This request for facilities use is based on the objectives of an existing NEUP award entitled - Irradiation Performance of Fe-Cr Base Alloys. The objective of the research in the main award is to conduct a coordinated set of experiments with post-irradiation examination and analyses that will provide significant new insight into the irradiation performance of ferritic alloys for advanced reactor applications. The research is based on neutron irradiation experiments on a matched set of ferritic alloys in a high flux test reactor. The irradiation exposures have been completed in a series of experiments in the Advanced Test Reactor (ATR).In the research proposed here, we intend to make positron annihilation spectroscopy measurements of materials irradiated in the ATR experiment. We had performed PAS experiments on the same set of materials following ion irradiations. In that work, there was a distinct relationship between the irradiation conditions – temperature and dose – and the response of the S and W parameters. This work would make similar measurements on neutron irradiated specimens from our ATR experiments. We would, again, determine if there are reasonable correlations between the irradiation conditions and microstructures and the positron annihilation results, as well as, between the neutron and ion results.