Abstract: A new interatomic potential in the framework of the modified embedded atom method (MEAM) to model U metal is presented. The potential acceptably reproduces the lattice parameters and cohesive energy of the orthorhombic αU. The relative stability of the experimentally observed phase at low temperatures with respect to several other structures (bct, bcc, simple cubic, tetragonal β Np, fcc and hcp) is also taken into account. Intrinsic point defect properties compare reasonably well with data from the literature. To determine the quality of the interaction, the potential is used to study a number of properties for the pure metal at finite temperatures and the results are compared with the available data. The obtained allotropic αU ↔ γU transformation and melting temperatures are in good agreement with experimental values. Based on the simulations, a new αU ↔ γU transformation mechanism is proposed.

Abstract: Using the force-matching method we develop an interatomic potential that allows us to study the structure and properties of α-U, γ-U and liquid uranium. The potential is fitted to the forces, energies and stresses obtained from ab initio calculations. The model gives a good comparison with the experimental and ab initio data for the lattice constants of α-U and γ-U, the elastic constants, the room-temperature isotherm, the normal density isochore, the bond-angle distribution functions and the vacancy formation energies. The calculated melting line of uranium at pressures up to 80 GPa and the temperature of the α-γ transition at 3 GPa agree well with the experimental phase diagram of uranium.

Abstract: Interaction for both pure Al and Al–U alloys of the MEAM type are developed. The obtained Al interatomic potential assures its compatibility with the details of the framework presently adopted. The Al–U interaction fits various properties of the Al2U, Al3U and Al4U intermetallics. The potential verifies the stability of the intermetallic structures in a temperature range compatible with that observed in the phase diagram, and also takes into account the greater stability of these structures relative to others that are competitive in energy. The intermetallics are characterized by calculating elastic and thermal properties and point defect parameters. Molecular dynamics simulations show a growth of the Al3U intermetallic in the Al/U interface in agreement with experimental evidence.

Abstract: We extend our recently developed interatomic potentials for UO2 to the fuel system (U,Pu,Np)O2. We do so by fitting against an extensive database of ab initio results as well as to experimental measurements. The applicability of these interactions to a variety of mixed environments beyond the fitting domain is also assessed. The employed formalism makes these potentials applicable across all interatomic distances without the need for any ambiguous splining to the well-established short-range Ziegler-Biersack-Littmark universal pair potential. We therefore expect these to be reliable potentials for carrying out damage simulations (and molecular dynamics simulations in general) in nuclear fuels of varying compositions for all relevant atomic collision energies.

Abstract: We provide a methodology for generating interatomic potentials for use in classical molecular-dynamics simulations of atomistic phenomena occurring at energy scales ranging from lattice vibrations to crystal defects to high-energy collisions. A rigorous method to objectively determine the shape of an interatomic potential over all length scales is introduced by building upon a charged-ion generalization of the well-known Ziegler-Biersack-Littmark universal potential that provides the short- and long-range limiting behavior of the potential. At intermediate ranges the potential is smoothly adjusted by fitting to ab initio data. Our formalism provides a complete description of the interatomic potentials that can be used at any energy scale, and thus, eliminates the inherent ambiguity of splining different potentials generated to study different kinds of atomic-materials behavior. We exemplify the method by developing rigid-ion potentials for uranium dioxide interactions under conditions ranging from thermodynamic equilibrium to very high atomic-energy collisions relevant for fission events.

Abstract: We studied structure and thermodynamic properties of cubic and tetragonal phases of pure uranium and U-Mo alloys using atomistic simulations: molecular dynamics and density functional theory. The main attention was paid to the metastable γ0-phase that is formed in U-Mo alloys at low temperature. Structure of γ0-phase is similar to body-centered tetragonal (bct) lattice with displacement of a central atom in the basic cell along [001] direction. Such displacements have opposite orientations for part of the neighbouring basic cells. In this case, such ordering of the displacements can be designated as antiferro-displacement. Formation of such complex structure may be interpreted through forming of short U-U bonds. At heating, the tetragonal structure transforms into cubic γs-phase, still showing ordering of central atom displacements. With rise in temperature, γs-phase transforms to γ-phase with a quasi body-centered cubic (q-bcc) lattice. The local positions of uranium atoms in γ-phase correspond to γs-phase, however, orientations of the central atom displacements become disordered. Transition from γ0 to γ can be considered as antiferro-to paraelastic transition of order-disorder type. \n\nThis approach to the structure description of uranium alloy allows to explain a number of unusual features found in the experiments: anisotropy of lattice at low temperature; remarkably high self-diffusion mobility in γ-phase; decreasing of electrical resistivity at heating for some alloys. In addition, important part of this work is the development of new interatomic potential for U-Mo system made with taking into account details of studied structures.

