Maria A. Okuniewski

First principles calculations for defects in U

Uranium (U) exhibits a high temperature body-centered cubic (bcc) allotrope that is often stabilized by alloying with transition metals such as Zr, Mo, and Nb for technological applications. One such application involves U-Zr as nuclear fuel, where radiation damage and diffusion (processes heavily dependent on point defects) are of vital importance. Several systems of U are examined within a density functional theory framework utilizing projector augmented wave pseudopotentials. Two separate generalized gradient approximations of the exchange-correlation are used to calculate defect properties and are compared. The bulk modulus, the lattice constant, and the Birch-Murnaghan equation of state for the defect free bcc uranium allotrope are calculated. Defect parameters calculated include energies of formation of vacancies in the α and γ allotropes, as well as self-interstitials, Zr interstitials, and Zr substitutional defects for the γ allotrope. The results for vacancies agree very well with experimental and previous computational studies. The most probable self-interstitial site in γ-U is the (110) dumbbell, and the most probable defect location for dilute Zr in γ-U is the substitutional site. This is the first detailed study of self-defects in the bcc allotrope of U and also the first comprehensive study of dilute Zr defects in γ-U.

Atomistic properties of γ uranium

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.

Molecular dynamics simulations of void and helium bubble stability in amorphous silicon during heavy-ion bombardment

A study of void and helium (He) bubble stability in amorphous silicon (a-Si) subjected to heavy-ion bombardment was conducted with molecular dynamics simulations. The effects of incident ion energy, incident ion direction, and He pressure were investigated. He bubbles with pressures equal to or greater than 0.1kbar were found to be stable during isotropic 2keV xenon (Xe) irradiation. Bubbles with pressures below this limit collapsed completely. On the other hand, voids and bubbles of all pressures were stable following unidirectional 2keV Xe bombardment. In this case, the voids and bubbles became elongated and resisted closure, a phenomenon attributed to the inability of liquid Si to wet the flat, low-curvature internal surfaces of the open-volume defect. The void closure rates varied from 55 to 180A∕dpa as the Xe projectile energy increased from 0.2keV to 2keV, respectively. An analytical model based on a viscous flow mechanism is presented to describe the behavior associated with the slowest closure rat...

Small-angle X-ray scattering measurements of helium-bubble formation in borosilicate glass

Small-angle X-ray scattering (SAXS) measurements have been performed to study helium-bubble formation in borosilicate glass. Helium was introduced by He+ implantation over an energy range of 1 to 2 MeV to give a uniform distribution over ∼1 µm depth. The implanted dose was varied from 9 × 1013 to 2.8 × 1016 ions cm−2, corresponding to a local concentration range of 40 to 11200 atomic parts per million (a.p.p.m.) averaged over the implantation depth. The SAXS response was fit with the Percus–Yevick hard-sphere interaction potential to account for interparticle interference. The fits yield helium-bubble radii and helium-bubble volume fractions that vary from 5 to 15 A and from 10−3 to 10−1, respectively, as the dose increased from 9 × 1013 to 2.8 × 1016 cm−2. The SAXS data are also consistent with maximum helium solubility with respect to bubble formation between 40 and 200 a.p.p.m. in the borosilicate glass matrix.

Vacancy Formation Enthalpy in Polycrystalline Depleted Uranium

Positron Annihilation Spectroscopy was performed as a function of temperature and beam energy on polycrystalline depleted uranium (DU) foil. Samples were run with varying heat profiles all starting at room temperature. While collecting Doppler-Broadening data, the temperature of the sample was cycled several times. The first heat cycle shows an increasing S-parameter near temperatures of 400K to 500K much lower than the first phase transition of 941K indicating increasing vacancies possibly due to oxygen diffusion from the bulk to the surface. Vacancy formation enthalpies were calculated fitting a model to the data to be 1.6± 0.16 eV. Results are compared to previous work [3,4].

Kinetics of the migration and clustering of extrinsic gas in bcc metals

We study the mechanisms by which gas atoms such as helium and hydrogen diffuse and interact with other defects in bcc metals and investigate the effect of these mechanisms on the nucleation of embryonic gas bubbles. Large quantities of helium and hydrogen are produced due to spallation and transmutation in structural materials in fusion and accelerator-driven reactors. The long time evolution of the extrinsic gas atoms and their accumulation at vacancies is studied using a kinetic Monte Carlo algorithm that is parameterized by the migration energies of the point defect entities. First-order reaction kinetics are observed when gas clusters with vacancies. If gas-gas clustering is allowed, mixed-order diffusion limited kinetics are observed. When dissociation of gas from clusters is allowed, gas-vacancy clusters survive to steady state while gas-gas clusters dissolve. We obtain cluster size distributions and reaction rate constants that can be used to quantify microstructural evolution of the irradiated metal.

Microstructural Evolution of Monolithic Fuel Foils During Processing

Residual stresses are expected in monolithic, aluminum clad uranium 10 weight percent molybdenum (U-10Mo) nuclear fuel plates because of the large mismatch in thermal expansion between the two bonded materials. Previous high energy x-ray diffraction measurements successfully profiled the residual stresses in the U-10Mo, but were unable to probe either the Al cladding or the 15micron Zr diffusion prevention barrier due to poor grain statistics. Neutron diffraction, with its inherently more divergent incident be alleviates this problem and, moreover, allowed for the determination of the dislocation density and texture in all three phases. Several samples were examined as a function of processing step and the phase stresses, dislocation density and texture are monitored with respect to the processing conditions.