Atomistic properties of γ uranium

Abstract

The properties of the body-centered cubic γ phase of uranium (U) are calculated using atomistic simulations. First, a modified embedded-atom method interatomic potential is developed for the high temperature body-centered cubic (γ) phase of U. This phase is stable only at high temperatures and is thus relatively inaccessible to first principles calculations and room temperature experiments. Using this potential, equilibrium volume and elastic constants are calculated at 0 K and found to be in close agreement with previous first principles calculations. Further, the melting point, heat capacity, enthalpy of fusion, thermal expansion and volume change upon melting are calculated and found to be in reasonable agreement with experiment. The low temperature mechanical instability of γ U is correctly predicted and investigated as a function of pressure. The mechanical instability is suppressed at pressures greater than 17.2 GPa. The vacancy formation energy is analyzed as a function of pressure and shows a linear trend, allowing for the calculation of the extrapolated zero pressure vacancy formation energy. Finally, the self-defect formation energy is analyzed as a function of temperature. This is the first atomistic calculation of γ U properties above 0 K with interatomic potentials.

Thermodynamic properties and phase transitions of γ and liquid uranium: QMD and classical MD modeling

“…At the densities 24.1 and 29.1 g/cm 3 the bct-U phase occurs at few 1000 K according to the results of CMD modeling. The isochore of density 29.1 g/cm 3 obtained by CMD modeling with the new EAM potential has two noticeable cusps near the temperatures 4000K and 6500K. We consider the former cusp as a consequence of bct-bcc polymorph transformation which was confirmed using the data for radial distance functions for the states at lower and higher temperatures (see 3(c)).…”

“…Due to the data for isochores the results from CMD calculation become close to the QMD data at temperatures higher than 2000 K. The reasons of discrepancies between CMD and QMD data at low temperatures are different. At the densities 18.3 and 20.0 g/cm 3 , thermal pressure of metastable bcc-U are overestimated (see also 2(b)). At the densities 24.1 and 29.1 g/cm 3 the bct-U phase occurs at few 1000 K according to the results of CMD modeling.…”

“…To this date both classical (CMD) [2,3,4,5] and quantum (QMD) [6,7] molecular dynamics have been used for investigation of uranium thermophysical properties. Although first-principles calculations give more accurate results, their immense computational cost makes the CMD approach more preferable for a study of phase diagrams of the actinides.…”

Atomistic modeling of the self-diffusion in γ-U and γ-U-Mo

“…In this study, we simulated γ U and U-Mo alloys with the classical MD method using our previously developed many body potentials [23][24][25], as well as the potential proposed by Beeler et al [26] to describe the properties of defects in γ U. We present the results found when considering diffusion in γ U and U-Mo with these potentials.…”

Impurity migration and effects on vacancy formation enthalpy in polycrystalline depleted uranium

“…When alloyed with Zr or Mo, the bcc γ phase can become stable at lower temperatures. A stable γ phase is desired for reactor engineering and design due to its isotropic swelling behavior [4,5].…”