Selective Oxidation of U(Al_xSi_1-x)₃ by Reaction with Water Vapor in the Temperature Range of 300-800 K

U-Al alloys are commonly used as nuclear fuel systems at research reactors [1-2]. In addition, Al based alloys can be used as fuel cladding materials. Silicon might be present as an impurity and diffusion can occur between the cladding and the fuel. Therefore, study of the interface between U and Al cladding under service conditions is most important to determine the fuel rods reliability. The U-Al-Si systems had been studied extensively [3]. Recently, a new ordered phase in the form of U(Al_xSi_1-x)₃ was discovered [4] and its oxidation properties were characterized at room temperature [5]. In the present study, the interaction of water vapor with U(Al_xSi_1-x)₃ surface was studied, at temperature range of 300-800 K, utilizing Auger electron spectroscopy (AES), X-Ray photoelectron spectroscopy (XPS) and direct recoil spectrometry (DRS).

The sample was arc-melted under Ar atmosphere to prepare the U(Al_xSi_1-x)₃ alloy from high purity uranium (99.99%), aluminum (99.999%) and silicon (99.999%). Metallurgical characterization utilizing scanning electron microscopy (SEM) and X-ray diffraction (XRD) revealed an ordered tetragonal phase (I4/mmm space group) of U(Al_xSi_1-x)₃, with x=0.57-0.58, surrounded by some Al at the grain boundaries (see also sample 3 at table 1, ref [4]). The experiments were performed in an ultra-high-vacuum (UHV) system, pumped by turbo-molecular and titanium sublimation pumps to a base pressure of ~2×10^-10 Torr. Prior to each experiment, the sample was sputter-cleaned at RT and then heated to the desired temperature up to 800 K. The changes in the composition of the clean surface during heating period (30-45 mins) were monitored by AES. The interaction of H₂O with the U(Al_xSi_1-x)₃ surface was studied by XPS and DRS at the end of each heat treatment after 1000 L H₂O exposure (1 L=1×10^-6 Torr×sec).

It was found that heating of the surface above 500 K, causes Al to segregate to the surface. The Al concentration at the surface reaches its maximal intensity at 700 K and then some decrease occurs up to 800 K due to segregation of silicon to the surface. The thickness of the Al layer formed on the surface at 700 K was calculated to be 0.7 nm. Exposing the surface to 1000 L of water vapor up to 500 K results in a mild oxidation of U and Al correlative to initial formation of Al₂O₃ and UO_2-x. However, above 600 K, XPS Al(2p) spectra indicates oxidation of the Al that segregated to the surface while the U(4f) peaks do not reveal any oxidation. It seems that the thin Al layer, formed on the surface, inhibits further oxidation of uranium. The Si(2s) XPS spectra, after exposure to 1000 L H₂O, shows only a minor oxidation up to 500 K, compared to uranium and aluminum. DRS results indicate that initially water molecules are fully dissociated, followed by adsorption of hydroxyls (OH). At the temperature range of 600-800 K only oxidation of Al occurs without hydroxyl adsorption.

References

1. L. Hofman, A Short Note on High Density Dispersion Fuel, Argonne, 1996.
2. L. Snelgrove, G.L. Hofman, M.K. Meyer, C.L. Trybus, T.C. Wiencek, Nucl. Eng. Des. 178 (1997) 119.
3. Noel, P. Rogl, Non-Ferrous Metal Ternary Systems. Selected Nuclear Materials and Engineering Systems: Phase Diagrams, Crystallographic and Thermodynamic Data, Springer Berlin Heidelberg, New York, 2007.
4. Rafailov, I. Dahan, and L. Meshi, Acta Crystallogr. Sect. B Struct. Sci. Cryst. Eng. Mater. 70, p.580–585 (2014).
5. Matmor, S. Cohen, G. Rafialov, M. Vaknin, N. Shamir, T. Gouder, and S. Zalkind, J. Nucl. Mater. 499 (2018) 29.