Microstructure and Durability of Stabilized High Burn-up Surrogate Fuel Pellets

The safe stabilization of damaged spent nuclear fuel (SNF) is essential for its long‑term management and disposal. This study examines how burn‑up level, composition, and sintering temperature influence the microstructure and aqueous durability of stabilized ceramic pellets fabricated from surrogate SNF powders. Surrogates representing high burn‑up conditions (35 and 55 GWd·tU−1) were sintered at 1,550°C or 1,700°C, then ground and tested using the Product Consistency Test A at 90°C for 7 days. Higher‑temperature sintering produced denser microstructures with smoother grains and lower porosity, while higher burn‑up increased surrogate content and slightly reduced densification. Leaching tests showed substantially reduced release of uranium and fission product surrogates from pellets sintered at 1,700°C, with each element’s behavior reflecting its microstructural association. ε‑phase particles enriched in Mo, Ru, and Pd were found to be highly resistant to dissolution. These results highlight the critical role of high‑temperature densification in enhancing chemical durability and support the use of stabilized pellets as robust waste forms for immobilizing damaged SNF, as represented by surrogate compositions in this study.