U(Al_xSi_1-x)₃ SURFACE INTERACTIONS WITH O₂ and H₂O

U-Al alloys can be used as fuel for research reactors, where the uranium is the burnable element and aluminum is present in the fuel matrix of dispersion fuels as well as in the fuel cladding. Silicon can be added as an alloying element to improve the castability and mechanical properties.

U(Al_xSi_1-x)₃ phase, with composition of U-25at%, Al-44at%, Si-31at% was casted and characterized by SEM+EDS and X-ray diffraction, revealing a tetragonal crystallographic structure.

The sample surface and its interaction with oxygen and water vapor were characterized utilizing Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS) and direct recoil spectrometry (DRS). On heating the sample in UHV up to 600K Al was found to segregate to the surface. Above 600K some Si was also found to segregate to the surface. Exposing the sputtered clean surface to oxygen and water vapor, up to 1000L at room temperature, revealed oxidation of the U and Al elements, with no evidence of Si oxidation. Comparison between the DRS, which is sensitive only to the uppermost layer and XPS results suggests that first a thin oxide layer is created at low exposures up to 5L and then inwards growth of the oxide occurs at higher exposure. During water adsorption the oxide formed is thinner and covered by hydroxyls. On exposing the surface to oxygen at room temperature after aluminum segregation, only a thin aluminum-oxide layer is formed on the surface.