Fully Atomistic Simulations of Hydrodynamic Instabilities and Mixing

The large-scale computational capabilities at LLNL make it possible to develop seamless connections from processes at the atomic scale to complex macroscopic phenomena such as hydrodynamic instabilities and turbulent mixing. Traditionally, these connections have been made by combining results from different scientific fields. For gases and fluids, atomic and molecular scattering cross sections must first be obtained and incorporated into Boltzmann transport equations. Their solution yields then transport coefficients which are input parameters for the Navier-Stokes equations for fluid dynamics. The latter are solved numerically with hydro-codes. For visco-elastic solids, on the other hand, atomistic simulations must first provide constitutive laws for the mobility and multiplication of dislocations and other crystalline defects. In turn, these laws are utilized to construct meso-scale models for plastic deformation. These models are then incorporated into hydro- and finite element codes to predict the macroscopic behavior of solid materials. Many of these intermediate steps can be bypassed with large-scale molecular dynamics simulations. For this purpose, codes have been developed in which trajectories of atoms or molecules are mapped onto continuum field descriptions for mass density, mass flow, stresses, and for temperature. It is now possible to compare directly and quantitatively atomistic simulations with predictions from hydro- and finite element codes and with experimental results

Toward a Deeper Understanding of Plutonium

Plutonium is a very complex element lying near the middle of the actinide series. On the lower atomic number side of Pu is the element neptunium; its 5f electrons are highly delocalized or itinerant, participating in metallic-like bonding. The electrons in americium, the element to the right of Pu, are localized and do not participant significantly in the bonding. Plutonium is located directly on this rather abrupt transition. In the low-temperature {alpha} phase ground state, the five 5f electrons are mostly delocalized leading to a highly dense monoclinic crystal structure. Increases in temperature take the unalloyed plutonium through a series of five solid-state allotropic phase transformations before melting. One of the high temperature phases, the close-packed face centered cubic {delta} phase, is the least dense of all the phases, including the liquid. Alloying the Pu with Group IIIA elements such as aluminum or gallium retains the {delta} phase in a metastable state at ambient conditions. Ultimately, this metastable {delta} phase will decompose via a eutectoid transformation to {alpha} + Pu{sub 3}Ga. These low solute-containing {delta}-phase Pu alloys are also metastable with respect to low temperature excursions or increases in pressure and will transform to a monoclinic crystal structure at low temperatures via an isothermal martensitic phase transformation or at slightly elevated pressure. The delocalized to localized 5f electron bonding transition that occurs in the light actinides surrounding Pu gives rise to a plethora of unique and anomalous properties but also severely complicates the modeling and simulation. The development of theories and models that are sufficiently sensitive to capture the details of this transition and capable of elucidating the fundamental properties of plutonium and plutonium alloys is currently a grand challenge in actinide science. Recent advances in electronic structure theory, semi-empirical interatomic potentials, and raw computing power have enabled remarkable progress in our abilities to model many of the anomalous properties of Pu. This special issue of the Journal of Computer-Aided Materials Design highlights a number of these advances in the area of the aging of plutonium. This aging is a long-term process due to the slow radioactive decay with a long half-life of 24400 years for the major isotope of plutonium. The challenge then is to predict the changes in properties of plutonium and its alloys from experimental results of plutonium aged only for a few decades and from theory and computational models that are build on a thorough, first-principle understanding of all the complex phenomena displayed by this material. We hope that progress and success of this enterprise will guide other endeavors in Computer-Aided Materials Design and prediction of materials performance

