Neptunium(V) and uranium(VI) sorption onto the zircaloy corrosion product zirconia (ZrO₂)

The interactions of long-lived radionuclides, such as uranium and the transuranium element
neptunium, with corroded phases in the near-field of a repository are crucial processes that
have to be taken into account in the safety assessment of a repository. Neptunium(V) and
uranium(VI) were chosen as representatives of pentavalent and hexavalent actinides,
respectively. Zirconia (ZrO₂) is the main corrosion product of the zircaloy cladding material of
spent nuclear fuel rods and constitutes as one of the first barriers encountered by mobilized
radionuclides. A comprehensive, multi-method approach was pursued to obtain a detailed
understanding of the Np(V)/U(VI) interactions at the zirconia-water interface. pH-dependent
batch sorption studies and isotherm experiments as well as spectroscopic techniques (EXAFS,
IR) were employed to gain information on the macroscopic and the molecular scale,
respectively. The derived information about Np(V)/U(VI) binding sites as well as number and
structure of the formed surface species were then used to constrain the parametrization of a
thermodynamic surface complexation model. The results of this work will contribute to more
reliable predictions about the environmental fate of Np(V)/U(VI) surrounding the near-field of
a repository.