In modern UO₂ nuclear fuels, often known as advanced fuels, the use of transition metal (TM) elements as dopants, such as Cr, have been shown to increase the in-reactor fuel performance over traditional non-doped variants. The improved fuel performance arises from enhanced grain growth phenomena of the fuel microstructure. These are dependent upon the chemistries of the dopants during high temperature sintering a part of fuel fabrication, in which specific conditions, i.e. the oxygen partial pressure and temperature, directly control the grain growth. Despite this and other advances in the science behind Cr- and other TM-doped UO₂ modern nuclear fuels, significant paucities of information remain regarding the mechanism for incorporation and formation of secondary phases. Pertinently prior to this investigation, there was no definitive conclusion as to the valence state and local environment of Cr and other TMs elements within the fuel matrix. As this presentation will demonstrate, this paucity of understanding originates from the complexity of chemical states adopted in the bulk ceramic material, which has commonly been investigated in literature. To ameliorate this and to provide novel investigative direction of the TM chemistry in UO₂ fuels, we have fabricated single crystals of Cr/Mn/V/Fe-doped UO₂ that are representative of the bulk fuel material. By studying these comparatively against the bulk material using a variety of advanced spectroscopic techniques (HERFD-XANES, EPR, EXAFS), we have been able to resolve the chemistries of these TM dopants conclusively and unambiguously in both fresh and spent fuel. Our results further corroborate previously thermodynamic models proposed for some of these materials. The results of this investigation will be discussed in detail in this contribution, focusing on the chemistries of particularly Cr, in addition to Mn, V and Fe and compared against current literature.

Recent studies have suggested that crystalline ceramic matrices, such as monazites and zirconia (ZrO₂) have a high potential to be used as immobilization matrices for radioactive waste. At room temperature, zirconia has a monoclinic (m) structure. At higher temperatures, tetragonal (t) and cubic (c) structures can be stabilized. The phase stabilization can also be achieved at ambient conditions by incorporating oversized cations. In addition, several metastable phases (t′, t′′, κ, and t*), can be formed for doped zirconia materials. Out of the several structural polymorphs, especially the cubic structure shows high radiation tolerance, which is important for host matrices containing radioactive elements. In the current study, cerium has been used as an analog for plutonium as these f-elements have identical cation radii and can be stabilized in the trivalent and tetravalent oxidation states. The zirconia samples were co-doped with a small amount of Eu(III) to allow for luminescence spectroscopic analyses of the solid phases. In a first step, the co-precipitation route was applied to synthesize Ce-doped zirconia samples over a wide Ce-concentration range. The phase composition of the samples was investigated with X-ray diffraction, and showed that the radiation tolerant cubic phase was stabilized only for samples with Ce concentrations above 75 mol% . At lower dopant concentrations, a mixture of different phases were present, including monoclinic in a low doping concentration range, tetragonal and tetragonal double prime phases appearing for intermediate Ce-concentrations. The latter phase was detected only by Raman spectroscopy, showing the presence of a defect band at 526.5 cm^-1. In addition, luminescence spectroscopy revealed structural changes in terms of different Eu environments in the t´´ and c samples. To stabilize the cubic phase for low tetravalent doping concentrations, trivalent yttrium (Y) was incorporated as a co-dopant. XRD and Raman analyses show that the cubic phase was stabilized when the concentration of Y was higher than 15 mol%. Finally, using the same co-precipitation route, a series of uranium-doped zirconia samples was synthesized. XRD investigations show a phase transformation from monoclinic to tetragonal and orthorhombic with increasing uranium doping. Identical to the Ce-doped samples, the pure cubic phase was stabilized only in the presence of Y for concentrations higher than 15 mol%. Discerning the crystal structure is crucial to understanding the properties of these phases. Although the binary zirconia systems with only one dopant show different phase compositions for Ce and U, the scenario changes when adding a trivalent co-dopant such as yttrium, which stabilizes the cubic phase both in the presence of uranium and cerium.

Preliminary solubility results for the pure cubic phase of uranium/cerium-doped zirconia co-doped with yttrium will be shown in the poster session.