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

Journal Article · Fri Feb 01 04:00:00 EST 2019 · Journal of Nuclear Materials

DOI:https://doi.org/10.1016/j.jnucmat.2018.11.022· OSTI ID:1496823

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.

The design of monolithic U-Mo 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 U-Mo ingot to final foil form has the potential to generate surface layers on the foil that differ from the bulk, metallic U-Mo. The interfacial properties between the U-Mo 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 surface composition of cold-rolled DU10Mo varied significantly between two foils, with a strongly Mo-depleted surface oxide layer observed on one foil but nominal Mo composition observed on the other. The efficacy of acid etching in HNO₃ is compared to that of electropolishing in H₂SO₄ to remove the surface 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.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 1496823
• Report Number(s): PNNL-SA-136846
• Journal Information: Journal of Nuclear Materials, Journal Name: Journal of Nuclear Materials Journal Issue: C Vol. 514; ISSN 0022-3115
• Publisher: Elsevier
• Country of Publication: United States
• Language: English