Microstructure-controlled aqueous dissolution of multi-dopant UO2 SIMFUEL across sintering and leaching conditions

Multi-element-doped UO 2 SIMFUEL pellets were fabricated by high-pressure compaction and sintering at 1550–1700 °C under a reducing atmosphere to simulate chemically complex, high-burnup-equivalent fuel compositions. SEM, XRD, EDS, and ICP-MS characterization revealed pronounced microstructural evolution with increasing sintering temperature. Sintering at 1700 °C promoted substantial grain coarsening and densification up to 96.3% theoretical density, reducing interconnected porosity and effective grain boundary area relative to pellets sintered at 1550 °C. This microstructural stabilization enhanced resistance to aqueous dissolution during one-year semi-dynamic leaching tests conducted at 25–90 °C in accordance with the ASTM C1308 protocol. Dopants partitioned into chemically resilient metallic ε-type phases and more soluble dopant-bearing oxide regions, giving rise to element-specific dissolution kinetics governed by phase chemistry and microstructural connectivity. Mo release reflected both normalization artifacts and genuinely faster dissolution at lower burnup. Beyond compositional effects, elevated leaching temperatures consistently accelerated elemental release and were accompanied by subtle lattice contraction, consistent with partial oxidation of the fluorite matrix. Collectively, these processing–microstructure–dissolution relationships provide mechanistic insights for improving the chemical durability of multi-component fluorite-structured nuclear fuels in aqueous environments.