Investigation of Np(V) sorption onto the zircaloy corrosion product ZrO₂


Investigation of Np(V) sorption onto the zircaloy corrosion product ZrO₂

Jessat, I.; Scheinost, A.; Roßberg, A.; Foerstendorf, H.; Jordan, N.

In a safety assessment of a high-level radioactive waste repository, interactions of radionuclides (e.g. neptunium(V)) with corroded phases in the near field of the repository have to be taken into account. The corrosion product of the zircaloy cladding material of spent nuclear fuel rods, namely zirconia (ZrO₂), potentially represents a first migration barrier towards mobilized radionuclide ions.
The interactions of Np(V) with monoclinic zirconia were studied at room temperature on a macroscopic and molecular scale. To gain comprehensive characteristics on the macroscopic level, batch sorption experiments were conducted to investigate the influence of different parameters (time, pH, Np(V) concentration and ionic strength). Np(V) showed increasing sorption to ZrO₂, starting from pH 3 and with a maximum uptake reached at pH ≥ 7. The Np(V) sorption is independent of ionic strength in the studied range (0.01 ‒ 0.1 M), indicating the formation of Np(V) inner-sphere sorption complexes on the ZrO₂ surface. Electrophoretic measurements further support this result by shifting the isoelectric point of ZrO₂ towards higher pH values in the presence of Np(V) compared to neat zirconia material.
Spectroscopic studies enable a deeper understanding of Np(V) sorption on a molecular scale. In situ Attenuated Total Reflection Fourier-transform Infrared spectroscopy potentially provides information on the number of sorption species, their denticity and the reversibility of the sorption process. From Extended X-ray Absorption Fine Structure spectroscopy access is given to the interatomic Np-Zr distances and the coordination numbers. Information gained on the macroscopic and molecular level will subsequently be used for surface complexation modelling (SCM) to parametrize a comprehensive description of the Np(V)-ZrO₂ system. This will contribute to a more reliable prediction of the environmental fate of neptunium(V).

Keywords: neptunium(V); sorption; zirconia; spectroscopy

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  • Lecture (Conference) (Online presentation)
    45th Scientific Basis for Nuclear Waste Management (SBNWM), 24.-29.10.2021, Cologne, Germany

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