Systematic investigations of uranium carbide composites oxidation from micro- to nano-scale: Application to waste disposal

The production of radioisotope beams at the ISOLDE (Isotope Separator OnLine DEvice) facility at CERN is achieved by irradiating target materials (e.g. uranium carbides and metal foils) with protons. The materials are usually operated at temperatures above 2000°C to promote isotope release. However, materials are rapidly degraded due to sintering which results in a loss of radioisotope beam intensity. Moreover, some refractory elements are particularly difficult to extract using the conventional high temperature approach due to their low vapor pressure and high boiling point.

The topic of this thesis deals with two types of target materials which were engineered to improve isotope release.

The first part of this thesis concerns new micro- and nano-structured uranium carbide materials. New nanomaterials were found highly pyrophoric and not compatible with long-term storage requirements. Thus, a safe process for their conversion into oxide is investigated. However, the oxidation mechanism is not fully understood, notably the relationship between the material characteristics and its oxidation kinetics. In this thesis, characterization and thermal analysis techniques were used to study the oxidation of five different uranium carbide materials. For each material, the mechanism of the reaction was determined and the reaction rate was modeled. The model predictions were evaluated against experimental data. The influence of the materials characteristics on the oxidation kinetics was notably discussed. The reaction was found generally controlled by the diffusion of oxygen through the uranium oxide product layer. However, nanomaterials presented a shift in the rate controlling mechanism from diffusion to surface. Onset oxidation temperatures between 150°C and 280°C and activation energies between 78.9 and 128.6 kJ/mol were observed. Both values were lower for nanomaterials, which highlights the importance of materials characteristics. Other parameters such as the concentration of carbon can affect the rate of the reaction by forming an additional passivation barrier around uranium carbide. The study provided a better understanding of the oxidation properties of uranium carbides. The approach is general and can be used to investigate other actinide materials in the future. In particular, the collected data can be used to design a safe process for the conversion and disposal of target materials at ISOL facilities worldwide.

The second part of this thesis dealt with the stability of graphene on metal foil target materials under irradiation. Refractory elements can be extracted from metal foils using reactive gases to form volatile molecules at room temperature. However, the gas could also react with the target material and prevent the extraction of isotopes. Protection of tantalum foils against corrosion and oxidation can be obtained using graphene coating. However, the stability of graphene under high energy proton beam irradiation was never evaluated. It