| AVS 63rd International Symposium & Exhibition | |
| Actinides and Rare Earths Focus Topic | Wednesday Sessions |
| Session AC+MI-WeA |
| Session: | Actinide and Rare Earth Theory (2:20-3:40 pm)/Nuclear Power, Waste Remediation and Applications (4:20-6:20 pm) |
| Presenter: | Ladislav Havela, Charles University, Prague, Czech Republic |
| Authors: | L. Havela, Charles University, Prague, Czech Republic M. Paukov, Charles University, Prague, Czech Republic F. Huber, European Commission, Joint Research Centre (JRC), Institute for Transuranium Elements T. Gouder, European Commission, Joint Research Centre (JRC), Institute for Transuranium Elements |
| Correspondent: | Click to Email |
High temperature bcc structure of Uranium (γ-U) can be retained to low temperatures by alloying with different d-metals. Such alloys are used, due to enhaced resitance to radiation damage and better mechanical properies comparing to α-U, as nuclear fuels. Alloying leads also to reduced surface reactivity. Such "stainless" uranium is much more resitant to oxidation, and reaction with hydrogen gas needs high H2 pressures. This raises questions about phenomena at the surface of such alloys. We performed a photoelectron spectroscopy study of the alloy U0.80Zr0.20, prepared by rapid solidification (splat cooling). The concentration of 20% Zr was chosen as the lowest concetration which still gives a safe margin for single-phase bcc phase. In particular, we studied the properties of the surface exposed to oxidation and/or annealing.
The surface in the initial state is Zr-rich (approx. 85% Zr), and the Zr excess is only slowly removed by Ar-ion sputtering. In the cleaned state the U-4f spectra indicate that the degree of the 5f localization is not changed between the α-U and γ-U phases, which can be understood considering that the U-U spacing even in the γ-U phase remains far below the Hill limit. Also the valence-band spectra in UPS exhibit only small differences with respect to pure U, which adopts the α-U phase.
Exposing the clean surface of U0.80Zr0.20 to O2, one observes relatively rapid oxidation. The dose of 5 Langmuir of O2 converts 35-40% of metal into a dioxide, while Uranium oxidizes slightly more than Zirconium.
Isochronal annealing for 900 s leaves the clean surface unchanged for temperatures below 500 K. However, above this temperature the concentration of Zr in the surface layer fast increases. Repeated cleaning and annealing cycles lead to the same picture of surface enriched by Zr. The oxide formation of such surface is much more sluggish. This is true particularly for U, which has difficulty to form UO2. The oxidation can be stimulated by elevated temperatures, pointing to the fact that such oxidation is modulated by diffussion of O thrugh the Zr overlayer. Still for 100 L of O2 at 473 K the large part of U remains in metallic state and the amount of oxide is lower than for room-temperature oxidation of stoichiometric surface after 5 L O2. The finding demosntrates that the U-Zr alloys are coated by Zr or Zr-rich film when exposed to 500-600 K and such film leads to a passivation of the surface.
This work was supported by the Czech Science Foundation under the Grant 15-01100S. The work at ITU was supported by the European FP7 TALISMAN project, under contract with the European Commission.