In this study, a pure FeNiCr (D302) model alloy has been investigated in parallel with an industrial 316L alloy. Both alloys have been irradiated in different conditions with 5 MeV Ni$^{2+}$ at high temperature (HT) 450°C, and to damage doses: 0.5 or 1 dpa (displacement per atom), or with 1.5 MeV $^4$He$^{2+}$ at room temperature (RT) and to damage dose between 1×10$^{-4}$ and 1×10$^{-2}$ dpa. The positron annihilation spectroscopy (PAS) implemented on a slow positron beam has been used to characterize vacancy defects. The results are compared to other analysis methods such as: TEM (Transmission Electron Microscopy) and APT (Atomic Probe Tomography) reported in [1]. Main results show that only single vacancies have been observed in the case of 316L alloy regardless the irradiation conditions, while vacancy clusters seem to be formed in the case of D302 alloy under HT irradiation conditions and from 1×10$^{-3}$ dpa at RT. The difference observed between 316L and D302 alloys could be linked to a role played by the alloying elements such as silicon.