The +2 oxidation state of Cr incorporated into the crystal lattice of UO2

Abstract

Doping by Cr is used to improve the performance of uranium dioxide (UO 2)-based nuclear fuel. However, the mechanism of structural incorporation of Cr remains unclear. Here, in order to understand this process on the atomic scale and the redox state of Cr in UO 2-based nuclear fuel, we performed intensive ab initio atomistic simulations of the Cr doped UO 2 matrix. We unexpectedly found that Cr in UO 2 exists as Cr 2+ species and not as the widely claimed Cr 3+. We re-evaluated previously published x-ray absorption near edge structure spectroscopy data and confirmed the computed redox state of Cr. Thermodynamic consideration shows that the favorable structural arrangement of Cr in UO 2 is given by a pair of associated Cr 2+ and oxygen vacancy. The realism of this doping mechanism is further demonstrated by a match to the measured maximum Cr solubility and small lattice contraction.

Chromium Doped UO2-Based Ceramics: Synthesis and Characterization of Model Materials for Modern Nuclear Fuels

“…At the same concentration from the present study, the experimentally determined Δ a / a derived from Rietveld refinements against XRD data of 0 and 1000 ppm Cr was −3.08 × 10 −3 for WCM −420 kJ/mol O 2 , –0.52 × 10 −3 for WCM −510 kJ/mol O 2 , −1.86 × 10 −3 for CPM −420 kJ/mol O 2 and −0.72 × 10 −3 for CPM −510 kJ/mol O 2 . Compared to the value reported by [ 24 ], the values were somewhat consistent for more reducing conditions, but an order of magnitude difference was observed for oxidizing. However as shown by this investigation, the solubility of Cr is oxygen potential dependent, such that varying the oxygen potential will result in variable amounts of Cr entering the matrix and subsequently effecting the lattice parameter to different degrees.…”

“…However as shown by this investigation, the solubility of Cr is oxygen potential dependent, such that varying the oxygen potential will result in variable amounts of Cr entering the matrix and subsequently effecting the lattice parameter to different degrees. Accordingly, it is difficult to corroborate the proposed structural redox scheme of Cr 2+ with U 5+ and an oxygen vacancy surrounding it by [ 24 ] via measurement of the lattice parameter due to its dependence on the oxygen potential. Subsequently, it is argued that for true determination of the structural and redox state within the matrix of Cr doped UO 2 , direct measurement of the crystal lattice ideally via grain or a single crystal samples is necessary.…”

“…Interest in the incorporation of Cr into UO 2 has recently expanded from that of applied SNF related to a more general inorganic structural-chemical related, as the mechanism for incorporation has been called into question by Sun et al (2020) [ 24 ]. It has been typically presumed that the use of Cr 2 O 3 coupled with its stable electronic configuration ([Ar]3 d 3 ) results in the trivalent state solely entering the ceramic, akin to the behavior of Ln 3+ in UO 2 .…”

“…It has been typically presumed that the use of Cr 2 O 3 coupled with its stable electronic configuration ([Ar]3 d 3 ) results in the trivalent state solely entering the ceramic, akin to the behavior of Ln 3+ in UO 2 . However, a recent investigation by [ 24 ] using ab initio simulations and re-evaluated XANES measurements by Mieszczynski et al (2014) [ 25 ] argued that instead of Cr 3+ within the UO 2 fuel matrix, Cr exists as Cr 2+ with U 5+ and an oxygen vacancy surrounding it. Further calculations by Cooper et al (2018) argue that under high temperatures, Cr can exist monovalently with excess oxygen vacancies facilitating this [ 26 ].…”

Electrode and Electrolyte Materials From Atomistic Simulations: Properties of LixFEPO4 Electrode and Zircon-Based Ionic Conductors

“…The first part in the square bracket reflects the vacancy migration probability contribution to the pre-exponential factor derived by Bukaemskiy et al (2021). The attempt frequency ν 0 was estimated from the computation of phonon spectra of the initial (IS) and transition states (TS), as (Beridze and Kowalski, 2014;Beridze et al, 2016;Kvashnina et al, 2018;Sun et al, 2020) we see strong dependence on the Fe redox state, with the U parameter for Fe(III) being ∼0.5 eV larger than for Fe(II). This is well-consistent with previous studies of FePO 4 and LiFePO 4 phases by Maxisch and Ceder (2006), who obtained 4.9 and 3.7 eV for both phases (taken as effective value, U eff = U − J), respectively.…”