Speciation and chemical behavior of molybdenum in uranium dioxide samples prepared by hydroxide precipitation

Abstract

This work aims to investigate the behavior of Mo in UO2±x for being an abundant fission product with a high fission yield and a complex speciation linked to its interaction with the fuel and other fission product elements. UO2-based model compounds containing different Mo contents (between 0 and 15 mol%) were synthesized by a wet-chemistry route using hydroxide precipitation. The recovered powders were converted to oxides, pelletized, and sintered to obtain densified pellets of UO2 incorporating Mo. PXRD analyses and Rietveld refinement calculations indicate that Mo has an almost negligible solubility in the fluorite structures of both UO2 and UO2+x samples. SEM, TEM, and EDX characterizations of the produced UO2 + Mo pellets revealed the homogeneous distribution of nanosized metallic Mo particles of spherical geometry inside and outside the UO2 grains and throughout the whole sample pellets, whatever the amount of Mo added, thus confirming that the solubility of Mo in the fluorite structure is way below 0.6 mol% Mo in accordance with the PXRD results. The microstructural properties of the UO2 + Mo pellets, including density, porosity, Mo particle size distribution, and UO2 grain size variation with Mo content, were also determined. The addition of Mo to UO2 reduced the UO2 grain size as compared to UO2 grains in pure pellets, and Mo thus plays an inhibiting role in the UO2 grain growth during sintering. The produced UO2 + Mo pellets exhibit a microstructure similar to that of the real spent nuclear fuel, except that the UO2 grains are smaller. The Mo metallic nanoparticles in these simplified UO2-based model compounds of controlled microstructure could be harnessed as surrogates of the Mo-rich ε-phase metallic nanoparticles in the real spent nuclear fuel for future studies.