9 research outputs found
Interatomic potentials for mixed oxide (MOX) nuclear fuels
We extend our recently developed interatomic potentials for UO_{2} to the
mixed oxide fuel system (U,Pu,Np)O_{2}. 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
Simulation of ion track ranges in uranium oxide
Direct comparisons between statistically sound simulations of ion-tracks and
published experimental measurements of range densities of iodine implants in
uranium dioxide have been made with implant energies in the range of 100-800
keV. Our simulations are conducted with REED-MD (Rare Event Enhanced
Domain-following Molecular Dynamics) in order to account for the materials
structure in both single crystalline and polycrystalline experimental samples.
We find near-perfect agreement between REED-MD results and experiments for
polycrystalline target materials.Comment: Eleven pages, four figures
Efficient Parallel Algorithm for Statistical Ion Track Simulations in Crystalline Materials
We present an efficient parallel algorithm for statistical Molecular Dynamics
simulations of ion tracks in solids. The method is based on the Rare Event
Enhanced Domain following Molecular Dynamics (REED-MD) algorithm, which has
been successfully applied to studies of, e.g., ion implantation into
crystalline semiconductor wafers. We discuss the strategies for parallelizing
the method, and we settle on a host-client type polling scheme in which a
multiple of asynchronous processors are continuously fed to the host, which, in
turn, distributes the resulting feed-back information to the clients. This
real-time feed-back consists of, e.g., cumulative damage information or
statistics updates necessary for the cloning in the rare event algorithm. We
finally demonstrate the algorithm for radiation effects in a nuclear oxide
fuel, and we show the balanced parallel approach with high parallel efficiency
in multiple processor configurations.Comment: 17 pages, seven figures, four table
Thiol density dependent classical potential for methyl-thiol on a Au(111) surface
A new classical potential for methyl-thiol on a Au(111) surface has been
developed using density functional theory electronic structure calculations.
Energy surfaces between methyl-thiol and a gold surface were investigated in
terms of symmetry sites and thiol density. Geometrical optimization was
employed over all the configurations while minimum energy and thiol height were
determined. Finally, a new interatomic potential has been generated as a
function of thiol density, and applications to coarse-grained simulations are
presented
Parallel TREE code for two-component ultracold plasma analysis
The TREE method has been widely used for long-range interaction {\it N}-body
problems. We have developed a parallel TREE code for two-component classical
plasmas with open boundary conditions and highly non-uniform charge
distributions. The program efficiently handles millions of particles evolved
over long relaxation times requiring millions of time steps. Appropriate domain
decomposition and dynamic data management were employed, and large-scale
parallel processing was achieved using an intermediate level of granularity of
domain decomposition and ghost TREE communication. Even though the
computational load is not fully distributed in fine grains, high parallel
efficiency was achieved for ultracold plasma systems of charged particles. As
an application, we performed simulations of an ultracold neutral plasma with a
half million particles and a half million time steps. For the long temporal
trajectories of relaxation between heavy ions and light electrons, large
configurations of ultracold plasmas can now be investigated, which was not
possible in past studies
Nanoscale oxidation and complex oxide growth on single crystal iron surfaces and external electric field effects
Oxidation of iron surfaces and oxide growth mechanisms have been studied using reactive molecular dynamics. Oxide growth kinetics on Fe(100), (110), and (111) surface orientations has been investigated at various temperatures and/or an external electric field. The oxide growth kinetics decreases in the order of (110), (111), and (100) surfaces at 300 K over 1 ns timescale while higher temperature increases the oxidation rate. The oxidation rate shows a transition after an initial high rate, implying that the oxide formation mechanism evolves, with iron cation re-ordering. In early stages of surface oxide growth, oxygen transport through iron interstitial sites is dominant, yielding non-stoichiometric wüstite characteristics. The dominant oxygen inward transport decreases as the oxide thickens, evolving into more stoichiometric oxide phases such as wüstite or hematite. This also suggests that cation outward transport increases correspondingly. In addition to oxidation kinetics simulations, formed oxide layers have been relaxed in the range of 600–1500 K to investigate diffusion characteristics, fitting these results into an Arrhenius relation. The activation energy of oxygen diffusion in oxide layers formed on Fe(100), (110), and (111) surfaces was estimated to be 0.32, 0.26, and 0.28 eV, respectively. Comparison between our modeling results and literature data is then discussed. An external electric field (10 MV cm−1) facilitates initial oxidation kinetics by promoting oxygen transport through iron lattice interstitial sites, but reaches self-limiting thickness, showing that similar oxide formation stages are maintained when cation transport increases. The effect of the external electric field on iron oxide structure, composition, and oxide activation energy is found to be minimal, whereas cation outward migration is slightly promoted
Toward Equatorial Planarity about Uranyl: Synthesis and Structure of Tridentate Nitrogen-Donor {UO<sub>2</sub>}<sup>2+</sup> Complexes
The reaction of UO<sub>2</sub>Cl<sub>2</sub>·3THF with the tridentate nitrogen donor
ligand 2,6-bis(2-benzimidazolyl)pyridine (H<sub>2</sub>BBP) in pyridine
leads to the formation of three different complexes: [(UO<sub>2</sub>)(H<sub>2</sub>BBP)Cl<sub>2</sub>] (<b>1</b>), [(UO)<sub>2</sub>(HBBP)(Py)Cl] (<b>2</b>), and [(UO<sub>2</sub>)(BBP)(Py)<sub>2</sub>] (<b>3</b>) after successive deprotonation of H<sub>2</sub>BBP with a strong base. Crystallographic determination of <b>1</b>–<b>3</b> reveals that increased charge through
ligand deprotonation and displacement of chloride leads to equatorial
planarity about uranyl as well as a more compact overall coordination
geometry. Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectra
of <b>1</b>–<b>3</b> at the U-4d edges have been
recorded using a soft X-ray Scanning Transmission X-ray Microscope
(STXM) and reveal the uranium 4d<sub>5/2</sub> and 4d<sub>3/2</sub> transitions at energies associated with uranium in the hexavalent
oxidation state. First-principles Density Functional Theory (DFT)
electronic structure calculations for the complexes have been performed
to determine and validate the coordination characteristics, which
correspond well to the experimental results