cluster for intrinsic conditions than under irradiation. We speculate that differences in the irradiation conditions and their impact on the Xe_U3O cluster can explain the wide range of diffusivities reported in experimental studies. However, all vacancy-­mediated mechanisms underestimate the Xe diffusivity compared to the empirical radiation-­enhanced rate used in most fission gas release models. We investigate the possibility that diffusion of small fission gas bubbles or extended Xe-­vacancy clusters may give rise to the observed radiation-­enhanced diffusion coefficient. These studies highlight the importance of U divacancies and an octahedron coordination of uranium vacancies encompassing a Xe fission gas atom. The latter cluster can migrate via a multistep mechanism with a rather low effective barrier, which together with irradiation-induced clusters of uranium vacancies, gives rise to the irradiation-enhanced diffusion coefficient observed in experiments.

• Research Organization: Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA)
• Contributing Organization: Idaho National Lab. (INL), Idaho Falls, ID (United States); Univ. of Florida, Gainesville, FL (United States)
• DOE Contract Number: AC52-06NA25396
• OSTI ID: 1291258
• Report Number(s): LA-UR-15-28086; TRN: US1601702
• Country of Publication: United States
• Language: English

Related Subjects

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS
36 MATERIALS SCIENCE
VACANCIES
FISSION PRODUCTS
URANIUM DIOXIDE
URANIUM
DENSITY FUNCTIONAL METHOD
IRRADIATION
COMPARATIVE EVALUATIONS
ATOMS
COMPUTERIZED SIMULATION
XENON
URANIUM IONS
ENTROPY
OXYGEN
BINDING ENERGY
CONCENTRATION RATIO
BUBBLES
DENSITY
RADIONUCLIDE MIGRATION
FISSION PRODUCT RELEASE
DIFFUSION
MOBILITY
PERFORMANCE
POTENTIALS
RETENTION
TRAPS
INTERSTITIALS
PHYSICAL RADIATION EFFECTS