Role of di-interstitial clusters in oxygen transport in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mtext>UO</mml:mtext></mml:mrow><mml:mrow><mml:mn>2</mml:mn><mml:mo>+</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>from first principles

Andersson, David; Watanabe, Taku; Deo, Chaitanya; Uberuaga, Blas P.

doi:10.1103/physrevb.80.060101

Cited by 58 publications

(82 citation statements)

References 41 publications

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“…OFT tends to agree with OFT +U in reproducing trends, but the quantitative details differ 6 [16]. Similarly here, the difference in energy between the two phases might change if calculated with DFT+V.…”

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confidence: 77%

Stress-induced phase transformation in nanocrystalline UO2

Desai

Uberuaga

2009

Scripta Materialia

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We report a stress-induced phase transfonnation in stoichiometric U02 from fluorite to a-Pb02 structure using molecular dynamics (MD) simulations and density functional theory (DFT) calculations. MD simulations, performed on nanocrystalline microstructure under constant-stress tensile loading conditions, reveal a heterogeneous nucleation of a Pb02 phase at the grain boundaries followed by the growth of this phase towards the interior of the grain. The DFT calculations confinn the existence of the a-Pb02 structure, showing that it is energetically favored under tensile loading conditions. Uranium dioxide (U02) is one of the most widely used materials for nuclear fuel because it is easy to fabricate and is highly stable under intense heat and radiation conditions in the reactor core. It has a high melting temperature (~3100 K) [1,2] but poor thermal conductivity. In the reactor, as heat is removed from the exterior regions of the U02 pellet via a coolant fluid, the temperature in the interior remains comparatively high, leading to large thermal gradients and a complex stress state across the fuel pellet.In addition, at high burnups and in presence of fission products, voids, and gas bubbles, the U02 matrix is likely to be under high stress. However, little is known about any stress-induced structural transformations U~. Experiments and simulations have confirmed that U0 2 , which is primarily found in the fluorite structure, transforms to the cotunnite structure under compression at 69 GPa [3]. However, to the best of our knowledge, there has been no examination of structural transformations under tension. In this work, we use two different computational techniques -molecular dynamics (MD) and density functional theory (DFT) -to study the structural changes of U0 2 under high stress tensile loading conditions. We find that U02 shows a stress-induced phase transformation from fluorite (space group 225) to the a-Pb02 structure (space group 60).The MD simulations were performed on idealized, nanocrystalline model structures of stoichiometric U0 2 • Under constant-stress tensile loading conditions of 4.5 GPa and at a temperature T 800 K, we observe the heterogeneous nucleation of U02 in the a-Pb02 structure at the grain boundaries (GBs) with the subsequent growth of this phase towards the grain interior. Newton's equation using a 5 th order predictor-corrector algorithm with a time step of M = 0.5xlO-15 s. In order to obtain thermally equilibrated GBs at the temperature of interest, the unrelaxed zero-temperature structure is gradually heated up to 3000 K, slowly cooled down in a step-wise fashion, and finally annealed at T 800 K for 7 ns. The temperature is maintained using a velocity-rescaling thermostat.In order to induce a tensile loading condition, a constant stress of 4.5 GPa is applied in the z-direction (see Fig. 2), which is normal to the columnar axis, by using the Parrinello-Rahman constant-stress technique [8]. Figure 1 illustrates the observed tensile the loading direction as a LUHvUUll During...

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confidence: 77%

Stress-induced phase transformation in nanocrystalline UO2

Desai

Uberuaga

2009

Scripta Materialia

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“…In order to simplify the calculation of defect properties the latter two contributions are ignored in the present study. Recent reports have shown that the LDA/GGA+U methodology correctly describes many of the relevant properties of UO 2 and UO 2+x 42,[44][45][46][47][48][49][50][51][52][53][54][55][56] . In accordance with earlier LDA+U studies the U and J values were set to U = 4.5 eV and J = 0.51 eV 46 .…”

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confidence: 99%

U and Xe transport in UO2±x: Density functional theory calculations

et al. 2011

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The detrimental effects of the fission gas Xe on the performance of oxide nuclear fuels are well known. However, less well known are the mechanisms that govern fission gas evolution. Here, in order to better understand bulk Xe behavior (diffusion mechanisms) in UO2±x we calculate the relevant activation energies using density functional theory (DFT) techniques. By analyzing a combination of Xe solution thermodynamics, migration barriers and the interaction of dissolved Xe atoms with U, we demonstrate that Xe diffusion predominantly occurs via a vacancy-mediated mechanism. Since Xe transport is closely related to diffusion of U vacancies, we have also studied the activation energy for this process. In order to best reproduce experimental data for the Xe and U activation energies, it is critical to consider the active charge-compensation mechanism for intrinsic defects in UO2±x. Due to the high thermodynamic cost of reducing U 4+ ions, any defect formation occurring at a fixed composition, i.e. no change in UO2±x stoichiometry, always avoids such reactions, which, for example, implies that the ground-state configuration of an O Frenkel pair in UO2 does not involve any explicit local reduction (oxidation) of U ions at the O vacancy (interstitial).

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“…The effects of chemical environment on the ground states and excitation spectra of f -electrons are particularly interesting, since they are responsible for splitting the otherwise 2J + 1-fold degenerate free-ion ground state 2S+1 L J , giving rise to rich physics and applications. Recently, actinide dioxides in the cubic fluorite structure have attracted renewed theoretical interest in the context of their use as nuclear fuels [1][2][3][4][5][6][7][8][9][10] . The crystal field (CF) method is a well established tool for describing the ligand environment of localized electrons.…”

Section: Introductionmentioning

confidence: 99%

Self-consistent density functional calculations of the crystal field levels in lanthanide and actinide dioxides

Zhou

Ozoliņš

2012

Phys. Rev. B

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Using a recently developed method combining a nonspherical self-interaction corrected LDA+U scheme and an on-site multi-body Hamiltonian [Phys. Rev. B 83, 085106 (2011)], we calculate the crystal field parameters and crystal field (CF) excitation levels of f -element dioxides in the fluorite structure with f n electronic configurations, including n = 1 (PaO2, PrO2), n = 2 (UO2), n = 3 (NpO2), and n = 4 (PuO2). It is shown that good agreement with experimental data (within approximately 10 to 20 meV) can be obtained in all cases. The properties of the multi-electron CF ground states are analyzed.

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Role of di-interstitial clusters in oxygen transport inUO2+xfrom first principles

Cited by 58 publications

References 41 publications

Stress-induced phase transformation in nanocrystalline UO2

Stress-induced phase transformation in nanocrystalline UO2

U and Xe transport in UO2±x: Density functional theory calculations

Self-consistent density functional calculations of the crystal field levels in lanthanide and actinide dioxides

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