Aqueous vs. high-temperature syntheses of crystalline zirconia (ZrO2) containing Cm3+

The zirconia (ZrO2) crystal structure can incorporate a variety of metal cations with differing oxidation states up to high dopant loadings, which is why the material has been considered as a potential host phase for the immobilization of especially actinide elements present in specific high-level waste streams. Furthermore, zirconia is the main corrosion product of the Zircaloy cladding material surrounding nuclear fuel rods. The corrosion of Zircaloy and subsequent formation of zirconia already occurs during reactor operation and is expected to proceed during long-term disposal of the spent nuclear fuel (SNF) assemblies. Thus, during final storage, zirconia may play an important role as the first retention barrier for released radionuclides. ZrO2 is monoclinic phase at ambient conditions, and transforms into tetragonal and cubic phases at high temperatures of around 1200 °C and 2370 °C, respectively. However, particle size effects, the incorporation of foreign ions such as the actinides, as well as high radiation fields are known to also influence the stability fields of the polymorphs.
In the present work, the incorporation of the trivalent actinide curium in the pristine, monoclinic ZrO2 structure has been investigated following synthesis (i) in aqueous solution at 80°C for several weeks [1], and (ii) at high temperatures (1000°C, 5h) [2]. The evolution of the ZrO2 crystal structure during synthesis was analyzed with powder x-ray diffraction, while the Cm-environment was studied via luminescence spectroscopy. For the syntheses, a hydrous zirconia phase was precipitated in the presence of Cm from alkaline NaCl solutions at pH 12. The precipitate was thereafter either re-suspended in 0.5 M NaCl at pH 5 or pH 12 and hydrothermally treated at 80°C for up to 117 days, or calcined at 1000°C for 5 hours. The hydrothermal samples at pH 12, show crystallization of the amorphous ZrO(OH)2 phase to a mixture of monoclinic and tetragonal ZrO2 after 16 d at 80°C. In contrast, the samples at pH 5 show no crystallinity even after 32 days. Luminescence emission spectra indicate the presence of two Cm-environments in the amorphous precipitate. With increasing crystallinity, a bathochromic shift and a narrowing of the emission spectra can be seen. The shift is untypically large, resulting in emission peak maxima at around 650 nm for crystalline ZrO2. A similar, equally pronounced shift is obtained for Cm incorporated into the monoclinic ZrO2 structure following calcination. Therefore, the actinide speciation seems to be identical in the solid phases obtained with the two different synthesis methods, at least for the fully crystalline solids. These combined results imply that actinides are incorporated into the crystal structure of ZrO2, even at low concentrations where no structural transformations take place, which in turn speaks for zirconia as a good retention barrier for released trivalent actinides from the SNF matrix.