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Oxygen Effects on Uranium Tested
Uranium and oxygen can form dozens of compounds, each with unique chemical properties. Researchers from Lawrence Livermore and the University of Michigan examined the formation of uranium–oxygen compounds following a nuclear explosion in different environments. Their study, published March 7, 2022, in Scientific Reports, found that both the cooling timescale and the local concentration of oxygen significantly influenced formation of uranium oxide species.
Determining the compounds likely to emerge post-detonation is crucial for refining computational models essential to Livermore’s predictive and nuclear forensic capabilities. As part of a Laboratory Directed Research and Development Program strategic initiative, a team led by Livermore chemist Mark Burton used a laser to ablate a small uranium metal target at different oxygen gas levels. Applying infrared and Raman spectroscopy to analyze particulates, the team found that ambient oxygen concentration strongly affected the type of compounds formed. In addition, the structural composition of uranium oxides depends on whether the plasma cools from 10,000˚C over the course of microseconds or milliseconds.
“We have improved our understanding of gas-phase chemical reactions between uranium and oxygen as hot plasmas cool,” says Livermore co-author and lead of the strategic initiative Kim Knight. “Our findings can inform models of nuclear explosions to refine predictive capabilities of particle formation and transport.” In a related project, team members developed a tabletop reaction chamber to study small amounts of uranium in reproducible experiments.
Contact: Mark Burton (925) 422-1618 (burton34@llnl.gov).
Seismic Models Improved
Scientists with Livermore’s Geophysical Monitoring Program have developed an improved Earth model for seismic waveform simulation. With Mondaic, a start-up from the Swiss Federal Institute of Technology in Zurich, Livermore employed full waveform inversion using the Lassen supercomputer to infer seismic models of the Earth for improved simulations. Their research, published July 7, 2022, in Journal of Geophysical Research, demonstrates new modeling capabilities by detailing the seismic structure of the Western United States.
Traditional seismic tomography relies on measurements derived from waveforms such as travel time and surface wave dispersion. However, limited knowledge of subsurface structure—3D variations in wave speed, attenuation, and density—has hindered higher accuracy seismic simulations. Research led by Livermore scientist Artie Rodgers inverted observed waveforms and iteratively updated models of subsurface Earth structure to reduce misfits between simulated and observed seismograms. This multiscale approach first fit long-period waveforms with long-wavelength Earth structure, then gradually reduced the minimum period to analyze finer scale structure. With Lassen, the researchers performed 256 model iterations for 72 earthquakes, updating the model to fit the real-world waveform data of 100,000 seismograms.
“The combination of Mondaic’s software for managing workflow, Lassen’s fast GPU-accelerated nodes, and Livermore’s high-performance computing ecosystem and support enabled us to perform many more inversion iterations and resolve more detailed structure than previous studies,” says Rodgers. The model’s ability to simulate waveforms from small seismic events makes it a promising resource for improving earthquake-explosion identification, aiding nuclear monitoring efforts.
Contact: Artie Rodgers (925) 423-5018 (rodgers7@llnl.gov).
Diamond Formation Detailed
A multi-institutional team headed by Livermore scientist Michael Armstrong capped off decades of theoretical and experimental efforts by observing the transformation of graphite into an elusive phase of diamond known as lonsdaleite on a picosecond timescale. The team’s results were published August 1, 2022, in Journal of Applied Physics.
Inside the familiar diamonds used in jewelry and industrial applications, carbon atoms are arranged in a cubic diamond crystal achieved by compressing graphite. Under immense, instantaneous pressures, such as those produced during meteorite impacts, carbon can instead assume the lonsdaleite phase with hexagonal diamond structure. The team replicated these cataclysmic criteria by bombarding graphite samples with picosecond (trillionths of a second) bursts of laser light to achieve 80 gigapascals of pressure (roughly 800,000 atmospheres). Using femtosecond (quadrillionths of a second) x-ray pulses emitted from the Matter in Extreme Conditions instrument at SLAC National Accelerator Laboratory, the team identified lonsdaleite’s mechanics of formation through time-resolved diffraction analysis.
“Lonsdaleite structure is likely an intermediate state in the phase transition (of graphite) to cubic diamond,” says Armstrong. The researchers observed formation of highly textured lonsdaleite crystals 20 picoseconds after compression that reverted to a distorted graphite structure upon release. Observations confirmed earlier conjecture that lonsdaleite’s formation is diffusionless, whereby atoms rearrange cooperatively and simultaneously. The study furthers understanding of dynamic materials response and enhances weapons physics.
Contact: Michael Armstrong (925) 423-5702 (armstrong30@llnl.gov).
