Spectroscopic Ellipsometry and White-Light Interferometry Investigation into Time-Dependent Oxidation Rates of Uranium in Pure Oxygen Yaakov Idell 1* , Wigbert Siekhaus 1 and William McLean II 1 1. Lawrence Livermore National Laboratory, Materials Science Division, Livermore, CA, USA. * Corresponding author: idell1@llnl.gov Material degradation of uranium resulting from the uranium-oxygen reaction has long been of great interest to the nuclear industry in hopes of solving long-term storage and disposal related concerns. Uranium reacts with oxygen as follows: +ቀ ଶା௫ ଶ ቁ ଶ →  ଶା௫ . At temperatures up to 200 °C, the oxide formed is a hyperstoichiometric uranium dioxide (UO2), with x in the range of 0.2 to 0.4 [1]; while at higher temperatures (>275 °C), U3O8 is formed in addition to the UO2. Initially, there is a thin protective oxide film present (<1 µm). The rate of oxidation for a thin oxide film follows a parabolic time-dependent growth rate that exhibits a growth rate behavior that is inversely proportional to the oxide thickness and is controlled by the diffusion of oxygen ions through the oxide [2]. During this period of oxide growth, elastic strain is gradually formed due to the oxide-metal density mismatch. At a certain oxide layer thickness, the strain reaches a critical level, and the oxide layer begins to crack. Upon the onset of cracking, the oxide growth is accelerated towards a linear or para-linear time dependence, which has been reported to occur at approximately 1 µm [3]. The oxidation kinetics for thick oxide films (>1 µm) is determined by both oxygen diffusion through a thin adherent oxide layer and the reaction of the outer surface of this oxide, which forms a porous unprotective outer layer [4]. It is not well understood precisely when this transition from a thin oxide to thick oxide occurs. Additionally, most of the previous studies regarding oxide growth in a uranium-oxygen system has been conducted with dry air [5]. Previous researchers have proposed because nitrogen does not react with uranium at low temperatures (<300°C); therefore, the uranium-dry air reaction is essentially the same as the uranium-oxygen reaction. We investigated the oxide layer kinetics through in-situ spectroscopic ellipsometry (SE) and white-light interferometry (WLI) to determine the thickness at which the time-dependent growth rates transition from parabolic to linear. Additionally, we report and discuss differences in the oxidation kinetics of uranium in a pure oxygen environment against the literature of dry air. The sample was inserted into an environmental cell that was capable of being heated up to 300 °C while flowing 99.95 % oxygen gas. Several experiments were conducted at different temperatures ranging from 60 °C to 185 °C in order to capture the transition thickness of the time-dependent growth rates from parabolic to linear. Prior to running in-situ SE measurements, it was necessary to experimentally determine the optical constants for uranium and UO2, which were determined to have excellent agreement with previously published values [6]. Figure 1 shows our experimentally determined oxidation growth rates where we observed only a parabolic growth rate. The activation energy of UO2 in pure O2 was determined to be 51.19 ± 4.25 ௞௃ ௠௢௟ with a rate equation of ln  = 9.637 − ଺ଵହ଻  . The associated oxide growth function in pure O2 was determined to be = 10.185 +  ఱభభఴళ.మ ఴ.యభర ቀ భ భబళశమ  భ ೅శమళయ ቁ ൫2.9229√ ൯. The initial data suggests that the parabolic-only oxidation growth rates might be associated with the characterization technique used. Previous studies have typically used x-ray diffraction or weight gain measurements to describe the oxide growth mechanics. These measurements are volume fraction 1550 doi:10.1017/S1431927619008481 Microsc. Microanal. 25 (Suppl 2), 2019 © Microscopy Society of America 2019 https://doi.org/10.1017/S1431927619008481 Downloaded from https://www.cambridge.org/core. IP address: 102.129.238.231, on 07 Aug 2019 at 10:14:36, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms.