It is well known that body-centered cubic metals have much longer swelling incubation periods than face-centered cubic metals during neutron or charged particle irradiation. In this paper, Fe9Cr model alloy was irradiated with 2 MeV self-ions to lower doses of 0.5, 1.0 and 3.0 dpa at room temperature. The evolution of microstructural defects was revealed by transmission electron microscopy and positron annihilation spectroscopy. Small interstitial dislocation loops were observed in the form of black spots and they were accompanied by a large number of vacancies after 0.5 dpa. Visible dislocation loops dominated the microstructure after irradiation with 1.0 dpa and 3.0 dpa, but the sharp decrement of the S value means that the vacancies should be annihilated gradually with the irradiation dose increasing. In addition, the vacancy status was further confirmed by the investigation of their occupation behaviors with low-energy deuterium atoms. The vacancies located around loops would be trapped and removed by the coarsening interstitial dislocation loops. We believe that the understanding of the interaction between vacancies and dislocation loops is a possible complementary mechanism, applicable for explaining the long stage of swelling incubation in bcc metals.