Paper SS1+AS+TF-ThM10
Characteristics and Stability of Oxide Films on Plutonium Surfaces

Thursday, November 12, 2009, 11:00 am, Room M

The oxidation of plutonium metal continues to be an area of considerable activity. The reaction characteristics have significant implications for production use, storage, and disposition of this reactive material. Developing an accurate physical model of the structures, oxidation states, and oxygen concentration gradients present during oxidation are essential to understanding this process. Traditionally, the stable oxides of plutonium have been thought to be plutonium sesquioxide (Pu₂O₃, O/Pu=1.5, Pu³⁺) and plutonium dioxide (PuO₂, O/Pu=2.0, Pu⁴⁺), existing in a layered structure on oxidized plutonium metal. Many studies of this system are performed under ultra-high vacuum (UHV) using surface sensitive techniques to probe oxidation states, electronic structure, and surface chemistry.

Here, we report on a detailed study using x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) to measure the relative concentrations of oxygen and plutonium, as well as the resulting oxidation states in the near-surface region. It has previously been shown that under UHV conditions, PuO₂ undergoes an auto-reduction reaction to the more stable Pu₂O₃. The auto-reduction of PuO₂ occurs as oxygen diffuses from the surface, through the oxide-film, to the metal-oxide interface.¹ The results of this study show a much greater than anticipated extent of auto-reduction and challenge the commonly held notion of the stoichiometric stability of Pu₂O₃ thin-films, especially in the presence of plutonium metal. The data indicates that a hypo-stoichiometric plutonium oxide (Pu₂O_3-y) exists at the metal-oxide interface. A new model of the plutonium/oxygen thin-film system will be proposed and its applicability to thicker-films will be discussed.