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The rapid expansion of electromagnetically launched thin metal rings is analyzed in detail, with particular emphasis on the determination of the expansion speed, currents, and specimen temperature as a function of launch voltage. Electrodynamic calculations for copper, tin, and lead rings are presented and compared with typical experimental results. Currents derived from Rogowski-probe records and expansion speeds measured with a velocity interferometer (VISAR) are found to be in excellent agreement with these calculations. Currents are large, of the order of tens of kiloamperes, and heating of the specimen can be significant. It is found that, for good conductors such as copper, the maximum useful strain rate will be limited by the Joule heating of the specimen. For materials with resistivities and strengths substantially larger than those of copper, direct expansion of a specimen ring is difficult. Use of a composite ring, in which a high-conductivity driver is used to launch a low-conductivity specimen, is discussed as a means of overcoming this limitation.

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In support of Stockpile Stewardship activities, accelerated aging tests on a plutonium alloy enriched with 7.3 atomic percentage of 238 Pu is underway using dilatometry at 35, 50, and 65°C and immersion density measurements of material stored at 50°C. Changes in density are expected from radiation damage in the lattice and helium in-growth. After twenty-five equivalent years of aging, the dilatometry data shows that the alloys at 35°C have expanded in volume by 0.11% to 0.12% and have started to exhibit a near linear expansion behavior primarily caused by the helium accumulation. The average He-tovacancy ratio from tested specimens was determined to be around 2.3. The model for the lattice damage and helium in-growth accurately represents the volume swelling at 35°C.The density converted from the dilatometry corresponds well to the decreasing density trend of reference plutonium alloys as a function of time.

First-principles electronic structure calculations based on the full-potential linear-muffin-tin-orbital method have been employed to study the contrasting effects of boron and hydrogen on the electronic structure of the I.lz ordered intermetallic Ni3Al. The total energy, the siteand I-projected densities of states, and the impurity-induced charge-density characteristics are calculated for various impurity configurations, to investigate the effects of local environment on the electronic structure. Total-energy calculations show that both boron and hydrogen impurities prefer to occupy octahedral interstitial sites that are entirely coordinated by six nickel atoms. Our results suggest that the underlying mechanism of the boron-induced strengthening in Ni3Al is the Ni-d and B-p hybridization between the nearestneighbor nickel and boron sites. This results in an enhancement of the intraplanar metallic bonding between the nickel atoms, an enhancement of interstitial bonding charge, and reduction of the bondingcharge directionality around the Ni atoms on the (001) NiAl planes. In contrast, hydrogen is found to enhance the bonding-charge directionality near some Ni atoms and to reduce the interstitial charge, suggesting that it promotes poor local cohesion. When both boron and hydrogen are present in Ni3Al, the dominant changes in the electronic structure are induced by boron and the charge distribution resembles that of Ni3Al+B. These results are broadly consistent with the notion of boron as a cohesion enhancer and hydrogen as an embrittler.