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.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 941398
- Report Number(s):
- UCRL-JRNL-230302; TRN: US200824%%587
- Journal Information:
- Journal of Computer-Aided Materials Design, vol. 14, no. 3, September 30, 2007, pp. 331-335, Vol. 14, Issue 3
- Country of Publication:
- United States
- Language:
- English
Similar Records
Toward a Deeper Understanding of Plutonium
Lawrence Livermore National Laboratory Annual Report 2006
Related Subjects
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