PhD thesis

Actinide incorporation in Zr-bearing phases: spectroscopic investigations of actinide-zirconia solid solutions.

Manuel Eibl
Prof. Dr. Thorsten Stumpf, Dr. Nina Huittinen (HZDR)
Surface Processes

Zirconium(IV) oxide is a corrosion product of the zircalloy cladding material surrounding nuclear fuel rods. In a final repository for spent nuclear fuel (SNF) where water has come into contact with the UO2 ceramic and surrounding fuel rods, dissolution of zircalloy and the formation of zirconia on the cladding surface may be accompanied by reactions with dissolved radionuclides from the SNF matrix. Actinide incorporation investigations in the corrosion and radiation resistant ZrO2 matrix have previously been conducted from a radionuclide conditioning point of view, i.e. where radionuclide immobilization within the zirconia ceramic is accomplished in high-temperature calcination or sintering procedures aiming at the long-term storage of radionuclides within the ceramic.
Building on these investigations the incorporation under SNF repository conditions where dissolution, coprecipitation and subsequent actinide incorporation reactions in zirconia occur in solution under slightly elevated temperatures will be investigated in this thesis which is why hydrothermal as well as high temperature synthesis will be employed.

ZrO2 has a monoclinic structure at ambient conditions but various metal ions such as Y3+, Ca2+, lanthanide and actinide ions can stabilize the oxide in its high temperature phases. Therefore the influence of synthesis conditions as well as the metal ion concentration on the zirconia structure and the nucleation process will be investigated using a combination of PXRD for phase determination and spectroscopic techniques to receive information on the local environment of the incorporated actinides. Incorporation of trivalent actinide dopants such as Cm3+ or their lanthanide analogues (e.g. Eu3+) will be investigated using time-resolved laser fluorescence spectroscopy (TRLFS). For the investigations with tetravalent actinides (Th4+, U4+ und Pu4+)x-ray absorption spectroscopy (XAS) will be complemented with TRLFS (U4+) and/or IR (oxidized forms of U and Pu) techniques. Finally, the structure and redox stability of the actinide-bearing zirconia phases will be investigated over time, as well as the stability of the synthesized phases under alpha particle irradiation.

The thesis will be done in cooperation with the University of Manchester.