Fundamental experimental determinations of thermal and radiolytic gas generation over actinide oxides

In the areas of actinide waste disposition and storage and medium- to long-term retrievable U and Pu materials storage, the issue of water and other small molecule interactions with pure or impure actinide oxide materials and metal has become a major concern. Small molecule reactions in these systems have led to changes in materials stoichiometry, containment corrosion and breaches, dispersal of material resulting from pressurization, and the collapse of sealed containers due to the formation of partial vacuum. The exact nature of these reactions and the resulting implications for medium- to long-term storage are not well understood, although some studies have attempted to explain them from a large body of observations and experiments. The interaction of water with actinide and actinide oxide surfaces constitutes the fundamental technical basis for understanding and predicting long-term actinide material storage behavior. In this paper some of the studies that pertain to these efforts are summarized with particular emphasis on gas generation and/or consumption events relevant to actinide materials storage. These studies include fundamental surface studies of actinide and actinide oxide materials with water vapor using a wide variety of physical chemical and surface-specific techniques. In addition, radical surface chemistry arising from radiolytic decomposition of adsorbed water molecules is experimentally examined using unique gas exposure techniques. Preliminary studies are described and compared with conventional molecular gas-solid chemical model systems. Where possible, the relevant results from the surface science studies are utilized in understanding the back reaction of H2 and O2 to form H2O at relevant pressures in actual storage scenarios (described by Morales)