Reflectance spectroscopy in analysis of UO2 scale: derivation of a kinetic model of uranium oxidation

Abstract

In this study, we analyzed the development of a compact oxide scale built in course of Uranium surface oxidation. The process was monitored by an in-situ acquisition of the reflectance interference peaks in the NIR-MIR. Dielectric properties of the growing oxide scale were derived in accord to the oscillator model. We used effective media approach to simulate heterogeneous dielectric content in the oxide-metal interface. Following dielectric parameterization, structural properties (e.g., scale thickness) of the proposed multi-scale scheme were calculated. As scale's growth process quantified, a valid kinetic model was proposed. Analysis showed that oxidation dynamics is governed by a multi-parabolic, true diffusion-limited mechanism of activation energy conveniently equaling the known anion diffusion enthalpy of 26 kcal/mol. The applied kinetic model suggested a setup of two consecutive oxide scales, characterized by differing anion diffusion rates. Though mathematical formalism presented a similar to the paralinear, time-dependent solution, here, in contrast to the classic paralinear assumption, both scales consisted of a compact, diffusion limited oxide barriers. As a result, the difference in anion flow across the outer and inner scale barriers assigned the overall, pseudo-linear rate constant-kl, of a negative (in contrast to the paralinear approach) value. Next, Uranium oxidation has been studied in the post-elastic domain. Markedly, upon breakaway of the compact oxide scale, classic paralinear behavior was reestablished for scale thickness of > or = 0.5 microm.

On the redox reactivity of doped UO2 pellets – Influence of dopants on the H2O2 decomposition mechanism

“…The activation energy for oxidation of UO 2 by O 2 is reported to vary between 67 and 126 kJ mol À1 . These data refer to the dry oxidation of UO 2 by oxygen [29]. Recently a quantum chemical study revealed the important role of the presence of water on the surface of UO 2 in what concerns the oxidation of this material [30].…”

Analytical Techniques to Characterize Scales and Deposits

“…In laboratory investigations of scaling phenomenon, reflectance FTIR can be used to follow the kinetics of scale growth, for example in the corrosion of uranium surfaces [30], but in situ monitoring of industrial processes has not been implemented. In laboratory investigations of scaling phenomenon, reflectance FTIR can be used to follow the kinetics of scale growth, for example in the corrosion of uranium surfaces [30], but in situ monitoring of industrial processes has not been implemented.…”

Ionic Diffusion Oxidation Model of Uranium

“…Although many researchers have empirically fit growth models to measured uranium oxidation behavior, none have modeled uranium oxidation using the basic physics of oxidation and known material properties. The dominant mechanism for oxidation of uranium at low temperatures and small oxide thicknesses is the motion of the ions through the generated electric field between the adsorbed oxygen ions at the surface and metal ions at the oxide–metal interface . Although a well‐established mechanism, theory‐driven models for the oxidation of uranium have not been formulated.…”

“…Although a well‐established mechanism, theory‐driven models for the oxidation of uranium have not been formulated. Until now only empirical oxidation models have been developed for uranium …”

“…Although most of these models are formed assuming that the metal diffuses through the oxide rather than the oxygen, they can typically be applied to both cases. Although these theoretical models have been developed for oxidation in general and multiple researchers point to the Cabrera–Mott theory as the driving mechanism for early oxidation of uranium, none have been directly applied to uranium oxidation.…”