Processing and Advanced Characterization of Pure and Ln-Doped UO2 Pellets Fabricated by Spark Plasma Sintering

Licentiate thesis, 2026

This thesis investigates how spark plasma sintering (SPS) parameters, powder characteristics, and lanthanide doping influence the densification and structural response of UO₂-based nuclear fuel materials. First, a systematic pressure–temperature sintering map was established for pure UO₂ using a low surface area starting powder. Densification increased with temperature and pressure but reached a plateau at ap proximately 93–94% of theoretical density. The results indicate a transition from pressure-assisted particle rearrangement to diffusion-limited pore elimination, with the final density strongly influenced by the characteristics of the starting powder. Second, five UO₂-based materials containing 1 wt% La, Ce, Pr, Nd, or Gd were synthesized by co-precipitation and consolidated by SPS. Relative densities of 96-98% were achieved. Synchrotron high-resolution powder diffraction confirmed retention of the fluorite structure without detectable secondary crystalline phases and revealed systematic changes in lattice parameter. Agreement with a hard-sphere model supports substitutional incorporation of the lanthanides and indicates that the lattice response is primarily governed by cation size and charge compensation. ICP–MS and laser ablation ICP–MS further confirmed compositions close to the targets and no pronounced large-scale compositional variation within the analysed regions. Together, the results demonstrate that control of powder characteristics, precursor chemistry, and SPS conditions is essential for producing dense and structurally well-defined surrogate nuclear fuel materials.