Defect Chemistry and Dissolution Behaviour of Doped Nuclear Fuels

Doped nuclear fuels, developed to improve safety and efficiency of standard UO2, are widely used in nuclear energy generation and continue to be developed for future, so-called accident tolerant or advanced doped, fuels. Small amounts of additive are doped into UO2 which, when fabricated under appropriate conditions, results in a larger grained microstructure, responsible for enhanced fuel performance. However, key understanding of the fundamental crystal chemistry of these materials, as well as the long-term durability in comparison to standard UO2 fuel, is lacking. A thorough understanding of both is required, firstly to aid further implementation of doped UO2 as nuclear fuel and, secondly, to ensure that the durability behaviour is well recognised and quantified for the purposes of developing radioactive waste management strategies.
Advanced high-flux, high-spectral purity X-ray absorption spectroscopy (XAS), corroborated by X-ray diffraction (XRD), Raman spectroscopy and high energy resolved fluorescence detection-XAS was used to directly evidence the crystal chemistry of both Cr- and Mn- doped UO2. Results confirm incorporation via substitution of Cr2+/Mn2+ dopant on the U4+ lattice site and concurrent formation of U5+ and Ov defects. Further analysis of sintered material highlighted that, for Cr-doped UO2, a degree of complexity is introduced to the system, evidenced by the mixed oxidation state (Cr2+/Cr3+), when sintered under reducing conditions. For Mn-doped UO2, no change in the local chemistry is observed upon sintering.
The effects of Cr2+/Cr3+ mixed oxidation state on dissolution behaviour was assessed in long-term dissolution experiments in a simplified simulant bicarbonate ground water solution. In comparison to un-doped UO2, a systematic decrease in the initial dissolution of rate of U as a function of increasing Cr content at 25 °C and 40 °C suggests that Cr improves the durability of UO2, hypothesised to be the result of galvanic coupling of Cr2+ with U6+. It was shown that at 60 °C, the bicarbonate solution-driven oxidative dissolution is dominant and no effect of Cr content is observed. Once dissolution is hindered by the thermodynamic processes that govern the solubility of U, the dissolution rate is suppressed due to solution saturation with respect to U and the eventual formation of secondary U-phases. In this regime, there is no change in the dissolution rate with Cr content. Highlighting the importance of microstructure in UO2 dissolution, a decrease in dissolution rate with increase grain size was also observed.
In addition to in-depth understanding of Cr-doped UO2 defect chemistry and dissolution behaviour, this work presents an initial synthesis and fabrication route for Mn-doped UO2 and a basis for understanding grain growth and ultimately the full potential of Mn-doped UO2 as a future nuclear fuel.