Characterization of surface layers formed on DU10Mo ingots after processing steps and high humidity exposure

• Published in: Journal of nuclear materials 2019-02, Vol.514 (C), p.28-39
• Main Authors: Kaspar, Tiffany C., Arendt, Christina L., Neal, Derek L., Riechers, Shawn L., Rutherford, Crystal, Schemer-Kohrn, Alan, Spurgeon, Steven R., Sweet, Lucas E., Joshi, Vineet V., Lavender, Curt A., Shimskey, Rick W.
• Format: Article
• Language: English
• Subjects: Acid etch; Acids; Air exposure; Atomic force microscopy; Barrier layers; Cladding; Cold rolling; Corrosion; Electropolish; Etching; Exposure; Foils; Hot rolling; Humidity; Ingots; Interfacial properties; Microscopy; Molybdenum; Morphology; Nitrides; Nuclear fuel elements; Nuclear fuels; Oxidation; Oxides; Photoelectron spectroscopy; Photoelectrons; Relative humidity; Scanning electron microscopy; Sulfuric acid; Surface layers; Surface oxide; Surface roughness; U-Mo; Uranium; USHPRR; Valence; XPS; Zirconium
• ISSN: 0022-3115; 1873-4820
• DOI: 10.1016/j.jnucmat.2018.11.022

The design of monolithic UMo fuel elements fabricated from low-enriched uranium for use in high-power research reactors requires bonding of the fuel foil to either Al cladding or a Zr barrier layer. Processing of the UMo ingot to final foil form has the potential to generate surface layers on the foil that differ from the bulk, metallic UMo. The interfacial properties between the UMo and Zr or Al cladding layers will then be determined by these surface layers. We use x-ray photoelectron spectroscopy, cross-sectional scanning electron microscopy, and atomic force microscopy to characterize the composition, oxidation state, and morphology of the surface layers that form after hot rolling and cold rolling depleted U–10 wt% Mo alloy (DU10Mo). A thick uranium nitride layer is observed after hot rolling, although its origin is likely from a previous processing step. The efficacy of acid etching in HNO3 is compared to that of electropolishing in H2SO4 to remove surface nitride and oxide layers, and both methods are found to be similarly effective. Both laboratory (low humidity) air exposure and longer rinse times in water are shown to promote the formation of surface oxide layers. Exposure of both acid-etched and electropolished DU10Mo foils to humid air (97% relative humidity) for six weeks results in formation of a thick oxide layer due to corrosion. The oxide layer on the acid-etched foil is thicker and more highly oxidized than the oxide layer that forms on the electropolished foil, and these differences in oxidation behavior are attributed to higher surface roughness on the acid-etched foil. In general, Mo is found to play a role as a sacrificial element, typically exhibiting a larger ratio of Mo6+/Mo4+ than U6+/U4+. This is unexpected, given the greater thermodynamic driving force to form U oxides than Mo oxides. •Surface characterization of monolithic UMo fuel elements.•Efficacy of acid etching vs. electropolishing to remove surface oxide.•Characterized by x-ray photoelectron spectroscopy and scanning electron microscopy.•Surface roughness enhances oxidation in high humidity environment.