A publication of the Office of Advanced Simulation & Computing, NNSA Defense Programs
NA-ASC-500-07—Issue 3
May 2007
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New Multiphase Plutonium Equation of State at Lawrence Livermore Improves Current High Standard and Aids Stockpile Certification
Replacing an equation-of-state (EOS) high standard set in the 1990s for weapons simulation at Lawrence Livermore (LLNL), a new multiphase plutonium (Pu) EOS will allow weapons designers to elaborate important weapons-relevant physics beyond current capabilities. The new multiphase EOS moves simulations from a few-phase, single-table quantum-based representation to a more accurate many-phase, multi-table advanced quantum representation, while confirming and retaining the best and most trusted features of the current baseline EOS.
A complete tables package for LLNL’s new multiphase EOS has been developed and implemented in LLNL hydrocodes, and application tests are in progress. General release of the tested tables is expected by October 1, 2007. As part of the NNSA Stockpile Stewardship Program, the EOS effort at Lawrence Livermore focuses on understanding, elaborating, and tabulating the thermodynamic properties of stockpile materials for programmatic application. The need for an improved Pu EOS is driven by the primary performance requirements of quantifying margins and uncertainties (QMU). Towards this end, LLNL seeks to exploit the fact that the thermodynamic properties of a material are determined at the atomistic (nanometer)-length scale where rigorous quantum-mechanical methods apply. Using advanced electronic structure and atomistic simulation techniques implemented on ASC platforms, together with recent accurate static and dynamic experimental data, LLNL developed a next generation multiphase Pu EOS intended for stockpile certification and QMU, completing an LLNL 2007 ASC level 2 milestone.
EOS advancements made include:
- Extensive and successful application of advanced first-principles electronic structure and quantum-based many-body inter-atomic potential methods to the calculation of the high-pressure phase diagram and the required cold, ion-thermal and electron-thermal EOS components of each individual phase. In addition, generalization and extension of these methods to the low-density phases of Pu capture important and strongly correlated behavior among f electrons.
- Validation of the low-temperature EOS by diamond-anvil-cell synchrotron x-ray data from the Advanced Photon Source at Argonne National Laboratory and of the high-temperature EOS from JASPER shock data taken at the Nevada Test Site.
- Incorporation of both well-known phases and recently discovered new phases and phase lines into the multiphase EOS. Certain long-standing issues and uncertainties concerning the Pu phase diagram have yielded to accurate diamond-anvil-cell measurements, while theory has predicted some new and unexpected behavior at high pressure.
- Implementation and testing of a unique multi-table EOS framework within the LLNL hydrodynamic simulation codes (“hydrocodes”) that will permit an optional treatment of phase kinetics, that is, nonequilibrium, time-dependent transitions between phases. The inherently modular nature of this framework will also allow easier future updates and repairs of the EOS.
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