Low-temperature oxidation behavior of fine UO2 powders in moisturized atmospheres

SCK•CEN Mentor

Leinders Gregory, gleinder@sckcen.be, +32 (0)14 33 31 63

Expert group

Fuel Materials

SCK•CEN Co-mentor

Delville Rémi, rdelvill@sckcen.be, +32 (0)14 33 31 65

Introduction

Globally the most used nuclear fuel material is uranium(IV) oxide (UO2) which is produced by powder metallurgical processing. Powders are first compacted into cylindrical pellets, and are subsequently sintered at high temperature to increase their density. UO2 powder shows a tendency to react with oxygen, especially if the grains have a high specific surface area. Uncontrolled uptake of oxygen is usually undesirable in the fuel production stage, for example, if oxidation can proceed continuously the higher oxide U3O8 is formed, and this crystallographic transformation is associated with a volume increase of about 36%. Such a volume increase can result in rupture of storage containers or vessels when not accounted for.

In recent years, active and successful research concerning the low-temperature (<300 °C) oxidation behavior of fine UO2 powders in dry oxidative atmospheres was conducted at SCKCEN [1-3]. By using high-end analytical equipment available in our laboratories, and techniques provided by synchrotron radiation facilities it was demonstrated that as a result of oxidation the sequential formation of higher oxides UO2+x, U4O9, U3O7, U3O8 and UO3 occurs. This process is, however, complicated by effects of particle size, oxidation temperature and type of gas atmosphere. Until now, only dry oxidative gasses were applied. However, the presence of moisture (H2O) may affect the oxidation process in ways that remain to be investigated and that are relevant for real-life applications. For this reason an experimental set-up which is capable to accurately humidify gasses was recently installed in our laboratories.

Objective

The goal of this Master's thesis will be to investigate the effects of oxidation under humid conditions in a variety of fine UO2 powders, at low temperatures. The candidate will need to prepare starting powders by applying both wet-chemical (dissolution, precipitation) and solid-state (drying, calcining) techniques. Characterization of these powders will consist of measuring specific surface area using the BET-method (N2 adsorption), assessing morphology via scanning electron microscopy (SEM), and analyzing the crystal structure via X-ray diffraction (XRD). The oxidation experiments will be performed and monitored in-situ using a simultaneous thermal analyzer (STA), which is capable to measure as a function of temperature changes in mass (thermogravimetry), reaction enthalpy (calorimetry), and evolved gasses (mass spectroscopy). All experimental work will be performed in the controlled areas of SCKCEN, providing safe manipulation of chemical and radioactive materials, and in laboratories equipped with the above mentioned analytical techniques.

 

References:

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

[2] G. Leinders, R. Delville, J. Pakarinen, T. Cardinaels, K. Binnemans, M. Verwerft, Inorg. Chem., 55 (2016) 9923.

[3] G. Leinders, R. Bes, J. Pakarinen, K. Kvashnina, M. Verwerft, Inorg. Chem., 56 (2017) 6784.

The minimum diploma level of the candidate needs to be

Academic bachelor

The candidate needs to have a background in

Chemistry , Physics