Overview of Modeling and Simulations of Plutonium Aging

Computer-aided materials research is now an integral part of science and technology. It becomes particularly valuable when comprehensive experimental investigations and materials testing are too costly, hazardous, or of excessive duration; then, theoretical and computational studies can supplement and enhance the information gained from limited experimental data. Such is the case for improving our fundamental understanding of the properties of aging plutonium in the nuclear weapons stockpile. The question of the effects of plutonium aging on the safety, security, and reliability of the nuclear weapons stockpile emerged after the United States closed its plutonium manufacturing facility in 1989 and decided to suspend any further underground testing of nuclear weapons in 1992. To address this, the Department of Energy's National Nuclear Security Administration (NNSA) initiated a research program to investigate plutonium aging, i.e., the changes with time of properties of Pu-Ga alloys employed in the nuclear weapons and to develop models describing these changes sufficiently reliable to forecast them for several decades. The November 26, 2006 press release by the NNSA summarizes the conclusions of the investigation, '...there appear to be no serious or sudden changes occurring, or expected to occur, in plutonium that would affect performance of pits beyond the well-understood, gradual degradation of plutonium materials'. Furthermore, 'These studies show that the degradation of plutonium in our nuclear weapons will not affect warhead reliability for decades', then NNSA Administrator Linton Brooks said. 'It is now clear that although plutonium aging contributes, other factors control the overall life expectancy of nuclear weapons systems'. The origin of plutonium aging is the natural decay of certain plutonium isotopes. Specifically, it is the process of alpha decay in which a plutonium atom spontaneously splits into a 5 MeV alpha particle and an 85keV uranium recoil. The alpha particle traverses the lattice, slowly loosing energy through electronic excitations, acquiring two electrons to become a helium atom, then finally coming to rest approximately 10 microns away with the generation of a few-hundred Frenkel pairs. The uranium recoil immediately displaces a couple-thousand Pu atoms from their original lattice sites. This process, which occurs at a rate of approximately 41 parts-per-million per year, is the source of potential property changes in aging plutonium. Plutonium aging encompasses many areas of research: radiation damage and radiation effects, diffusion of point defects, impurities and alloying elements, solid state phase transformations, dislocation dynamics and mechanical properties, equations of state under extreme pressures, as well as surface oxidation and corrosion. Theory, modeling, and computer simulations are involved to various degrees in many of these areas. The joint research program carried out at Lawrence Livermore National Laboratory and Los Alamos National Laboratory encompassed experimental measurements of numerous properties of newly fabricated reference alloys, archival material that have accumulated the effects of several decades of radioactive decay, and accelerated aging alloys in which the isotropic composition was adjusted to increase the rate of self-irradiation damage. In particular, the physical and chemical processes of nuclear materials degradation were to be studied individually and in great depth. Closely coupled to the experimental efforts are theory, modeling, and simulations. These efforts, validated by the experiments, aim to develop predictive models to evaluate the effects of age on the properties of plutonium. The need to obtain a scientific understanding of plutonium aging has revitalized fundamental research on actinides and plutonium in particular. For example, the experimental discovery of superconductivity in Pu-based compounds, the observation of helium bubbles in naturally aged material, and the measurement of phonon dispersion properties in gallium-stabilized delta plutonium have occurred in recent years. On the theory frontier, dynamic mean field theory calculated the phonon dispersion curves before the measurements were published and the application of spin-polarized density functional theory has resulted in reproducing the energies and densities of the light actinides and all plutonium phases in remarkable agreement with observed results. The delta, or face-centered-cubic phase, in particular, has been shown to have an anti-ferromagnetic spin configuration. Because this is in apparent contradiction to experiments that reveal no evidence of anti-ferromagnetic behavior, a lively scientific exchange of ideas and opinions among actinide researchers has taken place, and electronic structure theory and experiments for actinides has become exciting fields of research

DAFS contribution: the influence of dislocation density and radiation on carbon activity and phase development in AISI 316. [LMFBR]

The objective of this effort is to identify the role of each major element in the microchemical evolution of AISI 316 and the dependence of that role on preirradiation treatment and parameters such as neutron energy and flux, temperature and stress

Atomic-volume variations of (alpha)-Pu alloyed with Al, Ga, and Am from first-principles theory

First-principles methods are employed to calculate the ground-state atomic densities (or volumes) of {alpha}-Pu alloyed with Al, Ga, and Am. Three configurations for the alloying atom are considered. (1) It is located at the most open and energetically most favorably site. (2) It is located in the least open site. (3) It is randomly distributed within the {alpha}-Pu matrix. When alloyed with Al or Ga, {alpha}-Pu behaves similarly, it expands considerably for configurations (2) and (3), while for (1) only small changes of the density occurs. Interestingly, for Am the alloying effects are quite different from that of Al and Ga. Small expansion is noted for the ordered configurations (1) and (2), whereas for the disordered (3), only insignificant changes of the density take place. The bonding character is thus differently influenced in Pu by the addition of Al and Ga on one hand and Am on the other. This is consistent with the view that Al and Ga stabilize the {delta} over the {alpha} phase in Pu by a different mechanism than Am, as has been discussed in recent publications

