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

Abstract

Results of investigations of the self diffusion in gamma uranium and metallic U-Mo alloys are presented. Calculations are performed using the method of atomistic modeling with the help of interatomic potentials based on the embedded atom model and its modifications. Proposed potentials are verified by cal culating thermodynamic and mechanical properties of uranium and U-Mo alloys. The formation energies of point defects and atomic diffusivities due to the diffusion of defects are calculated for gamma uranium and alloy containing 9 wt % molybdenum. Self diffusion coefficients of uranium and molybdenum are evaluated. Based on the data obtained, it has been concluded that the experimentally observed features of the self diffu sion in gamma uranium can be explained by the prevalence of the interstitial mechanism.

Formation mechanism of gas bubble superlattice in UMo metal fuels: Phase-field modeling investigation

“…The elastic interaction among gas bubbles and the cladding constraint in monolithic fuels might be important for gas bubble nucleation as well as growth kinetics. Thermodynamic and kinetic properties of UMo alloys and defects including Xe, vacancies, and U interstitials in bcc U single crystals have been investigated by experiments [25][26][27][28] and theoretical simulations with density functional theory (DFT) [29][30][31][32][33] and the molecular dynamics (MD) method [31,[34][35][36]. The results show that Xe substitution is stable; the dumbbell configurations of U interstitials along [100] and [110] are energetically favored; and the migration barrier of U interstitials is about 0.1 eV, which is much smaller than that of vacancies (0.5eV).…”

“…Generally speaking, the diffusion of vacancies, interstitials, and gas atoms and gas bubble evolution can be simulated by solving the Cahn-Hilliard and Allan-Cahn equations, which are similar to conventional phase-field models [43]. However, both experiments and MD simulations show that interstitials in UMo alloys have a much larger diffusivity than that of vacancies [28,36,55]. With the migration barriers, 0.1 eV for interstitials and 0.5 eV for vacancies in bcc UMo alloys, the diffusivities can be calculated, resulting in a diffusivity of interstitials about five orders of magnitude larger than that of vacancies at 400 K. The time step solving the diffusion equations of vacancies, interstitials, and Xe atoms can be estimated by…”

“…. Molecular dynamics method was used to study the U self-diffusion in  U and  UMo alloys [36,68] with EAM [35]and MEAM potentials [31]. The assessment of U self-diffusion data from experiments [28,55] and the atomistic simulations show that the migration barrier of vacancy and U interstitial are about 0.5 eV and 0.25 eV, respectively.…”

“…The assessment of U self-diffusion data from experiments [28,55] and the atomistic simulations show that the migration barrier of vacancy and U interstitial are about 0.5 eV and 0.25 eV, respectively. From the extrapolation of the self-diffusion coefficients [36] at low temperature T = 400 K, the interstitial diffusivity is approximately ) / ( 10 [8][9][10][11][14][15][16][17][18]. Although the experiments and MD simulations shows that the self-diffusivity of U interstitials in bcc UMo is much larger than that of vacancies there is very limited data of U interstitial mobility anisotropy in UMo alloys.…”

Atomistic simulations of thermodynamic properties of Xe gas bubbles in U10Mo fuels

“…The properties of defectsin bcc U single crystals have been investigated via experiments [4,[11][12][13], density functional theory (DFT) [14][15][16][17][18], and molecular dynamics (MD) method [1,16,19,20].The results show that 1) Xe is stableas a substitutional defect, and Xe migrates through vacancy-assisted mechanisms, presumably as a complex consisting of one Xe and two vacancies (XeV 2 complex), 2) the dumbbell configurations of U interstitials along the [100] or [110] direction are energetically favored, and 3) the migration barrier of U interstitials is about 0.1 eV, which is much smaller than that of vacancies (0.5eV).The defect generation and spatial distribution during cascades in bcc U were simulated using MD methods [21,22].The results show that most of the surviving interstitials form a [100] or [110] dumbbell.Almost all the interstitials remain isolated, while vacancies tend to cluster into polyhedral voids [22].The equation of state (EOS) of the Xe gas phase has been examined by experiments [23][24][25] and atomistic simulations [26]. Very recently, Xiao et al simulated the pressure inside Xe bubbles with different Xe concentrations in bcc U10Mo alloys [27].…”