Editorial: Structures and Properties of Fluorite-related Systems for Nuclear Applications

Gianguido Baldinozzi^*Thierry Wiss^*

• European Commission, Joint Research Centre (Germany), Karlsruhe, Germany

research itself is a complex system and that results are best studied by looking at them from many perspectives. Quoting Edgard Morin: "The paradigm of complexity thus stands as a bold challenge to the fragmentary and reductionistic spirit that continues to dominate the scientific enterprise". Nuclear materials are undoubtedly a field that faces this great challenge.The application of diffraction, microscopy and spectroscopy techniques allows researchers to understand remarkable details of polymorphic and morphotropic phase transformations in pristine samples, but also in the out-of-equilibrium conditions typical of radiation damage. Understanding and modelling microstructural evolution and phase stability under chemical and physical conditions relevant to applications, but also chemistry and structural evolution at interfaces, is of paramount importance to understand and design materials and material interfaces with radiation, where temperature gradients and chemical phenomena can span multiple length and time scales. These observations and models provide valuable information for the fabrication and performance of advanced nuclear fuels, especially those containing minor actinides. In these complex systems, understanding thermodynamics, transport and chemical behaviour remains challenging, but it also provides opportunities for developing new approaches for nanoscale characterisation and simulation. The collective contributions of these 6 articles provide a snapshot and underscore the potential of techniques and models in advancing our understanding of advanced nuclear ceramic materials with fluorite-related structures.Leonid Burakovsky and co-authors at Los Alamos National Laboratory (USA) have modelled the Liquidus Curve of Uranium-Plutonium Mixed Oxide (MOX) System currently considered as reference fuel for some of the generation IV fast breeder reactors. The key factor determining the performance and safety of fuel such as MOX is its operational limits in the application environment which are closely related to material's structure and thermodynamic stability. They are in turn closely related to the ambient (zero pressure) melting point (Tm). In their study, Burakovsky et al. present a theoretical model for the melting curve (liquidus) of an ideal mixture of pure UO2 and PuO2. The model has the merit to link the liquidus points with an equation of state of the single oxides and of the mixture, which is certainly an original and useful approach. It has only one free parameter which must be determined independently. The examples of the application of the model to real mixtures, Si-Ge and MOX, considered in their work clearly demonstrate that, although the model is not based on rigorous thermodynamic arguments, it is reliable and relatively easy to apply in practice, in contrast to more complicated and more time consuming Calphad calculations.In a second paper, Leonid Burakovsky and co-authors also from Los Alamos National Laboratory present a Quantum Molecular Dynamic study on the higher-temperature portion of the phase diagram of the uranium-oxygen system including the Ambient Melting Behavior of Stoichiometric Uranium Oxides. As UO2 is easily oxidized during the nuclear fuel cycle it is important to have a detailed understanding of the structures and properties of the oxidation products. Experimental work over the years has revealed many stable uranium oxides including UO2 , U4O9 (UO2.25), U3O7 (UO2.33), U2O5 (UO2.5), U3O8 (UO2.67), and UO3 , all with a number of different polymorphs. These oxides are broadly split into two categories, fluorite-based structures with stoichiometries in the range of UO2 to UO2.5 and less dense layered-type structures with stoichiometries in the range of UO2.5 to UO3. While UO2 is well characterized, both experimentally and computationally, there is a paucity of data concerning higher stoichiometry oxides in the literature. In their work Burakovski et al. determine the ambient melting points of all the six stoichiometric uranium oxides listed above and compare them to the available experimental and/or theoretical data. They demonstrate that a family of the six ambient melting points map out a solid-liquid transition boundary consistent with the high-temperature portion of the phase diagram of uranium-oxygen system. Jarrod Lewis from University of Bristol (UK) and co-authors, assessed the Charge-Lattice Coupling and the Dynamic Structure of the U-O Distribution in UO2+x by examining the different structures and behaviours of UO2+x observed in crystallographic and local structure by Extended X-ray Absorbance Fine Structure (EXAFS) measurements of pristine UO2.0, or irradiated with protons or helium or at multiple temperatures bulk U4O9 and U3O7 and thin film U4O9-δ on an epitaxial substrate. The disorder caused by irradiation is mostly limited to increased widths of the existing U-O/U pair distributions with any new neighbour shells being minor. As has been previously reported, the disorder caused by oxidative addition to U4O9 and U3O7 is much more extensive, resulting in multisite U-O distributions and greater reduction of the U-U amplitude with different distributions in bulk and thin film U4O9. In addition to indicating that these anomalies only occur in the mixed valence materials, this work confirms the continuous rearrangement of the U-O distributions from 10-250 K. Although these variations of the structure are not observed in crystallography, their prominence in the EXAFS indicates that the dynamic structure underlying these effects is an essential factor of these materials.Also investigation effects on non-stoichiometry, Clotilde Gaillard from Institut de Physique des 2 Infinis (France) and co-authors presented new insights on the study of the UO2+x/U4O9 equilibrium in UO2 as a function of the hyperstoichiometry by coupling HERFD-XANES at the uranium M4-edge and micro-Raman spectroscopy mapping. While XANES allowed measuring the uranium speciation in the samples, Raman spectroscopy was used to characterize individually the composition and localization of the different oxide phases. As the O/U increases, they could evidence the formation of a network of U4O9 crystallized inside UO2+x grains. The variation of the UO2+x phase hyperstoichiometry (x) was then evaluated as a function of the sample oxidation.Thierry Wiss from Joint Research Centre of the European Commission and co-authors have measured the heat capacity of alpha-damaged uranium, plutonium, and americium mixed dioxide (Uu, Puv, Amw)O2±x samples during thermal annealing. The excess of heat released was assessed and the recovery stages associated with various defects described by integrating results from transmission electron microscopy, helium desorption spectroscopy, thermal diffusivity, and XRD annealing studies. It is shown that different defect-annealing stages could be singled out. It could also be evidenced that the excess of energy stored in defects tends to saturate after rather low damage levels, but that, with increasing radiogenic helium production, another contribution of stored energy appears which can be attributed to the formation of He-defect complexes that cannot be annihilated until higher temperatures are reached.Finally, Jennifer Yao and co-authors from Pacific Northwest National Laboratory (USA) described the development of a Novel in situ Particle attached Microfluidic Electrochemical Cell based on a vacuum compatible microfluidic electrochemical cell (E-cell) for investigating the redox behaviour of uranium dioxide. Experiments using bulk amounts of radioactive material can be costly and may require shielded hot cell facilities. In contrast, the amount of radioactive material used in a single test could be significantly reduced by using microfluidic techniques, allowing electrochemical experiments possibly to be conducted outside a shielded facility. The different approaches for building a microfluidic E-cell are described, that all use UO2 as the working electrode, with the ability to characterize the corroding materials in situ. The authors found that embedding UO2 particles in a polyvinylidene fluoride binder was the most effective method, and further demonstrated that particle-based electrodes can provide an effective and low-cost solution for microfluidic electrochemical applications. The in-situ microfluidic E-cell offers a promising method for investigating the corrosion of UO2 and other materials while reducing the amount of materials needed for analysis to microgram levels.TEM bright field image of an irradiated UO2 fuel at 75 GWd.t -1 showing that the harsh irradiation conditions result in a microstructural re-organisation with the formation of low-angle grain boundaries (in this case) and of extended defects (dislocation lines/tangles, loops) as well as fission gas bubbles while the fluorite structure is kept as confirmed by electron diffraction.The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.GB: Conceptualization, Writingreview and editing. TW: Writingreview and editing.The author(s) declare that no financial support was received for the research and/or publication of this article.