Effect of surface characteristics and environmental aging on wetting of Cr-coated Zircaloy-4 accident tolerant fuel cladding material

Abstract

With the goal of increasing the performance and safety of existing nuclear power plants, the research and development of Accident Tolerant Fuels (ATFs) are of high priority within the nuclear community. Two widely studied ATF concepts are the replacement of currently used zirconium alloy for other cladding materials that provide resistance to hydrogen production during accident conditions or the deposition of protective functional coatings on the Zircaloy cladding that mitigate the risk. While different protective coating options are being considered, Cr-coating is predominant. In this study, Cr-coating on Zircaloy-4 substrates were produced by Plasma Assisted Physical Vapor Deposition (PAVD), aiming at investigating their surface characteristics that may have an impact on the efficiency of heat transfer. The thickness, structure, and mechanical properties of the coatings were examined by scanning electron microscopy, focused ion beam, X-ray diffraction, and microindentation. Furthermore, surface topography, surface energy conditions, and surface chemistry were assessed using contact profilometry, atomic force microscopy, contact angle goniometry, and X-ray photoelectron spectroscopy. The static contact angle reduced from 75° for as-received substrates to almost 20° after coating. This significant increase in wettability observed after chromium deposition on Zircaloy changed with aging. For instance, over time, a hydrophobic recovery on the plasma-treated surfaces was detected. Correlations between surface chemistry and surface energy studies helped to explain the variation in contact angle with aging time. Results demonstrated that the formation of chromium oxides and hydroxides, and other carbonaceous species affected the surface coating affinity with water upon ambient exposure.