Characteristics and Stability of Oxide Films on Plutonium Surfaces

The oxidation of plutonium (Pu) metal continues to be an area of considerable activity. The reaction characteristics have significant implications for production use, storage, and disposition of this radiological 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 Pu 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 Pu metal. Many of the notions of the layered Pu oxide model are based on extrapolations of data acquired on bulk oxides with only a few supported with results from actual Pu oxide film studies. This dissertation involves 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. A model to fit the XPS data has been developed for the accurate and reliable quantification of oxide film stoichiometries (O2-/PuTot), and a relative sensitivity factor (RSF) has been determined. The influence of temperature, pressure and exposure time on oxide film characteristics was investigated. The results indicate that, like PuO2, the sesquioxide is not stable on a clean metal substrate under reducing conditions, resulting in substoichiometric films (Pu2O3-y). The Pu2O3-y films prepared exhibit a variety of stoichiometries as a function of preparation conditions. This study show a much greater than anticipated extent of auto-reduction of PuO2 and challenge the commonly held notion of the stoichiometric stability of Pu2O3 thin films, especially in the presence of plutonium metal. The fate of gallium and carbon impurities after oxidation of plutonium surfaces was probed. Both remain at the metal-oxide interface upon oxidation of the metal. A new model of the plutonium/oxygen thin-film system will be proposed and its applicability to thicker-films will be discussed