PhD Defense | Selim Uygur | Process impact on cation distribution and phase purity of sintered (U,Ce)O2±x.

Process impact on cation distribution and phase purity of sintered (U,Ce)O2±x.

The most common and technologically mature nuclear fuel form for fast neutron reactors is high-plutonium, hypostoichiometric MOX, written as (U₁₋yPuᵧ)O₂₋x (y ≥ 20%, O/M < 2). During cooling after sintering, these materials can undergo phase separation into two fluorite (fcc) phases with different oxygen contents, producing lattice mismatches that may cause macroscopic cracks, reduced thermal conductivity, and other safety concerns. Additionally, low-oxygen phases may accelerate low-temperature oxidation, making it difficult to maintain the desired composition during fabrication and storage. Thus, phase stability and fabrication of mixed uranium–plutonium oxide (MOX) fuels for fast neutron reactors is an active domain of research. Because plutonium is highly radiotoxic and requires specialized glovebox facilities, many preliminary studies are performed using surrogate materials. This work therefore focused on the U–Ce–O system, where cerium (Ce) is commonly used as a surrogate for plutonium due to similar oxidation states (Ce³⁺/Ce⁴⁺ vs. Pu³⁺/Pu⁴⁺), ionic radii, and solid-solution behavior in fluorite-type oxides.

A key challenge in studying these materials is achieving homogeneous cation distribution in the solid solution, since local heterogeneities can interfere with measurements aimed at understanding oxygen-related phase behavior. The main goal of the PhD was therefore to develop a reliable fabrication process for (U,Ce)O₂±x solid solutions that ensures uniform cation distribution and crystallographic phase purity.

Two initial fabrication routes were studied:

1. Dry route: high-energy milling of UO₂ and CeO₂ powders.
2. Wet precipitation route: co-precipitation of uranyl nitrate and cerium nitrate in ammonia.

Characterization using X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) showed that the dry route produced cation heterogeneities and impurity phases, including Ce₄.₆₇(SiO₄)₃O, caused by an unexplained silicon contamination. In contrast, the wet precipitation route yielded sharper diffraction peaks and a more homogeneous cation distribution, yet some residual contaminations remained unexplained.

The research was therefore expanded to investigate additional liquid-to-solid conversion techniques designed to improve cation homogeneity:

3. Internal gelation microsphere production from mixed metal nitrates.
4. Modified direct denitration, which avoids liquid waste.
5. Direct thermal denitration on ash-free cellulose filter paper, a simplified novel process.

Materials produced by these routes were characterized using XRD, EDS, and electron probe microanalysis (EPMA) to assess phase purity and elemental distribution.

Overall, the work aimed to establish robust fabrication methods for homogeneous (U,Ce)O₂±x solid solutions, enabling accurate phase studies and ultimately supporting the development of high-plutonium MOX fuels for fast reactors.

Promoter:

• Jef Vleugels (KU Leuven)

SCK CEN mentors:

• Rémi Delville