Hello, A new project on the LCRC cluster has been requested. Please forward the information on to the LCRC Allocation sub-committee. Applicant's name: Zhigang Mei Applicant's institution: ANL Applicant's division: NE Project Name: DFT-defects-UO2 Project title: Nanoscale Modeling of Defects in Materials for Energy Applications Associated funding: ANL LDRD Other Systems: Science: The technological importance of actinide dioxides, AO2 (A=U, Pu or Np), largely originates from their application in nuclear reactor fuels. Today many reactor use UO2 as the primary fuel component. High-energy particle (self)irradiation in materials causes damage by creating point defects that can form defect structures such as small clusters, voids, and stacking faults, ultimately leading to material failure. The damage tolerance of a material depends not only on the number of defects created during irradiation but also on their long-time kinetic evolution. Therefore, understanding the evolution of highly interacting defects is very important. Experimentally is difficult to capture the irradiation-induced short time-scale atomistic process; computer simulations are particularly adept at capturing such dynamical process. Evaluating the concentration of point defects in actinide oxides requires a complex evaluation of the energy of formation of each defect type. The traditional mass action law is widely used to calculate the concentration of lattice defects. This formalism assumes that the defects do not interact, and that each defect occupies only one lattice site. However, complex defect structures can occupy multiple sites and the strong Coulomb and elastic repulsions may exclude other complexes from nearby sites. This is a source of great uncertainty in any statistical description, especially when the defect complexes are anisotropic. Roughly, the concentration of point defects increases with the temperature and the departure from stoichiometry, and decreases with pressure. Uranium oxide, UO2+x, which is the basis for most nuclear fuels used in Light Water Reactors, has a fluorite structure and hosts a large set of point defects to include oxygen vacancies, uranium vacancies, oxygen interstitials, as well as more complex defects such as small polarons or Willis defects. In UO2+x, uranium can exhibit valence changes from U4+ to U5+ leading to a wide range of departure from stoichiometry x, up to UO2.3. As first shown by neutron diffraction studies, this is mainly due to excess oxygen ions that are incorporated at interstitial sites and to a lesser degree to uranium vacancies. However, most current experimental and theoretical treatments of uranium oxide focus on isolated, non-interacting defects, which is consistent with small departures form stoichiometry, usually x < 0.1, and provides good descriptions of the thermochemistry of the oxide at low temperatures. The purpose of this project is to develop an understanding of defect formation in UO2+x at high defect concentrations and departures from stoichiometry, 0.1 < x < 0.3. Such an understanding will allow us to develop models of the free energy of the oxide that are valid at high temperatures, consistent with potential accident scenarios in the nuclear reactor. This study involves a comprehensive analysis of the oxide electronic and defects structure at nano-scale and using Density Functional Theory (DFT) and Ab-Initio Molecular Dynamics (AIMD). Since UO2+x is thermodynamically stable over a wide range of oxygen content in which the nature of the majority of defects may change greatly, we will examine the free energy of formation of a variety of defects types using DFT and AIMD. As opposite to previous studies, we will examine the particular cases of closely interacting defects and determine the energetics of these structures. For example, at large oxygen contents, where oxygen interstitial migration is preponderant, we will calculate the formation and activation energy for oxygen atoms diffusion as function of temperature and defect concentration. We will validate the advanced QM methods by predicting lattice constants, thermodynamics and other properties of compounds in the U-O phase diagram. Project description: To achieve the objectives of the study we will run standard quantum mechanical packages such as VASP and CP2K. These theoretical and computational tools have evolved to the point where calculations involving hybrid functionals can be carried out on high performance computers. We propose to apply these advanced quantum mechanical methods to model defects in compounds of non-stoichimetric uranium oxide, UO2+x, for the case of high defect concentrations, which is consistent with large departures from stoichiometry 0.1 < x ,0.2 . Since it is difficult to correlate the activation energy with defect formation or migration energies, we will calculate entropic contributions to the free energy of various interacting defects to evaluate properties that are suitable for experimental validation, such as oxygen self and chemical (tracer) diffusivity in UO2+x. The LCRC’s high-performance computing cluster Fusion provides an outstanding combination of cutting-edge computational power and the specific electronic structure calculation software needed for the current project. Due to the strongly correlated nature of U-5f electrons, more accurate methods, such as the LDA+U method, hybrid density functionals, the self-interaction corrected methodology (SIC) and dynamical mean field theory (DMFT) are required to overcome the limitations of conventional LDA/GGA method. These new methods require an order magnitude more of computational time to study the defects formation in UO2+x by DFT calculations. To go beyond point defects and study defect clustering and voids, we will use the AIMD capability of VASP and CP2K to study the formation and evolution of these defects in realistic time scale. Metadynamics is an accelerated molecular dynamics technique which generates a free energy surface for reaction pathways. It is an extremely powerful to technique to explore the mechanisms of chemical reactions. CP2K is a program to perform atomistic and molecular simulations of solid state, liquid, molecular, and biological systems. The Quickstep module of CP2K will used to perform accelerated ab initio molecular dynamics. Quickstep exploits linear scaling method to compute the total energy of the system containing 1000s of atoms and orbital transformation methods for very efficient Ab Initio Molecular Dynamics. We will calculate the structure and thermodynamics of UO2 to establish the reliability of U GTH pseudopotential and the hybrid functional. Based on these results, we will model oxygen frenkel pairs and Schottyky defects with the pseudopotential and functional as an additional check. Once the reference calculations are complete, we will calculate the free energy surface for the formation of the Oxygen Frenkel pair in UO2+x with metadynamics. These simulations will require approximately 100ps of ab initio molecular dynamics (physical time). For a unit cell containing 100 atoms this calculation should require 300,000 core-hours of CPU, based on benchmark calculations. The reference calculations will require a relatively trivial amount of CPU time. There will be three person involved in this projected. Dr. Marius Stan is a Senior Scientist in the Nuclear Engineering Division at ANL and a Senior Fellow of the Computational Institute at University of Chciago. Dr. John Low is a Computational Scientist in Mathematics and Computer Science division at ANL. Dr. Zhi-Gang Mei is a newly hired postdoc in the Nuclear Engineering Division at ANL. Project URL: Requested allocation: 400000 Q1: 100000 Q2: 100000 Q3: 100000 Q4: 100000 Justification: The requester has used undetermined amount hours of their initial startup project. In addition to approving an initial amount, please specify a Category and Subcategory for this project. For a list of the current selection of approved categories, please see: https://wiki.lcrc.anl.gov/wiki/Processes/Categories Once the Allocation committee has approved the project, please go to the Project Management page to create it: https://accounts.lcrc.anl.gov/projects.php Thank You, The LCRC Accounts System