Single Crystal Spectroscopic Studies of Cr doped UO₂—Resolving the Cr Valence and Chemical State of Cr in Chromia Doped Nuclear Fuels

As a part of modern nuclear fuels, the use of transition metal elements as dopants, such as Cr, has been shown to increase the in-reactor fuel performance over traditional non-doped variants. The small size of the Cr cation relative to the U⁴⁺ cation results in a low solubility of Cr into UO₂ of approximately 750-1000 ppm depending on the oxygen potential, where higher oxygen potential leads to improved solubility of Cr [1]. Despite this and other advances in the science behind Cr doped UO₂ modern nuclear fuels, significant paucities of information remain such as even the mechanism for incorporation. Pertinently prior to this investigation, there is no definitive conclusion as to the valence state and local environment of Cr within the fuel matrix [2],[3]. Notwithstanding this, we have managed to fabricate Cr single crystals and have studied these using a combination of single crystal X-ray diffraction, electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy (HERFD-XAS/EXAFS). Accordingly, we provide conclusive resolved measurements of the Cr chemical states within the UO₂ matrix, assigning both valence and local structure behaviour. We have further systematically examined the incorporation of Cr doping above and below the solubility limit, providing detailed observation of changes to the lattice parameters and microstructure via synchrotron X-ray powder diffraction measurements and electron microscopy respectively. The results of this investigation will be discussed in detail in linking the local atomic behaviour of Cr in UO₂ to microstructure behaviour in addition to the relevance they have to both modern fuels and spent nuclear fuel disposal.<br/><br/>1. Kegler, P., et al., <i>Chromium Doped UO2-Based Ceramics: Synthesis and Characterization of Model Materials for Modern Nuclear Fuels.</i> Materials, 2021. <b>14</b>(20): p. 6160.<br/>2. Cooper, M.W.D., C.R. Stanek, and D.A. Andersson, <i>The role of dopant charge state on defect chemistry and grain growth of doped UO2.</i> Acta Materialia, 2018. <b>150</b>: p. 403-413.<br/>3. Sun, M., J. Stackhouse, and P.M. Kowalski, <i>The+ 2 oxidation state of Cr incorporated into the crystal lattice of UO 2.</i> Communications Materials, 2020. <b>1</b>(1): p. 1-8.