Thermodynamic assessment of the uranium–oxygen system

Guéneau, C.; Baichi, Mehdi; Labroche, D.; Chatillon, C.; Sundman, Bo

doi:10.1016/s0022-3115(02)00878-4

Cited by 199 publications

(145 citation statements)

References 54 publications

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“…59 Using the LDA+U approximation, we calculated the formation energy of U 4 O 9 to be −1.4 eV. By comparing our calculated value with the experimental value (−1.8 eV) 50 , we derive a correction of −0.4 eV. Assuming that this correction is constant (measured per interstitial oxygen ion) for all UO 2+x compositions, which is reasonable if the correction is related to the O 2 molecule and filling of the O 2p orbitals, we can apply the same value to the calculated interstitial formation energy.…”

Section: Indirect Determination Using U4o9 Compoundsmentioning

confidence: 90%

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First-principles calculations of uranium diffusion in uranium dioxide

et al. 2012

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The present work reports first-principles DFT+U calculations of uranium self-diffusion in uranium dioxide (UO2), with a focus on comparing calculated activation energies to those determined from experiments. To calculate activation energies, we initially formulate a point defect model for UO2±x that is valid for small deviations from stoichiometry. We investigate five migration mechanisms and calculate the corresponding migration barriers using both the LDA+U and GGA+U approximations. These energy barriers are calculated using the occupation matrix control scheme that allows one to avoid the metastable states that exist in the DFT+U approximation. The lowest migration barrier is obtained for a vacancy mechanism along the 110 direction. This mechanism involves significant contribution from the oxygen sublattice, with several oxygen atoms being displaced from their original position. The 110 vacancy diffusion mechanism is predicted to have lower activation energy than any of the interstitial mechanisms and comparison to experimental data for stoichiometric UO2 also confirms this mechanism.

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Section: Indirect Determination Using U4o9 Compoundsmentioning

confidence: 90%

“…50 It should be noted that in this composition range, stoichiometric variations are dominated by clustering phenomena (hence the use of the term effective). Similarly, the lowering of the formation energy as function of increasing non-stoichiometry (x) follows from increased clustering.…”

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confidence: 99%

First-principles calculations of uranium diffusion in uranium dioxide

et al. 2012

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“…We therefore assume that uranium initially condenses and is incorporated into the glass as U(VI), the most highly ionized state observed in solid U-O phases [69,70]. Under this hypothesis, we might interpret Event 3 glass to have been quenched before equilibration between the Fe(II) and U(VI) could occur, while the glasses from Event 1 and Event 2 cooled over a slightly longer time, allowing some Fe(II) oxidation…”

Section: Discussionmentioning

confidence: 99%

Chemical speciation of U, Fe, and Pu in melt glass from nuclear weapons testing

Pacold

Lukens

Booth

et al. 2016

Journal of Applied Physics

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Nuclear weapons testing generates large volumes of glassy material that influence the transport of dispersed actinides in the environment and may carry information on the composition of the detonated device. We determine the oxidation state of U and Fe (which is known to buffer the oxidation state of actinide elements and to affect the redox state of groundwater) in samples of melt glass collected from three U.S. nuclear weapons tests. For selected samples, we also determine the coordination geometry of U and Fe, and we report the oxidation state of Pu from one melt glass sample. We find significant variations among the melt glass samples, and in particular, find a clear deviation in one sample from the expected buffering effect of Fe(II)/Fe(III) on the oxidation state of uranium. In the first direct measurement of Pu oxidation state in a nuclear test melt glass, we obtain a result consistent with existing literature that proposes Pu is primarily present as Pu(IV) in post-detonation material. In addition, our measurements imply that highly mobile U(VI) may be produced in significant quantities when melt glass is quenched rapidly following a nuclear detonation, though these products may remain immobile in the vitrified matrices. The observed differences in chemical state among the three samples show that redox conditions can vary dramatically across different nuclear test conditions. The local soil composition, associated device materials, and the rate of quenching are all likely to affect the final redox state of the glass. The resulting variations in glass chemistry are significant for understanding and interpreting debris chemistry, and the later environmental mobility of dispersed material.

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“…The chemical and physical properties of uranium oxides have been studied for many years using various techniques (Guéneau et al, 2002). Small changes in stoichiometry produce considerable variation in uranium oxides, where uranium atoms can be in more than one oxidation state.…”

Section: Introductionmentioning

confidence: 99%

Determination of U3O8 in UO2 by infrared spectroscopy

et al. 2017

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The oxygen-uranium (O-U) system has various oxides, such as UO 2 , U 4 O 9 , U 3 O 8 , and UO 3 . Uranium dioxide is the most important one because it is used as nuclear fuel in nuclear power plants. UO 2 can have a wide stoichiometric variation due to excess or deficiency of oxygen in its crystal lattice, which can cause significant modifications of its proprieties. O/U relation determination by gravimetry cannot differentiate a stoichiometric deviation from contents of other uranium oxides in UO 2 . The presence of other oxides in the manufacturing of UO 2 powder or sintered pellets is a critical factor. Fourier Transform Infrared Spectroscopy (FTIR) was used to identify U 3 O 8 in samples of UO 2 powder. UO 2 can be identified by bands at 340 cm -1 and 470 cm -1 , and U 3 O 8 and UO 3 by bands at 735 cm -1 , 910 cm -1 , respectively. The methodology for sample preparation for FTIR spectra acquisition is presented, as well as the calibration for quantitative measurement of U 3 O 8 in UO 2 . The content of U 3 O 8 in partially calcined samples of UO 2 powder was measured by FTIR with good agreement with X-rays diffractometry (XRD).

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Thermodynamic assessment of the uranium–oxygen system

Cited by 199 publications

References 54 publications

First-principles calculations of uranium diffusion in uranium dioxide

First-principles calculations of uranium diffusion in uranium dioxide

Chemical speciation of U, Fe, and Pu in melt glass from nuclear weapons testing

Determination of U3O8 in UO2 by infrared spectroscopy

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