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Here, we report on fracture behaviors of Al–Mg–Si alloys (LT21 Al) decommissioned from a research reactor in service for 30 years. Microscopic characterizations show that the initial microstructures of the long-term low-dose-rate neutron-irradiated LT21 Al are dominated by a large number of micrometer-sized voids and second-phase particles, different from short-term high-dose-rate neutron-irradiated materials. Quasi-static tensile and shock spall experiments on the irradiated and unirradiated LT21 Al reveal considerable irradiation-induced softening (larger than 40%) in both the tensile and spall strength. We attribute the irradiation softening to the over-aging process promoted by irradiation enhanced diffusion during the long-term service. Postmortem characterizations on shock-recovered samples indicate that the long-term low-dose-rate neutron irradiation-induced microstructures (micrometer-sized voids and second-phase particles) facilitate nucleation and growth of the dynamic damage in irradiated materials, thus playing an important role in the change in facture behaviors. [ABSTRACT FROM AUTHOR]