The paper deals with a detailed study of He-filled cavity ensemble development in ODS-EUROFER steel implanted with 10 keV helium ions to a high peak concentration of 8.5 × 10 3 appm both with and without simultaneous irradiation with 4 MeV gold ions, which allowed us to strongly vary the ratios of dpa/He introduction. The subsequent transmission electron microscopy examination reveals excellent radiation stability of He-implanted sample in the single-beam implantation mode. In contrast, after the simultaneous dual-beam irradiation a bubble-to-void transition was observed for bubbles that were associated with yttria nanoparticles. The relative importance of different He bubble families observed in the He-implanted samples for the swelling accumulation is quantitatively assessed, emphasizing the potential risks of abrupt swelling acceleration in the case of bubble-to-void transition launched by nanoparticles. A model of bubble-to-void transition for gas bubbles associated with spherical second-phase particles is developed and used to rationalize experimental observations.