| AVS 58th Annual International Symposium and Exhibition | |
| Actinides and Rare Earths Focus Topic | Thursday Sessions |
| Session AC+SS-ThM |
| Session: | The Surface Science of Actinides and Rare Earths |
| Presenter: | David Pugmire, Los Alamos National Laboratory |
| Authors: | D.L. Pugmire, Los Alamos National Laboratory H.G. Garcia Flores, Los Alamos National Laboratory D.P. Moore, Los Alamos National Laboratory A.L. Broach, Los Alamos National Laboratory P. Roussel, Atomic Weapons Establishment, UK |
| Correspondent: | Click to Email |
An understanding of the oxidation and corrosion processes of plutonium metal at room temperature is important to the safe, effective use and storage of this reactive metal. The oxidation rate for the δ-phase stabilized, plutonium/gallium alloy (a commonly employed alloy) can be significantly affected by a number of parameters including the gallium content and the composition of the oxidizing atmosphere (O2, O2/H2O, H2O). The nature of plutonium oxidation has typically been thought of as the growth of a dioxide (PuO2) overlayer on the metal to a thickness at which the film begins to spallate (μ's). Based on thermodynamic arguments, it has been pointed out that a relatively thin layer of the sesquioxide (Pu2O3) should exist at the dioxide/metal interface for thick oxide films.
Historically, the oxidation/corrosion of plutonium has been studied by oxygen uptake of samples at elevated temperatures inferred from mass gain measurements. Accuracy of these experimental setups likely limited measurements to oxide films thicker than ~0.05 to 0.1 μ (50 - 100 nm). This is at the upper-limit of the thicknesses typically observed for plutonium oxide films. Little work has been published for studies of plutonium oxide thin-films (< 50 nm) on metal substrates. Additionally, very little is known about the role that gallium plays during the oxidation of the alloy other than it can significantly slow the rate.
We report here our studies of the initial stages of plutonium oxidation with O2 in the thin-film regime with x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The results indicate that not only does a Pu2O3 layer exist in thin plutonium oxide films, but that the sesquioxide exists as a substoichiometric species on a metal substrate, and is probably best described as Pu2O3-y. It also appears that the oxide thin-film is comprised mainly of the sesquioxide species, with PuO2 accounting only for a relatively thin portion of the overall oxide film thickness. While the surface sensitive techniques employed here suffer from relatively high limits of detection, we have also been able to qualitatively, and in some cases quantitatively, study the behavior of gallium during the oxidation of the δ-plutonium alloy. The gallium content relative to plutonium is observed to decrease within the oxide film during oxidation, with the displaced gallium moving to the oxide/metal interface to form a thin gallium rich region. These new results will be compared and contrasted with existing literature. Additionally, how these results have altered our understanding of the Pu/O thin-film system and the oxidation/corrosion of plutonium will be discussed.