PNNL

Abstract

This study, conducted from November 2025 to September 2026, investigates the individual and combined effects of He+ and D2+ irradiations on the microstructural evolution, Li redistribution, and D retention in ?-LiAlO2 pellets. Experiments were performed at 773 K using He+-only, D2+-only, and sequential He+ and D2+ irradiations in three regions of a pellet to fluences of 2E17 He+/cm2, 2E17 D+/cm2, and 2E17 (He++D+)/cm2, respectively, at an ion flux of ~1E13 (ions/cm2)/s. A second pellet was irradiated under the same conditions but at a lower flux of ~4E12 (ions/cm2)/s for all three irradiation conditions. The results show higher non-gas D retention following sequential He+ and D2+ irradiation than following D2+-only irradiation at both dose rates, suggesting that He-vacancy clusters form and provide effective D trapping sites. Lower dose rate resulted in increased D retention, consistent with greater time for D diffusion and trapping at stable defects. At the high dose rate, He+-only irradiation produced He blisters, a polycrystalline region containing nanoscale precipitates and cavities, and, tentatively, a deeper featureless crystalline region in which non-gas D remains trapped. D2+-only irradiation produced a crystalline surface layer with cavities, followed by a region containing large, faceted LiAl5O8-type precipitates surrounded by cavities. Preferential cavity formation was observed at grain boundaries. Sequential He+ and D2+ irradiation produced an amorphous surface layer with a high density of cavities and substantial Li depletion, followed by precipitate-cavity formation. These results indicate that the sequential microstructure arises from coupled damage accumulation, amorphization, gas transport, Li redistribution, precipitation, and cavity evolution rather than a simple superposition of the individual irradiation responses. Reducing the sequential irradiation dose rate from 3.8E-4 to 1.5E-4 dpa/s suppressed or inhibited Li depletion, amorphization, and the formation of large blisters while preserving a crystalline surface layer. Precipitate and cavity formation, including preferential cavity accumulation at grain boundaries, remained evident. These observations indicate that lower dose rates provide greater opportunity for defect recombination, migration to sinks, and dynamic recovery. Overall, the results establish that He+ irradiation-induced He-vacancy clusters enhance D trapping, while irradiation dose rate controls the balance between defect accumulation and recovery and therefore governs the Li redistribution and microstructural evolution. The findings demonstrate the importance of gas–defect interactions, defect-sink behavior, and dose rate in determining the irradiation response of ?-LiAlO2 pellets and provide a mechanistic basis for understanding tritium retention and microstructural evolution in neutron-irradiated ?-LiAlO2 under reactor-relevant conditions.