Experimental Investigation of Physical Properties of Simulated Burn-up Nitride Fuels

Sammanfattning

Uranium nitride simulated burn-up fuel pellets were fabricated from powders synthesised via the hydriding-nitriding-denitriding route using arc-melted U-Zr, U-Th, and U-Zr-Th ingots. The pellets were consolidated using Spark Plasma Sintering. Two compositions – approximately 5 at.% and 20 at.% of the secondary metals – were investigated for each system, resulting in six distinct compositions. Quantitative phase analysis and structural characterisation were performed on both the as-fabricated powders and sintered pellets using Rietveld refinement of synchrotron powder X-ray diffraction patterns.The crystallographic refinements, combined with Scanning Electron Microscopy, enabled precise quantification of the extent of solid solution formation between UN and the secondary metals. Secondary phases formed during powder processing and sintering were also identified and quantified. Notably, complex phases were observed in the U-Th-N-O system, indicating interactions that warrant further investigation. Observations also suggest a potential stabilisingrole of thorium in extending the range of stoichiometry over which UN remains stable under sub-stoichiometric U conditions. Furthermore, an unidentified phase was detected in the U-Th-Zr-N-O system.The detailed compositional and structural data obtained in this study provide a basis for future investigations of the thermal and mechanical properties of these materials within the SUNRISE project.