NuFuel & MMSNF 2015

First Workshop on Research into Nuclear Fuel in Europe
and Materials Modeling and Simulation for Nuclear Fuels Workshop
Karlsruhe, Germany, November 16th to 18th, 2015

Updated: Tue 08 Dec 2015, 14:27

Talk 5.4: Simulation of fission gas diffusion in UO2 nuclear fuel

David Andersson, Xiang-Yang Liu, Romain Perriot, Giovanni Pastore, Michael R. Tonks, Blas P. Uberuaga and Christopher R. Stanek
  • Los Alamos National Laboratory, Los Alamos, NM USA

Abstract

In UO2 nuclear fuel, the retention and release of fission gas atoms such as xenon (Xe) are important for nuclear fuel performance. We use multiscale simulations to determine fission gas diffusion mechanisms as well as the corresponding rates in UO2 under both intrinsic and irradiation conditions [1,2]. Density functional theory (DFT) calculations are used to study formation, binding and migration energies of small clusters of Xe 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[2]. The predictions are compared to available experimental data. The importance of the large XeU3O 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 XeU3O cluster gives Xe diffusivities that are higher for intrinsic conditions than under irradiation over a wide range of temperatures. Under irradiation the fast-moving XeU3O 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 XeU3O cluster for intrinsic conditions than under irradiation. We speculate that differences in the irradiation conditions and their impact on the XeU3O 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 model 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 diffusivity. These studies highlight the importance of U divacancies, surface termination and non-stoichiometry for the cluster migration properties. Finally, diffusion of Xe is strongly impacted by the temperature distribution in the fuel rod and, in this context, studies of the contribution from phonon-spin scattering [3] as well as other mechanisms to the reduction of UO2 thermal conductivity will be briefly discussed.

References:
  1. D. A. Andersson, P. Garcia, X.-Y. Liu, G. Pastore, M. Tonks, P. Millett, B. Dorado, D. R. Gaston, D. Andrs, R. L. Williamson, R. C. Martineau, B. P. Uberuaga, C. R. Stanek, “Atomistic modeling of intrinsic and radiation-enhanced fission gas (Xe) diffusion in UO2±x: Implications for nuclear fuel performance modelling”, J. Nucl. Mater. 451, 225 (2014).
  2. D. A. Andersson, M. R. Tonks, L. Casillas, S. Vyas, P. Nerikar, B. P. Uberuaga, C. R. Stanek, ”Multiscale simulation of xenon diffusion and grain boundary segregation in UO2”, J. Nucl. Mater. 462, 15 (2015).
  3. K. Gofryk, S. Du, C. R. Stanek, J. C. Lashley, X.-Y. Liu, R. K. Schulze, J. L. Smith, D. J. Safarik, D. D. Byler, K. J. McClellan, B. P. Uberuaga, B. L. Scott and D. A. Andersson, “Anisotropic thermal conductivity in uranium dioxide”, Nature Comm. 5, 4551 (2014).