Investigation of the effect of the microstructure upon the oxidative long-term dissolution of uranium dioxide: a fundamental approach including synthesis and corrosion of simplified model systems as well as electron microscopy and atomistic simulation
Mechanical Properties of Metallic Materials and their Microstructural Origins
Mineralogy, Petrology and Geochemistry
Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
Final Report Abstract
The project explored the influence of lanthanoid doping and microstructure on the oxidative corrosion of UO₂-based model systems, contributing to the safety research of spent nuclear fuels. The study focused on the hypothesis that grain boundaries play a crucial role in overall corrosion. Doped UO₂ ceramics were produced and analysed for relationships between microstructure, doping, and chemical stability. Grain boundaries and grain orientations were examined using conventional scanning electron microscopy (SEM), elemental micro analysis (energy dispersive X-ray spectroscopy, EDS), and electron backscatter diffraction (EBSD) to assess their impact on dissolution. Advanced transmission electron microscopy (TEM) was employed for high-resolution detailed analyses. The project successfully produced samples with up to 99% theoretical density; the porosity level decreased with increasing doping levels for both Nd- and Gd-doping. Analysis of grain size distributions indicated a narrowing only at the highest doping levels (8 at.% relative to U), while lower doping levels had negligible effects on the microstructure. EBSD investigations revealed typical cubic system orientations in all samples, with consistent grain boundary types and statistical distributions. Special coherent (CSL) grain boundaries of pure UO₂, relevant for simulations, were found to exhibit coherence only over regions less than 100 nm. Random high-angle grain boundaries showed uranium deficiency in both pure and doped samples, with additional Nd enrichment at these boundaries in doped specimens. Accelerated dissolution experiments in H2O2 were run for 336 hours. For pure UO2, a plateau in uranium concentration was reached after 96 hours, whereas all doped samples reached the plateau after just 24 hours. The degree of dissolution decreased with increasing doping, with Nd-doped samples demonstrating higher stability compared to Gd-doped ones. Electron microscopic observations were able to establish a relationship between the reactivity of grain boundaries, surfaces, and the level of doping. Atomistic and continuum simulations were carried out to determine the point-defect chemistry of grain boundaries in UO2. Since UO2 typically exhibits oxygen excess, two defect models were considered: U5+ cations charge compensated by (i) oxygen interstitials or (ii) uranium vacancies. Atomistic calculations yielded the segregation energies of these defects to a specific grain boundary; the resulting data were used as input in a continuum model of defect concentrations in the grain-boundary region. Based on the results, a mechanism for the preferential oxidative corrosion of grain boundaries was proposed: U5+ cations are enriched in, and near to, the grain boundaries. Furthermore, a means of mitigating preferential corrosion of grain boundaries was suggested, involving the doping of UO2 with acceptor cations.
Publications
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Accelerated Dissolution of Doped UO2 -Based Model Systems as Analogues for Modern Spent Nuclear Fuel Under Repository Conditions, MRS Fall Meeting 2022 - 46th Symposium on Scientific Basis for Nuclear Waste Management, SBNWM2022, Boston, USA. (Conference Presentation)
Kegler, P., Klinkenberg, M., Bukaemskiy, A., Thümmler, R., Deissmann, G., Brandt, F. & Bosbach, D.
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Detailed investigation of the microstructure of UO2 based model systems for spent nuclear fuel, MRS Fall Meeting & Exhibit, - 46th Symposium on Scientific Basis for Nuclear Waste Management, SBNWM2022, Boston, USA, 2022 (Conference Presentation)
Thümmler, R., Barthel, J., Klinkenberg, M., Wolf, M., Kegler, P., De Souza, R., Mayer, J., Bosbach, D. & Brandt, F.
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UO2 based model systems for spent nuclear fuel: microstructure and oxidative dissolution, 31st Spent Fuel Workshop, Barcelona, Spain, 2022 (Presentation)
Thümmler, R., Klinkenberg, M., Barthel, J., Kegler, P., Mayer, J., Bosbach, D. & Brandt, F.
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UO2 based model systems for spent nuclear fuel: microstructure and oxidative dissolution, Helmholtz Energy Young Scientist Workshop, Frankfurt, Germany, 2022 (Poster)
Thümmler, R., Klinkenberg, M., Barthel, J., Kegler, P., Mayer, J., Bosbach, D. & Brandt, F.
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Deconvoluting Cr states in Cr-doped UO2 nuclear fuels via bulk and single crystal spectroscopic studies. Nature Communications, 14(1).
Murphy, Gabriel L.; Gericke, Robert; Gilson, Sara; Bazarkina, Elena F.; Rossberg, André; Kaden, Peter; Thümmler, Robert; Klinkenberg, Martina; Henkes, Maximilian; Kegler, Philip; Svitlyk, Volodymyr; Marquardt, Julien; Lender, Theresa; Hennig, Christoph; Kvashnina, Kristina O. & Huittinen, Nina
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Research for the Safe Management of Nuclear Waste: Microscopic Approaches, Helmholtz Energy Conference 2023, Koblenz, Germany. (Conference Presentation)
Brandt, F., Klinkenberg, M., Thümmler, R., Barthel, J. & Bosbach, D.
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The effect of Nd and Gd doping on the microstructure of UO2-based model systems for spent nuclear fuel, Scientific Basis of Nuclear Waste Management, SBNWM 2023, Cologne, Germany, (Conference Presentation)
Thümmler, R., Barthel, J., Klinkenberg, M., Kegler, P., Mayer, J., Bosbach, D. & Brandt, F.
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High resolution grain boundary investigations of UO2 based model systems for spent nuclear fuel. Goldschmidt2024 abstracts. Geochemical Society.
Klinkenberg, Martina; Barthel, Juri; Mayer, Joachim; Thümmler, Robert; Bukaemskiy, Andrey; Brandt, Felix; Kegler, Philip & Bosbach, Dirk
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Synthesis and detailed investigation of the microstructure of Nd- or Gd-doped UO2 based model systems for spent nuclear fuel. Goldschmidt2024 abstracts. Geochemical Society.
Brandt, Felix; Kegler, Philip; Bosbach, Dirk; Barthel, Juri; Mayer, Joachim; Thümmler, Robert; Bukaemskiy, Andrey & Klinkenberg, Martina
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Grain-Boundary Corrosion in UO2+δ from a Defect Chemical Perspective: A Case Study of the Σ5(310)[001] Grain Boundary. ACS Applied Materials & Interfaces, 17(5), 7906-7915.
Wolf, Matthew J.; Usler, Adrian L. & De, Souza Roger A.
