Magnetite surfaces for radionuclide retention using DFT+U

In many countries, spent nuclear fuel and high-level radioactive waste from operation of nuclear power plants will be stored in deep geological repositories. Thick steel casks are used to prevent the release of radionuclides into the environment. Steel is known to slowly corrode under repository conditions forming mixed iron oxides e.g. magnetite (Fe3O4). It is expected that after a long corrosion period of several ten thousand years, the steel casks may breach and long-lived radionuclides (e.g. plutonium (Pu), technetium (Tc)) may get in contact with the steel oxidation products. Such interaction can result in the formation of surface complexes or incorporation of the radionuclides into the crustal structures. The pre-dominance of one or another mechanism will strongly affect the retention and transport of long-lived hazardous radionuclides. The mechanism of these reactions, especially the structural relationships at the solid-liquid interface and adsorption kinetic, are poorly understood at the atomistic scale. This project further investigates the redox sensitive uptake of Tc and Pu at the surface of magnetite under incorporation of these elements into the magnetite structure combining atomistic simulations and X-ray absorption spectroscopy1,2.
The necessary preparatory step in this study is the identification of dominant low index surfaces on nano-magnetite particles and their termination at the relevant conditions. The atomistic simulations based on the Kohn-Sham density functional theory (DFT) are performed using the open source CP2K code. For highly correlated chemical elements such as iron (Fe), a correction due to the highly localized 3d electrons is necessary and can be achieved by utilizing the DFT+U method. Initially, the Hubbard U parameter has been determined similar to earlier investigations3 by comparing the experimental cell constants and the band gap. The preferential magnetite orientation plane (111) with six different surface terminations are examined as function of oxygen or water fugacity.

1 R. Kirsch, D. Fellhauer, M. Altmaier, V. Neck, A. Rossberg, T. Fanghänel, L. Charlet and A. C. Scheinost, Environmental Science & Technology, 2011, 45, 7267–7274.
2 E. Yalç𝚤ntas, A. C. Scheinost, X. Gaona and M. Altmaier, Dalton Transactions, 2016, 45, 17874–17885.
3 A. Kéri, R. Dähn, M. Krack and S. V. Churakov, Environmental Science & Technology, 2017, 51, 10585–10594.