Oxidation behaviour of uranium and neptunium in stabilised zirconia |
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| Authors: | Marcus Walter Joseph Somers Boris Brendebach |
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| Affiliation: | a European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, D-76125 Karlsruhe, Germany
b Forschungszentrum Karlsruhe, Institut für Nukleare Entsorgung (INE), P.O. Box 3640, D-76021 Karlsruhe, Germany |
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| Abstract: | Yttria stabilised zirconia (YSZ) based (Zr,Y,U)O2−x and (Zr,Y,Np)O2−x solid solutions with 6 and 20 mol% actinide were prepared with Y/Zr ratios ranging from 0.2 to 2.0 to investigate uranium and neptunium oxidation behaviour depending on the oxygen vacancies in the defect fluorite lattice. Sintering at 1600 °C in Ar/H2 yields a cubic, fluorite-type structure with U(IV) and Np(IV). Annealing (Zr,Y,U)O2−x with Y/Zr=0.2 at 800 °C in air results in a tetragonal phase, whereas (Zr,Y,U)O2−x with higher Y/Zr ratios and (Zr,Y,Np)O2−x retain the cubic structure. XANES and O/M measurements indicate mixed U(V)-U(VI) and Np(IV)-Np(V) oxidation states after oxidation. Based on X-ray diffraction, O/M and EXAFS measurements, different oxidation mechanisms are identified for U- and Np-doped stabilised zirconia. In contrast to U, excess oxygen vacancies are needed to oxidise Np in (Zr,Y,Np)O2−x as the oxidation process competes with Zr for oxygen vacancies. As a consequence, U(VI) and Np(V) can only be obtained in stabilised zirconia with Y/Zr=1 but not in YSZ with Y/Zr=0.2. |
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| Keywords: | Actinide Transmutation Nuclear waste disposal Conditioning Zirconia Pyrochlore EXAFS |
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