We investigate the role of radiation-induced defects in the microstructural evolution of Fe-Cr model alloys with two approaches; a positron annihilation measurement and computer simulations. We prepared Fe-Cr model alloys with different Cr contents which were irradiated with 2 MeV-electrons at the JAEA accelerator at Takasaki in Japan. The production of vacancy-type defects was verified by positron annihilation lifetime spectroscopy. The average positron lifetimes of the irradiated samples were increased by about 14% when compared to the unirradiated one. The ratio curves, derived from a Doppler broadening measurement, did not reveal a prominent change between the irradiated pure Fe and Fe-Cr alloys, which implies that there was no formation of a vacancy-Cr complex. We performed computer simulations by using the Metropolis Monte Carlo method to predict a possible microstructure composed of Cr and vacancies. As expected, the simulation also revealed that no vacancy-Cr complexes were formed. Both results support a small amount of radiation-induced swelling in the Fe-Cr alloys.