In Situ Raman Oxidation of Undoped and Zr-Doped UO₂

<br/>Uranium dioxide is used as an analogue for used nuclear fuel (UNF) doped with small amounts of lanthanide and rare-earth elements to simulate fission products. Structural and thermodynamic data from these materials can be used to help predict the long-term behavior of UNF stored in geological repositories where elevated temperatures, oxidizing environments and eventual groundwater contact may accelerate UNF degradation.<br/>Raman spectroscopy is a particularly useful tool for studying the structures of uranium oxides and defects induced by processes like doping, irradiation, and oxidation.<br/>In this work, the Raman spectra of undoped UO₂ and Zr-doped UO₂ are presented from in-situ heating experiments performed in air at 250°C. After 1 hour, the undoped sample begins to show spectral features indicative of U₄O₉, whereas immediately after heating the Zr:UO₂ sample shows features of U₄O₉ and U₃O₈. U₃O₈ features were not observed in the undoped sample even after &gt;10 hours of heating. Accelerated oxidation of Zr:UO₂ compared to undoped UO₂ is consistent with previous work. Optical images of the Zr:UO₂ captured during ramping from 250°C to 400°C show accelerated oxidation along grain boundaries leading to cracking along grain boundaries and eventually intergranular cracking. Raman mapping and principal component analysis together indicate U₃O₈ forms along grain boundaries while grain interiors remain primarily U₄O₉. Additionally, using 455 nm Raman excitation proved most useful for in-situ experiments; 532 nm gave poor spectra from U₄O₉ and 785 nm gave poor spectra from U₃O₈.