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: Bubble_U-X Project title: Multiscale modeling of fission gas diffusion and bubble formation in metallic fuel U-X (X=Zr, Mo) Associated funding: LDRD Other Systems: Carbon at CNM: 370000 core-hours Fusion at LCRC: 270000 core-hours Clusters at NE division: no time limit Science: As a key component for nuclear reactor, the development of advance nuclear is a continuous effort. Compared with oxide (UO2) fuels, metallic fuels have the advantage of much higher heat conductivity, and have the potential for the highest fissile atom density. Argonne has a long history in developing and testing new metallic fuels such as U-Zr based fuel alloys for fast reactors and U-Mo fuels for research reactors. The swelling of metallic fuel is known to be severe compared with oxide fuels, and understanding and minimizing the swelling presents multiple challenges. The formation of gas bubbles from fission product (Xe, Kr) and decay product (He) is the main cause of the swelling. The formation of gas bubbles not only impacts the microstructure of the fuel, but also leads to degradation of its thermal and mechanical properties. To fully understand and accurately predict the microstructure evolution under irradiation, it is very important to study t he diffusivity of fission gas in the fuel and elucidate the underlying formation mechanism of the fission gas bubbles. Computational modeling of gas bubble nucleation and growth is useful for understanding and ultimately developing strategies to reduce the swelling of the fuel. Due to the extremely low solubility of fission product (Xe, Kr) in the metallic fuel, the stable nucleus of the bubble is very small. The formation energy and binding energy of small size clusters can be investigated by first-principles methods. Then, based on the calculated binding energy of small size clusters, rate theory (cluster dynamics) or kinetic Monte Carlo (kMC) method can be employed to reveal the behavior of bubble nucleation and growth. A multiscale method, including Density Functional Theory (DFT), Molecular Dynamics (MD), and kinetic Monte Carlo will be used to analyze the experimental data, develop material property models, and predict the material behavior in extreme conditions. These studies will provide unique opportunities for model validation and simulation benchmarking as well as correlating modifications in materials induced by neutron and ion irradiation. Project description: In this project, different scale simulation method will be used, including DFT, MD, and kMC computational methods to study the diffusion behaviors of fission gas (Xe, Kr) and the formation of gas bubbles in in metallic fuel U-X (X=Mo, Zr). Computationally efficient codes will be used in this study, such as VASP for DFT calculations, LAMMPS for MD simulations and SPPARKS for kMC simulations. U alloys exhibit several allotropes. At low temperature, the dominant phase in the outer zone of U metallic fuel is α phase with body-center-cubic (bcc) structure. Therefore, we will focus on α phase of U-X alloys. For DFT calculations, VASP can scale well up to a few hundred of atoms. Such size of supercell will be enough to investigate the formation energy and migration barrier of vacancies, interstitials and fission gas in the fuel. A typical supercell 7x3x3 with 252 atoms will be used for the calculation of defect formation energy and migration barrier. Nudg ed elastic band (NEB) method implemented in VASP will be adopted to predict the migration barriers of all the defects. Meanwhile, the binding energy of small Xe-vacancy clusters is important to understand the nucleation and growth behavior of fission gas bubbles. Even larger supercells are required for such calculations. The predicted energetic information along with the structural data will be used to develop interatomic potentials for MD simulations of the fuels. Due to the limitation of supercell size in the first-principles calculation, the stable nucleus for vacancy clusters, or voids, cannot be included at present. However, MD method can be used to directly simulate the nucleation and growth of voids and gas bubble with the newly developed interatomic potential based Embedded Atom Model (EAM). Typical cell size for this kind of MD simulations requires a few millions of atoms or even more. The classical MD code LAMMPS is able to run in parallel on thousands of CPU-cores , which can dramatically reduce the running time needed. The nucleation and growth of gas bubbles in fuels involves diffusion of defects and fission gas over a long period of time, typically in the nanosecond-microsecond regime. Standard MD method is not efficient enough to run such kind of simulations. The kMC method will be used to bridge the time gap. With the DFT calculated defect formation energy, migration barrier and binding energy of Xe-vacancy clusters, new computational models will be developed for the kMC simulations in order to study the nucleation and growth behaviors of fission gas bubbles in metallic fuels. The Modified kMC code SPPARKS, which can run efficiently on parallel clusters, will be used for these simulations. The total amount computational resource needed to finish all the calculations for this project is approximately 450,000 core-hours. The team includes Abdellatif M. Yacout, Marius Stan, Zhi-Gang Mei, Yun Di, and Walid M. Mohamed. Project URL: Requested allocation: 440000 Q1: 110000 Q2: 110000 Q3: 110000 Q4: 110000 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