XAFS and XRD studies on tetravalent actinides in zirconia- and zircon-based ceramics

Ceramic material is under discussion to be an alternative to borosilicate glass for the immobilization of nuclear waste. The corrosion resistance of ceramic material can increase over several magnitudes compared to glass. A homogenous mixture of the actinide and the ceramics precursor is essential to supress phase segregation which may weaken the dissolution resistance under the conditions of a nuclear waste repository. We investigated different sol-gel preparation routes of zirconia (ZrO2) and zircon (ZrSiO4) based ceramics.
Laboratory studies were accompanied by X-ray absorption spectroscopy (XAS) performed at the Rossendorf Beamline (ROBL). The beamline has broadened its experimental capacities with a 6-circle diffractometer which was used in this study for powder X-ray diffraction (XRD) experiments. Among the synthesis attempts, an acetate-base route seems to support the homogeneity of the precursor for zirconia-based ceramics, most likely because An(IV) and Zr(IV) show the same complexes in the sols [1,2]. X-ray absorption spectroscopy reveals that acetate supported An(IV) clusters are structurally very close to the zirconia structure units [3] and remain obviously intact when entering the zirconia ceramics. X-ray powder diffraction measurements show that this synthesis route reduces phase segregation in zirconia ceramics during thermal treatment. The host lattice needs a certain flexibility to tolerate the introduction of An(IV) ions. However, zirconia has a rigid structure which limits the intercalation of An(IV). The synthesis of zircon-based ceramics is faced with the problem that ZrSiO4 has no extended phase range in the ZrO2-SiO2 system and appears therefore in equilibrium with one of the limiting species. However, zircon shows a large structural flexibility. Therefore, zircon-based ceramics can be synthesized in a way that it forms unlimited solid solutions with An(IV).