Integrated study of experiment and first-principles computation for the characterization of a corium type ZrO₈ complex in a Zr-doped fluorite UO₂

We, for the first time, observe ZrO₈ complex in Zr-doped UO₂, which is a corium structure, using experimental characterization integrated with first-principle computational validation. Atomic level structure of U_1−yZr_yO₂ pellets (y = 0.01, 0.03, 0.05, and 0.1) is identified using Raman spectroscopy measurement and X-ray diffraction pattern analysis. The lattice constants shrink with increasing Zr doping levels, which consistently represents in the positive shift of T_2g Raman vibration peak around 445 cm⁻¹. More interestingly, conventionally unknown new Raman peak appears around 598 cm⁻¹, which has not been observed in neither a pure ZrO₂ nor UO₂ doped with tetravalent elements other than Zr. We unveil that the new peak originates from ZrO₈-type complex formed on the fluorite UO₂. Our study provides precise understanding on the formation mechanisms and material properties of the corium in the hypervalent oxide.