We propose to investigate the porosity development and evolution in zirconium-niobium alloys, samples of irradiated and non-irradiated Zr-xNb (x = 0.2, 0.4, 0.5, 1.0) and Zircaloy-4 irradiated under 2-MeV rastering proton beam to 1.0 dpa at 350 °C at the University of Wisconsin Ion Beam Laboratory. These samples were then corroded in water at 320 °C for up to 120 days total exposure. Samples were archived at varying total exposures, resulting in a range of catalogued pre-transition oxide thicknesses for each irradiated alloy. As part of the proposed RTE, Positron Annihilation Lifetime Spectroscopy (PALS) and Doppler Broadening Spectroscopy (DBS) will be performed on these prepared samples at the Positron Intense Beam Facility at North Carolina State University. These results will be used to characterize and compare the pore density between the irradiated and non-irradiated samples at different depths within the thin oxide layer. It is noted that this would be the first time that PAS is used on pre-irradiated, corroded, specimen. Preliminary study performed by PI’s group at the NCSU facility confirmed that the sensitivity to the thin film oxide layer is too low using a positron point source because most of the annihilation events occur within the bulk material, resulting in a low signal-to-noise ratio. By using a positron beam condition, long-lifetime positronium can be detected with better resolution and sensitivity to investigate porosity in thin-film zirconium oxides. 12 samples will be analyzed, 4 pre-irradiated ZrNb samples corroded up to 120 days, 4 pre-irradiated Zircaloy-4 samples corroded up to 120 days and 4 unirradiated ZrNb and Zircaloy-4 samples corroded up to 120 days to serve as benchmark. The samples are already available and, after discussion with the NCSU PAS group, 4 weeks of beamtime are required to accommodate these 12 samples.

This research directly supports the aims of the DOE-NE’s goals to: (i) develop advanced fuel claddings for the deployment of safer and more economical Light Water Reactors, (ii) develop validation data for advanced modelling tools to enhance in-reactor predictive modelling of fuel cladding integrity, (iii) assess the benefits of using ion irradiation to mimic neutron irradiation in the overall objective of using ion irradiation for licensing purposes, by addressing current challenges in understanding fuel cladding integrity under corrosion/irradiation conditions. A significant limiting factor in burnup extension of LWR’s fuel assembly is the hydriding induced by the corrosion reaction. Indeed, the beneficial effect of Nb on the in-reactor corrosion kinetics and hydrogen pickup of ZrNb LWR’s fuel claddings is still not understood and not properly modelled. It is thus critical to further our understanding of Nb effect on fuel cladding corrosion under irradiation to inform predictive models under development in CASL and NEAMS programs and to develop better alloys for advanced LWRs as part of the Accident Tolerant Fuel campaign, in which coated zirconium candidate represents a short term deployment solution. In addition, this study aims at characterizing for the first time pore density in oxide of pre-irradiated specimen to compare to unirradiated specimen in two different alloys showing drastic differences in their corrosion rates under irradiation. This represents a unique opportunity for the research team to study irradiated materials and compare with the data already obtained using TEM Fresnel contrast imaging. This project and RTE proposal are parts of a larger on-going program to better understand the effect of irradiation on corrosion of nuclear materials in the PI research group and associated research programs. On the broad scale, this study will provide crucial information on the effect of alloying elements on corrosion under irradiation and on the corrosion mechanism.