Low-temperature oxidation of fine UO2 powders

Globally the most used nuclear fuel material is uranium(IV) oxide (UO2) and it is produced by powder metallurgical processing. Powders are first compacted into cylindrical pellets, which are subsequently sintered at high temperature to increase their density. The UO2 nuclear fuel cycle is also inseparable from wet-chemical routes: uranium is leached from ore bodies and is further purified via, for example, ion exchange or solvent extraction techniques. Additionally, used nuclear fuel can be reprocessed to separate the elements which contain fissile nuclides (U, Pu) from the fission products. Such reprocessing involves the dissolution of used fuel. Ultimately, the compounds produced via wet-chemical routes −irrespective of their nature, “fresh” or reprocessed− are converted into fine oxide powders.

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, and in very fine UO2 powders exposure to normal air at room temperature may even lead to a pyrophoric reaction. Passivation treatments are required to decrease the reactivity of such UO2 powders. Additionally, 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. Evidently, the reactivity of UO2 towards oxygen may pose a risk. Despite continued research in this domain the oxidation process is still not completely understood, especially in fine powders and at low temperatures (< 100 °C).

In this PhD thesis the reaction between UO2 and oxygen has been investigated under conditions related to nuclear fuel production and to storage of UO2 powders and pellets. The research focused on assessing the crystal structure of the different phases which are formed, and also evaluated the mechanisms and kinetics involved in the oxidation reaction. Results of this study allow a better understanding of the issues related to UO2 oxidation, and can be applied to improve the handling of UO2 powders and to evaluate the effects of storage conditions.