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

For reactors operating on a fast neutron spectrum one of the most commonly foreseen fuel type is high Pu-content hypostoichiometric MOX fuel, i.e. (U1-yPuy)O2-x with y≥20% and O/M<2.00. In hypo-stoichiometric MOX phase separation into two f.c.c. phases with different oxygen content can occur during cooling after sintering. The separation in two phases with different lattice parameters can induce severe macroscopic cracks that can potentially affect fuel properties and cause in-pile safety issues (e.g. decrease in thermal conductivity). The presence of low-oxygen phase may also accelerate low-temperature oxidation, making it difficult to keep a constant hypo-stoichiometry during fabrication and storage. The study of U-Pu-(MA)-O phase diagrams and the material properties of various phases occurring in fast reactor MOX under various physical conditions is a very active field of research. Due to its radiotoxicity and overall hazardous nature, the handling of Plutonium(Pu) requires specialized, and strictly regulated, glovebox laboratory facilities. Consequently, many studies are first conducted on surrogate materials, in particular Ln in the form of (U, Ln)O2±x, and subsequently transferred onto the actual work with Pu.

The subject of this PhD work was originally signed up and started on the “Phase stability studies on Uranium-Plutonium Mixed Oxides with high Plutonium content for fast reactors”. Since the initially offered infrastructure to fabricate and study Pu-MOX was not made available, the work provisionally began on the U-Ce-O system, which is the most commonly used U-Pu-O surrogate. Of particular importance is the phase domain in which fluorite (Fm-3m) type (U,Ce)O2±x solid solutions form. In literature, Ce is an often used surrogate for Pu. Ce mimics many behaviors and properties of Pu throughout various stages of analogous fabrication processes to obtain (U,Pu)O2±x. Ce3+ and Ce4+ generally are reported to be good analogues in aqueous solutions to Pu3+ and Pu4+, respectively. The ionic radii of Ce4+ and Pu4+ in an 8-fold coordinated solid state environment are similar and both Ce and Pu are known to be fully miscible in (U1-yMy)O2-x solid solutions with M={Pu, Ce} over the entire y range, while in the range between ambient temperature and commonly applied sintering temperatures (1473 K to 1973 K), the U-Ce-O phase system is mimicking the U-Pu-O system remarkably well in the composition range UO2-MO2-M2O3, and partly in the higher oxide domain.

Cation driven local heterogeneities in fcc microstructures interfere with measurements results intended to study anion driven phase characterization studies. Thus fabricating mixed oxide solid solutions with cation distribution homogeneity is a prerequisite to conduct phase studies on the latter. In literature, cation distribution homogeneity on the studied (U,Ce)O2±x solid solutions is very often reported based on X-Ray Diffraction (XRD) patterns acquired on sintered (U,Ce)O2±x. These patterns are analyzed for phase purity in terms of a single fluorite type face centered cubic (fcc, Fm-3m) phase, the resulting lattice parameters are compared to literature values, as well as observations on possible peak-broadening and peak-asymmetry are made. When present, the latter become particularly well-distinguishable at high 2θ angles and hint to a deviation from cation and/or anion homogeneity. In literature on (U,Ce)O2±x, such details are often underreported and/or acquisition ranges exclude the higher angle 2θ range.

The main goal of this PhD was to demonstrate a robust fabrication process for sintered (U,Ce)O2±x solid solutions that would yield a reproducible homogeneous cation distribution and crystallographic phase purity. For this two classical routes were initially investigated to fabricate mixed oxide powders precursor powders: (I) MIMAS type dry milling and mixing of as-received UO2 and CeO2 via a high-energy mill, and (II) the precipitation of aqueous Uranyl Nitrate (UNS) and Ce(NO3)3 mixtures in ammonia excess. Characterization of sintered (U,Ce)O2±x pellets obtained from these precursors was done via XRD patterns and Energy Dispersive Spectroscopy (EDS) for elemental mapping of pellet cross-sections. Notable XRD peak artifacts were observed in pellets from the dry route (I), whereas in addition to the main fcc (U,Ce)O2±x  phase also the hexagonal Ce4.67(SiO4)3O minor phase was identified. Elemental analysis confirmed that sintered material from the dry route exhibited notable cation heterogeneities together with an unexplained Si presence, whose source could not be found with certainty at the time of discovery. The wet precipitation route (II) resulted in very sharp peaks with overall clean diffractograms, while elemental analysis revealed a very homogeneous overall cation distribution with some notable features nevertheless.This resulted in the decision to expand the study to a number of liquid to solid conversion techniques based on aqueous actinide nitrate mixtures, which generally are considered superior in obtaining a high cation homogeneity in the solid solution. Among the most researched of such routes is the microsphere production via internal gelation of mixed metal nitrates (route III). For the latter already fabricated and partially characterized microspheres were available from an earlier PhD. These had exhibited however anomalies in their XRD patterns that were not yet explained. A detailed first of a kind characterization study linking elemental analysis of polished microspheres and XRD results on crushed bulk amounts was conducted on material from this route (III). For later Pu MOX application, the absence of liquid waste was an optional but desirable property for alternative fabrication routes. Therefore, the modified direct denitration was selected as route (IV) that would promise a high degree of cation homogeneity while avoiding liquid waste altogether. Based on the observations made on route (IV), a last effort was made to radically simplify the flow-sheet for a novel fabrication process, route (V), that consisted of the direct thermal denitration of mixed metal nitrates on pure cellulose ash-free filter paper. Depending on the observations made on sintered materials from various routes, additional characterizations with an Electron Probe Microanalyzer (EPMA) for elemental analysis were made.

The characterization results on abovementioned fabrication routes recurringly revealed that even aqueous liquid-to-solid conversion techniques that were generally reported to be highly dependable in yielding cation distribution homogeneity, did not always do so. In fact, depending on the fabrication parameters used for each route, very significant and systematic cation distribution heterogeneity features and patterns were observed. The abovementioned outcomes pushed the focus of the PhD towards understanding the sources and critical parameters, leading to these cation distribution heterogeneities in each of the studied routes. For each fabrication route, the fabrication parameters suspected to be relevant for the observed heterogeneity patterns were identified and remedies were proposed. This narrowed down the potential domain of parameters of follow-up studies, while also allowing the development of additional remedies in the form of process modifications. Lastly, an unanticipated yet recurring observation in sintered (U,Ce)O2±x samples obtained from routes I, II, III and IV, was the presence of highly Ce-enriched features which defied the very well-established knowledge on the U-Ce-O phase diagram. Very often, Si was identified as a completely unexpected critical contaminant coinciding with the latter. Thus the impact of Si presence on the microstructure of (U,Ce)O2±x during sintering and the relationship with high local concentrations of Ce in the latter was studied. A very likely source of unwanted Si was localized by analysis of results and dedicated experiments and a simple decontamination protocol was demonstrated.