Atomic interactions between plutonium and helium.

An essential issue in gallium (Ga)-stabilized fcc-phase plutonium ({delta}-Pu) is the formation of helium (He) voids and bubbles emanating from the radiolytic decay of the Pu. The rate of formation of He voids and bubbles is related to the He-defect formation energies and their associated migration barriers. The size and shape distributions of the bubbles are coupled to these critical migration processes. The values of the defect formation energies, internal pressure, and migration barriers can be estimated from atomistic calculations. Complicating this picture is the destruction of He-filled voids and bubbles by subsequent radiolytic decay events. The present study concerns the construction of the necessary potential energy surfaces for the Pu-He and He-He interactions within the modified embedded atom method (MEAM). Once fully tested, the potentials will be used to estimate the He-defect formation energies and barriers to the migration of these defects for both interstitial and substitutional He on an fcc Pu lattice. The He-He interactions are modeled from ab initio electronic structure calculations for the He{sub 2} dimer and the equilateral He, trimer. The experimental data and the electronic structure calculations on He{sub 2} agree very well. These data were fit to a Rose function fn{sub R}(x) = A P({alpha}x) exp(-{alpha}x), where P is a polynomial, x = R/R{sub 0}-1, R is the bond length, and R{sub 0} is its equilibrium value. The fits are very satisfactory. Both linear (P = 1+{alpha}x, zeroth-order Rose) and rational (P = 1+{alpha}x+a{sub 3} ({alpha}x){sup 3}/(1+x) first-order Rose) polynomials in the Rose function were tried. The more flexible rational form does improve the fit, but only marginally. Only the linear form was used thereafter. The resulting MEAM potential was used to predict the behavior of the linear trimer and the fcc cold compression curve. The results are shown in Fig. 2 and appear to be satisfactory. The compression regions of the curves are of particular interest for several reasons. First, an octahedral interstitial He atom in anfcc Pu lattice with a lattice constant of 4.64 {angstrom} has a nearest Pu neighbor distance of 2.32 {angstrom}. This distance is in the compressive region of the potential energy curve. Second, the compressive region will partially determine the internal pressure of He-filled voids and bubbles. Third, the shape of the He-filled voids will be influenced by the compression region of the potential. The Pu-He interactions are also modeled from ab initio electronic structure calculations, this time only for the PuHe dimer. The lowest-energy spin state of the dimer appears to be the S=7/2 state with a 'Stuttgart small-core RECP/6-31 g' basis. Two electronic structure methods were tried which would bound the extremes of the Pu-He interaction. One was the local density approximation (LDA), which tends to overestimate binding strength. It gives a well depth of 0.08 eV and a bond length of 3.6 {angstrom}. The other used the Becke-3-Lee-Yang-Parr (B3LYP) exchange-correlation energy functional, which tends to underestimate binding strength. It predicts no binding at any separation. For purposes of fitting to the Rose functional form, a well depth of 0.03 eV and bond length of 4.8 {angstrom} was used. The bond length exceeds the cutoff distance that will be used in future simulations to limit the maximum range of the atomic interactions and is effectively purely repulsive. Furthermore, the dimer information is sufficient to determine the painvise part of the Pu-He MEAM potential, but not the effective electron density that determines the many-body part of the potential, the embedding functions F{sub He} and F{sub Pu} in Fig. 3. The effective electron density, as well as determining which of the two dimer curves (LDA or B3LYP) is preferable, will be decided by comparing simulation results to known information about He bubble formation rates at elevated temperatures and estimates of Me bubble sizes. Initial simulations suggest that an interstitial He defect, based on either the LDA or the B3LYP dimer curve with a He:Pu density ratio of 0.04, will not remain at an octahedral site as in other fcc metals such as nickel. The He defect may also form a split interstitial with a Pu atom. The details remain to be determined

Point-detect production and migration in plutonium metal at ambient conditions

Modeling thermodynamics and defect production in plutonium (Pu) metal and its alloys, has proven to be singularly difficult. The multiplicity of phases and the small changes in temperature, pressure, and/or stress that can induce phase changes lie at the heart of this difficulty, In terms of radiation damage, Pu metal represents a unique situation because of the large volume changes that accompany the phase changes. The most workable form of the metal is the fcc (6.) phase, which in practice the 6 phase is stabilized by addition of alloying elements such as Ga or AI. The thermodynamically stable phase at ambient conditions is the between monoclinic (a-) phase, which, however, is approximately 20 % lower in volume than the 6 phase. In stabilized Pu metal, there is an interplay between the natural swelling tendencies of fcc metals and the volume-contraction tendency of the underlying phase transformation to the thermodynamically stable phase. This study explores the point defect production and migration properties that are necessary to eventually model the long-term outcome of this interplay

