Phase behavior of oxidized Ce and Gd-doped (U,Zr)O₂

Re-criticality analysis of the fuel debris at the Fukushima Dai-ichi Nuclear Power Plant is the key step to ensure the safe retrieval and storage of the fuel debris. Knowledge of the amount and distribution of Pu and Gd within the fuel debris greatly contributes to such analysis as they directly affect the fission-chain reaction. However, little is known about how Pu-doped and Gd-doped (U,Zr)O₂ solid solutions oxidize and whether phases concentrated in Pu or Gd form. In this study, CeO₂ is used as a surrogate material for PuO₂ because of the similarities in their crystal structures and valence states. (U_0.9-xZr_0.1Ce_x)O₂ and (U_0.9-xZr_0.1Gd_x)O₂ solid solutions are prepared by sintering under an argon atmosphere and oxidized at 1073 K in air for 2 hours to simulate heavily oxidized fuel debris. Samples doped with 5 at% Ce and Gd contain only an orthorhombic-U₃O_8-x phase after oxidation, but its diffraction peaks' intensities decrease as the amount of dopant increases. The phase transformation of (U_0.9-xZr_0.1Gd_x)O₂, with further oxidation, is found to be cubic-(U,Zr,Gd)O_2+x → orthorhombic-(U,Zr,Gd)₃O_7±x → orthorhombic-(U,Zr,Gd)₃O_8-x. SEM/EDS analysis reveals that Ce and Gd are uniformly distributed in the (U_0.9-xZr_0.1RE_x)O₂ (RE = Ce, Gd) samples after oxidation. (author)