Cr-doped UO₂ is a leading accident tolerant nuclear fuel where the complexity of Cr chemical states in the bulk material has prevented acquisition of an unequivocal understanding of the redox chemistry and mechanism for incorporation of Cr in the UO₂ matrix. To resolve this, we have used electron paramagnetic resonance, high energy resolution fluorescence detection X-ray absorption near energy structure and extended X-ray absorption fine structure spectroscopic measurements to examine Cr-doped UO₂ single crystal grains and bulk material. Ambient condition measurements of the single crystal grains, which have been mechanically extracted from bulk material, indicated Cr is incorporated substitutionally for U⁺⁴ in the fluorite lattice as Cr⁺³ with formation of additional oxygen vacancies. Bulk material measurements reveal the complexity of Cr states, where metallic Cr (Cr⁰) and oxide related Cr⁺² and Cr⁺³₂O₃ were identified and attributed to grain boundary species and precipitates, with concurrent (Cr⁺³_xU⁺⁴_1-x)O_2-0.5x lattice matrix incorporation. The deconvolution of chemical states via crystal vs. powder measurements enables the understanding of discrepancies in literature whilst providing valuable direction for safe continued use of Cr-doped UO₂ fuels for nuclear energy generation.

In the present study we have investigated the complexation of uranyl(VI) with chloride and fluoride using luminescence spectroscopy (TRLFS, time-resolved laser-induced fluorescence spectroscopy). At 25 °C (298.15 K), in the presence of 0–0.175 M fluoride, the first single-component emission spectra for all four uranyl(VI)-fluoride complexes, i.e. UO₂F⁺, UO₂F₂, UO₂F₃ ⁻, and UO₂F₄ ²⁻ could be extracted. Based on the aqueous speciation derived from the TRLFS data, log K* values at I = 1 M were calculated for all these complexes and extrapolated to infinite dilution using the SIT approach. In the case of chloride, however, quenching of the uranyl(VI)-luminescence hampered the experiments. Thus, uranyl(VI)-complexation was studied with TRLFS at liquid nitrogen temperatures. Samples were prepared at 25 °C (298.15 K) with chloride concentrations ranging from 0 to 1.0 M followed by instantaneous freezing and subsequent luminescence spectroscopic measurements at −120 °C (153.15 K). This allowed for the determination of the first luminescence spectra for the UO₂Cl⁺ complex with the TRLFS method. The chloride quench reaction was further studied in the temperature range 1–45 °C (274.15–318.15 K) using Stern–Volmer analysis. By applying the Arrhenius and the Eyring equations we obtained the first thermodynamic parameters for the dynamic quench process, i.e. the activation energy (E_a = 55.0 ± 12.9 kJ mol⁻¹), enthalpy (ΔH^‡ = 52.5 ± 13.0 kJ mol⁻¹), and entropy (ΔS^‡ = 103.9 ± 42.8 J mol⁻¹ K⁻¹).