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Title: Overview of Modeling and Simulations of Plutonium Aging

Abstract

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 themore » 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.« less

Authors:
;
Publication Date:
Research Org.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
941398
Report Number(s):
UCRL-JRNL-230302
TRN: US200824%%587
DOE Contract Number:
W-7405-ENG-48
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Computer-Aided Materials Design, vol. 14, no. 3, September 30, 2007, pp. 331-335; Journal Volume: 14; Journal Issue: 3
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; 75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; ACTINIDES; AGING; ALLOYS; ALPHA DECAY; ALPHA PARTICLES; COMPUTERIZED SIMULATION; DECAY; ELECTRONIC STRUCTURE; EQUATIONS OF STATE; FCC LATTICES; MATERIALS TESTING; MEAN-FIELD THEORY; MECHANICAL PROPERTIES; NUCLEAR WEAPONS; PHASE TRANSFORMATIONS; PLUTONIUM; PLUTONIUM ISOTOPES; POINT DEFECTS; RADIATION EFFECTS; RESEARCH PROGRAMS

Citation Formats

Schwartz, A J, and Wolfer, W G. Overview of Modeling and Simulations of Plutonium Aging. United States: N. p., 2007. Web.
Schwartz, A J, & Wolfer, W G. Overview of Modeling and Simulations of Plutonium Aging. United States.
Schwartz, A J, and Wolfer, W G. Tue . "Overview of Modeling and Simulations of Plutonium Aging". United States. doi:. https://www.osti.gov/servlets/purl/941398.
@article{osti_941398,
title = {Overview of Modeling and Simulations of Plutonium Aging},
author = {Schwartz, A J and Wolfer, W G},
abstractNote = {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.},
doi = {},
journal = {Journal of Computer-Aided Materials Design, vol. 14, no. 3, September 30, 2007, pp. 331-335},
number = 3,
volume = 14,
place = {United States},
year = {Tue Apr 24 00:00:00 EDT 2007},
month = {Tue Apr 24 00:00:00 EDT 2007}
}
  • Here, the aim of this study is to present several approaches that have been used to model the behavior of radioactive materials (specifically Pu) in contaminated wounds. We also review some attempts by the health physics community to validate and revise the National Council on Radiation Protection and Measurements (NCRP) 156 biokinetic model for wounds, and present some general recommendations based on the review. Modeling of intake via the wound pathway is complicated because of a large array of wound characteristics (e.g. solubility and chemistry of the material, type and depth of the tissue injury, anatomical location of injury). Moreover,more » because a majority of the documented wound cases in humans are medically treated (excised or treated with chelation), the data to develop biokinetic models for unperturbed wound exposures are limited. Since the NCRP wound model was largely developed from animal data, it is important to continue to validate and improve the model using human data whenever plausible.« less
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  • The efficient usage of parallel computers and workstation clusters for biologically motivated simulations depends first of all on a dynamic redistribution of the workload. For the development of a parallel algorithm for the Perma model of aging we have used a dynamic load balancing library, called PLB. It turns out that PLB manages a nearly balanced load situation during runtime taking only a low communication overhead. We compare different architectures like parallel computers and nondedicated heterogeneous networks, and give some results for large populations.
  • Electronic properties of semiconductor devices are sensitive to defects such as second phase precipitates, grain sizes, and voids. These defects can evolve over time especially under oxidation environments and it is therefore important to understand the resulting aging behavior in order for the reliable applications of devices. In this paper, we propose a kinetic Monte Carlo framework capable of simultaneous simulation of the evolution of second phases, precipitates, grain sizes, and voids in complicated systems involving many species including oxygen. This kinetic Monte Carlo model calculates the energy barriers of various events based directly on the experimental data. As amore » first step of our model implementation, we incorporate the second phase formation module in the parallel kinetic Monte Carlo codes SPPARKS. Selected aging simulations are performed to examine the formation of second phase precipitates at the eletroplated Au/Bi{sub 2}Te{sub 3} interface under oxygen and oxygen-free environments, and the results are compared with the corresponding experiments.« less