An Empirical Charge Transfer Potential with Correct Dissociation Limits

The empirical valence bond (EVB) method [J. Chem. Phys. 52, 1262 (1970)] has always embodied charge transfer processes. The mechanism of that behavior is examined here and recast for use as a new empirical potential energy surface for large-scale simulations. A two-state model is explored. The main features of the model are: (1) Explicit decomposition of the total system electron density is invoked; (2) The charge is defined through the density decomposition into constituent contributions; (3) The charge transfer behavior is controlled through the resonance energy matrix elements which cannot be ignored; and (4) A reference-state approach, similar in spirit to the EVB method, is used to define the resonance state energy contributions in terms of "knowable" quantities. With equal validity, the new potential energy can be expressed as a nonthermal ensemble average with a nonlinear but analytical charge dependence in the occupation number. Dissociation to neutral species for a gas-phase process is preserved. A variant of constrained search density functional theory is advocated as the preferred way to define an energy for a given charge.Comment: Submitted to J. Chem. Phys. 11/12/03. 14 pages, 8 figure

Multiscale simulations of alloy phase stability

First principles, atomic scale and continuum level models are combined to predict thermodynamic properties of alloys and stability of phases. Many-body interactions, as well as vacancies, defects, and non-stoichiometry are included in the modeling process and the structural stability of hypothetical phases is evaluated. The resulted thermodynamic functions and phase diagrams are integrated in a casting simulation computer program. The process of relating microscopic modeling results to the macroscopic heat transfer and phase equilibrium calculations is detailed to emphasize the self-consistency of the approach and to identify the potential sources of errors. The sequence: data acquisition, modeling, prediction experimental validation, is illustrated for several recent results in actinide based alloys

EFFECTS OF MATERIALS STRENGTH ON STRONGLY-SHOCKED NONENERGETIC MATERIALS

The role of materials strength in changing the shock dynamics in strongly-shocked nonenergetic materials is still a matter of investigation because materials strength properties become convoluted with other materials properties and the shock strength. The regime under consideration here is one in which the material in question is shocked strongly enough to be treated as a fluid, but not strongly enough to be treated as a simple fluid. The present work takes a case-study approach in which two models of the constitutive properties of the complex fluid are applied to shock instability for two different polymeric materials. The intent here is to obtain some measure of the sensitivity of the model predictions to variations in the complex fluid constitutive properties. The linear time-regime in a Richtmyer-Meshkov instability is modeled with the viscosity dependence of Mikaelian and the nonlinear time-regime is modeled with an aerodynamic viscous-drag model. Each combination of materials and models will be examined as a function of shock strength, Atwood number, and variation in materials constitutive properties. Although the these models are NOT the most advanced, they are useful for illustrating orders of magnitude

Atomistic models of point defects in plutonium metal.

The aging properties of plutonium (Pu) metal and alloys are. driven by a combination of materials composit ion, p rocessing history, and self-irradiat ion effects . Understanding these driving forces requires a knowledge of both t h ermodynamic and defect properties of the material . The multiplicity of phases and the small changes in tempe rat u re, pressure, and/or stress that can induce phase changes lie at the heart of these properties . In terms of radiation damage, Pu metal represents a unique situation because of the large volume chan ges that accompany the phase changes . The most workable form of the meta l is the fcc (S-) phase, which in practice is stabi l ized by addit io n of a ll oying el eme n ts s u c h as Ga or Al. The thermodynamically stable phase at ambient conditions is the monoclinic (a-) phase, which, however, is 2 0 % lower i n volume th an the S phase . In stabilized Pu metal, there is an in t er play between th e n atu ral swe l li n g tendencies of fcc metals and the volume-contraction tendency of the u n d erlyin g thermodynamicall y stable phase. This study exp lores the point d efect pr operties that are necessary to model the long-term outcome of this interplay

BioTIME:a database of biodiversity time series for the Anthropocene

Abstract Motivation: The BioTIME database contains raw data on species identities and abundances in ecological assemblages through time. These data enable users to calculate temporal trends in biodiversity within and amongst assemblages using a broad range of metrics. BioTIME is being developed as a community‐led open‐source database of biodiversity time series. Our goal is to accelerate and facilitate quantitative analysis of temporal patterns of biodiversity in the Anthropocene. Main types of variables included: The database contains 8,777,413 species abundance records, from assemblages consistently sampled for a minimum of 2 years, which need not necessarily be consecutive. In addition, the database contains metadata relating to sampling methodology and contextual information about each record. Spatial location and grain: BioTIME is a global database of 547,161 unique sampling locations spanning the marine, freshwater and terrestrial realms. Grain size varies across datasets from 0.0000000158 km² (158 cm²) to 100 km² (1,000,000,000,000 cm²). Time period and grain: BioTIME records span from 1874 to 2016. The minimal temporal grain across all datasets in BioTIME is a year. Major taxa and level of measurement: BioTIME includes data from 44,440 species across the plant and animal kingdoms, ranging from plants, plankton and terrestrial invertebrates to small and large vertebrates. Software format: .csv and .SQL