Composition and structure of fission product precipitates in irradiated oxide fuels: Correlation with phase studies in the Mo-Ru-Rh-Pd and BaO-UO2-ZrO2-MoO2 systems

Composition and crystal structure of fission product precipitates in irradiated oxide fuels were studied by X-ray microanalysis and X-ray diffraction using instruments shielded for α-contamination and β-γ-radiation. Pin cross-sections, fuel micro-samples from 300 μm hollow drillings and residues from the dissolution of irradiated material in HNO3 were investigated. The metallic phases found are hcp epsilon-Ru(Mo, Tc, Rh, Pd) solid solutions with broad variations in concentration of the components, bcc β-Mo(Tc, Ru) and fcc α-Pd(Ru, Rh). The dominating ceramic precipitate is composed of (Basub(1-x)Srsub(x)Cssub(y))(U, Pu, RE, Zr, Mo)O3 which crystallizes in the cubic perovskite type. The Mo fraction of these phases is related to the local oxygen potential of the fuels. The in-pile observed results agree well with phase studies in the quaternary Mo-Ru-Rh-Pd system where complete solid solubility exists between hcp Ru and the hexagonally stabilized Mo-Rh and Mo-Pd phases. Agreement is further attained with phase studies in the pseudoquaternary BaO-UO2-ZrO2-MoO2 system which is characterized by a cubic perovskite phase Ba(U, Zr, Mo)O3. (orig.)