Abstract: A new interatomic potential for a uranium–molybdenum system with xenon is developed in the framework of an embedded atom model using a force-matching technique and a dataset of ab initio atomic forces. The verification of the potential proves that it is suitable for the investigation of various compounds existing in the system as well as for simulation of pure elements: U, Mo and Xe. Computed lattice constants, thermal expansion coefficients, elastic properties and melting temperatures of U, Mo and Xe are consistent with the experimentally measured values. The energies of the point defect formation in pure U and Mo are proved to be comparable to the density-functional theory calculations. We compare this new U–Mo–Xe potential with the previously developed U and Mo–Xe potentials. A comparative study between the different potential functions is provided. The key purpose of the new model is to study the atomistic processes of defect evolution taking place in the U–Mo nuclear fuel. Here we use the potential to simulate bcc alloys containing 10 wt% of intermetallic Mo and U2Mo.

Abstract: In this work we studied the pressure-induced phase transition between different structures of uranium mononitride: cubic Fm-3m-structure and rhombohedral R-3m-structure. We used molecular dynamics together with a new interatomic potential developed for this purpose. We estimated phase diagram of uranium mononitrde in a wide range of temperature and pressure using thermodynamic and mechanical criteria of stability. From simulations we see that at zero temperature the phase transition Fm-3m -> R-3m takes place at pressure about 35 GPa, which agrees well with the available experimental and theoretical data. Results of the calculations show that the lattice of rhombohedral phase becomes close to cubic structure with increase in temperature.

Abstract: Uranium-silicide (U-Si) fuels are being pursued as a possible accident tolerant fuel (ATF). This uranium alloy fuel benefits from higher thermal conductivity and higher fissile density compared to uranium dioxide (UO2). In order to perform engineering scale nuclear fuel performance simulations, the material properties of the fuel must be known. Currently, the experimental data available for U-Si fuels is rather limited. Thus, multiscale modeling efforts are underway to address this gap in knowledge. In this study, a semi-empirical modified Embedded-Atom Method (MEAM) potential is presented for the description of the U-Si system. The potential is fitted to the formation energy, defect energies and structural properties of U3Si2. The primary phase of interest (U3Si2) is accurately described over a wide temperature range and displays good behavior under irradiation and with free surfaces. The potential can also describe a variety of U-Si phases across the composition spectrum.

Abstract: A semi-empirical Modified Embedded Atom Method (MEAM) potential is developed for application to the high temperature body-centered-cubic uranium–zirconium alloy (γ-U–Zr) phase and employed with molecular dynamics (MD) simulations to investigate the high temperature thermo-physical properties of U–Zr alloys. Uranium-rich U–Zr alloys (e.g. U–10Zr) have been tested and qualified for use as metallic nuclear fuel in U.S. fast reactors such as the Integral Fast Reactor and the Experimental Breeder Reactors, and are a common sub-system of ternary metallic alloys like U–Pu–Zr and U–Zr–Nb. The potential was constructed to ensure that basic properties (e.g., elastic constants, bulk modulus, and formation energies) were in agreement with first principles calculations and experimental results. After which, slight adjustments were made to the potential to fit the known thermal properties and thermodynamics of the system. The potentials successfully reproduce the experimental melting point, enthalpy of fusion, volume change upon melting, thermal expansion, and the heat capacity of pure U and Zr. Simulations of the U–Zr system are found to be in good agreement with experimental thermal expansion values, Vegard's law for the lattice constants, and the experimental enthalpy of mixing. This is the first simulation to reproduce the experimental thermodynamics of the high temperature γ-U–Zr metallic alloy system. The MEAM potential is then used to explore thermodynamics properties of the high temperature U–Zr system including the constant volume heat capacity, isothermal compressibility, adiabatic index, and the Grüneisen parameters.

Abstract: We have developed an improved uranium dioxide interatomic potential by fitting to forces, energies, and stresses of first principles molecular dynamics calculations via a genetic algorithm approach called Iterative Potential Refinement (IPR). We compare the defect energetics and vibrational properties of the IPR-fit potential with other interatomic potentials, density functional theory calculations, and experimental phonon dispersions. We find that among previously published potentials examined, there is no potential that simultaneously yields accurate defect energetics and accurate vibrational properties. In contrast, our IPR-fit potential produces both accurate defects and the best agreement with the experimental phonon dispersion and phonon density of states. This combination of accurate properties makes this IPR-fit potential useful for simulating UO2 in high temperature, defect-rich environments typical for nuclear fuel. Additionally, we verify that density functional theory with a Hubbard U correction accurately reproduces the experimentally derived UO2 phonon density of states.

Abstract: We provide a methodology for generating interatomic potentials for use in classical molecular-dynamics simulations of atomistic phenomena occurring at energy scales ranging from lattice vibrations to crystal defects to high-energy collisions. A rigorous method to objectively determine the shape of an interatomic potential over all length scales is introduced by building upon a charged-ion generalization of the well-known Ziegler-Biersack-Littmark universal potential that provides the short- and long-range limiting behavior of the potential. At intermediate ranges the potential is smoothly adjusted by fitting to ab initio data. Our formalism provides a complete description of the interatomic potentials that can be used at any energy scale, and thus, eliminates the inherent ambiguity of splining different potentials generated to study different kinds of atomic-materials behavior. We exemplify the method by developing rigid-ion potentials for uranium dioxide interactions under conditions ranging from thermodynamic equilibrium to very high atomic-energy collisions relevant for fission events.