Formation of multicomponent alloy particles in doped ceria under I2+ ion irradiation and thermal annealing

Highlights: • Work provides new understanding of the early stages of alloy particle formation in nuclear fuel in the presence of fission gas. • Doped CeO₂ was used as a surrogate for UO₂ along with implantation with fission gas to avoid the challenge of handling radioactive material. • The combined effects of heavy recoil damage and elevated temperature led to multicomponent precipitate formation • Iodine segregated to extended defects. • Re, the surrogate for ⁹⁹Tc, was found in precipitates formed at 900 °C Multicomponent alloys resembling the ε-phase particles in nuclear fuel have the potential to immobilize ⁹⁹Tc in a stable waste form. Formation of such multicomponent alloys under extreme conditions is studied in a non-activated model system: CeO₂ doped with 2 wt.% Mo, 1.5 wt.% Ru, 0.75 wt.% Pd, 0.5 wt.% Re and 0.25 wt.% Rh. The doped CeO₂ films were irradiated with I²⁺ ions (610°C, 10¹⁶ and 5×10¹⁶ I²⁺/cm²). For selected samples post-irradiation heat treatment was conducted (900°C, 1100°C). Analytical transmission electron microscopy revealed the formation of 5-20 nm precipitates containing Pd, Mo, and Re sometimes associated with cavities. Iodine was found to segregate to grain boundaries and cracks. After heat treatment at 1100°C, the CeO₂ matrix had recrystallized. Precipitates containing Pd, Mo, Re, and Ru were observed near the interface with the polycrystalline yttria-stabilized zirconia substrate. It follows that Pd is the most mobile element, followed by Mo and Re. While irradiation promotes precipitation, high-temperature effects are more significant.