Oxidation and corrosion of UO₂ pellets under storage conditions

Host group: Fuel Materials (FMA)Expert Group, Belgian Nuclear Research Centre (SCK CEN), Industriezone Boeretang Zuid, Boeretang 190 - 2400 Mol – Belgium

Supervisors: Mr. Alejandro Vieyra Huerta and Dr. Gregory Leinders

Introduction

Uranium dioxide (UO₂) is the most used nuclear fuel in power reactors in the form of pellets, to which doping agents are sometimes added to improve its efficiency in the reactor (e.g., use of rare earth elements to increase reactor burnup) [1,2]. UO₂ can undergo oxidation, even at room temperature, resulting in the formation of U₃O₈, which is the most thermodynamically stable oxide [3]. The transformation of UO2 into U3O8 causes a volume expansion of about 36 vol.%, which can be problematic to maintain confinement in storage containers or recipients such as fuel rods. Therefore, to ensure safety during fabrication, interim storage or final disposal of UO2 fuel a deep understanding of the uranium–oxygen (U–O) system is required.

The oxidation behavior of UO2 powders has been extensively studied within the FMA expert group. Specifically, the effects of specific surface area and temperature on oxidation rates and mechanisms in dry, oxidizing atmosphere were analyzed [4,5]. Currently, the additional effect of moisture in the gas atmosphere is being investigated. Such research is relevant in the context of intermediate storage and final disposal of nuclear fuel, where scenarios include the eventual contact of water with fuel pellets.

Objectives

To investigate the oxidation and corrosion of UO₂ fuel pellets, with or without additional doping elements, under different external conditions (temperature, controlled moisturized and dry atmospheric conditions). The student will be enrolled in the Fuel Labs of SCK CEN and carry out powder metallurgical synthesis steps, including mixing of UO2 powders, compaction of powders into cylindrical compacts, and high temperature sintering to achieve dense pellets. Selected pellets will be subjected to oxidation experiments, either in bulk, or using in-situ thermogravimetric analysis. Characterization will include techniques such as X-ray diffraction and optical or electron microscopy, allowing to analyze microstructural changes resulting from oxidation or corrosion reactions at the surface.

References

[1] N. Rodríguez-Villagra, S. Fernández-Carretero, A. Milena-Pérez, L.J. Bonales, L. Gutiérrez, J. Cobos, H. Galán, J. Nucl. Mater., 606 (2025) 155635.

[2] S. García-Gómez, J. Giménez, I. Casas, J. Llorca, J. De Pablo, Appl. Surf. Sci., 629 (2023) 157429.

[3] P. Taylor, D.D. Wood, A.M. Duclos, D.G. Owen, J. Nucl. Mater., 168 (1989) 70.

[4] G. Leinders, J. Pakarinen, R. Delville, T. Cardinaels, K. Binnemans, M. Verwerft, Inorg. Chem., 55 (2016) 3915.

[5] G. Leinders, T. Cardinaels, K. Binnemans, M. Verwerft, Inorg. Chem., 57 (2018) 4196.