Reversible expansion of gallium-stabilized (delta)-plutonium

It is shown that the transient expansion of plutonium-gallium alloys observed both in the lattice parameter as well as in the dimension of a sample held at ambient temperature can be explained by assuming incipient precipitation of Pu{sub 3}Ga. However, this ordered {zeta}-phase is also subject to radiation-induced disordering. As a result, the gallium-stabilized {delta}-phase, being metastable at ambient temperature, is driven towards thermodynamic equilibrium by radiation-enhanced diffusion of gallium and at the same time reverted back to its metastable state by radiation-induced disordering. A steady state is reached in which only a modest fraction of the gallium present is arranged in ordered {zeta}-phase regions

Dynamic Strength of Metals in Shock Deformation

It is shown that the Hugoniot and the critical shear stress required to deform a metal plastically in shock compression can be obtained directly from molecular dynamics simulations without recourse to surface velocity profiles, or to details of the dislocation evolution. Specific calculations are shown for aluminum shocked along the [100] direction, and containing an initial distribution of microscopic defects. The presence of such defects has a minor effect on the Hugoniot and on the dynamic strength at high pressures. Computed results agree with experimental data

Modified Rate-Theory Predictions in Comparison to Microstructural Data

Standard rate theory methods have recently been combined with experimental microstructures to successfully reproduce measured swelling behavior in ternary steels around 400 C. Fit parameters have reasonable values except possibly for the recombination radius, R{sub c}, which can be larger than expected. Numerical simulations of void nucleation and growth reveal the importance additional recombination processes at unstable clusters. Such extra recombination may reduce the range of possible values for R{sub c}. A modified rate theory is presented here that includes the effect of these undetectably small defect clusters. The fit values for R{sub c} are not appreciably altered, as the modification has little effect on the model behavior in the late steady state. It slightly improves the predictions for early transient times, when the sink strength of stable voids and dislocations is relatively small. Standard rate theory successfully explains steady swelling behavior in high purity stainless steel

Quantifying structural damage from self-irradiation in a plutonium superconductor

The 18.5 K superconductor PuCoGa5 has many unusual properties, including those due to damage induced by self-irradiation. The superconducting transition temperature decreases sharply with time, suggesting a radiation-induced Frenkel defect concentration much larger than predicted by current radiation damage theories. Extended x-ray absorption fine-structure measurements demonstrate that while the local crystal structure in fresh material is well ordered, aged material is disordered much more strongly than expected from simple defects, consistent with strong disorder throughout the damage cascade region. These data highlight the potential impact of local lattice distortions relative to defects on the properties of irradiated materials and underscore the need for more atomic-resolution structural comparisons between radiation damage experiments and theory.Comment: 7 pages, 5 figures, to be published in PR

Isochronal annealing effects on local structure, crystalline fraction, and undamaged region size of radiation damage in Ga-stabilized δ\delta-Pu

The effects on the local structure due to self-irradiation damage of Ga stabilized $\delta$-Pu stored at cryogenic temperatures have been examined using extended x-ray absorption fine structure (EXAFS) experiments. Extensive damage, seen as a loss of local order, was evident after 72 days of storage below 15 K. The effect was observed from both the Pu and Ga sites, although less pronounced around Ga. Isochronal annealing was performed on this sample to study the annealing processes that occur between cryogenic and room temperature storage conditions, where damage is mostly reversed. Damage fractions at various points along the annealing curve have been determined using an amplitude-ratio method, standard EXAFS fitting, and a spherical crystallite model, and provide information complementary to previous electrical resistivity- and susceptibility-based isochronal annealing studies. The use of a spherical crystallite model accounts for the changes in EXAFS spectra using just two parameters, namely, the crystalline fraction and the particle radius. Together, these results are discussed in terms of changes to the local structure around Ga and Pu throughout the annealing process and highlight the unusual role of Ga in the behavior of the lowest temperature anneals.Comment: 13 pages, 10 figure