Thermal air oxidation of UO2: Joint effect of precursor's nature and particle size distribution

Abstract

The rate and extent of the air oxidation of UO2 are of paramount importance due to its relevance in the management of spent nuclear fuel at dry storage facilities. In particular, the formation of U3O8 requires special interest because it could compromise the integrity of the fuel cladding. In an attempt to reproduce the behavior of used fuel (in which the presence of fission products and Pu will also influence the oxidation), unirradiated UO2 was studied in this work. UO2 oxidation is strongly dependent on physical parameters of the starting material, although some parameters are not fully-studied. In the present study, the nature of the UO2 starting material and the particle size distribution on thermal oxidation temperature were evaluated in detail. Since UO2 oxidation follows a complex kinetic mechanism comprising various crystalline phases, which composition and quantity change influenced by temperature and oxidant gas phase, oxidation conditions were set to favor the formation of U3O8, due to its practical importance, as well as to study the impact of the physico-chemical properties of raw material in UO2 oxidation. Various powdered sample substrates ranging from synthetic UO2 (prepared via ADU route) to grinded and sieved fragments of fresh UO2 pellets were oxidized in the same conditions. The average O/U atomic ratios of the samples were calculated by assuming total conversion to U3O8 from data of thermogravimetric analysis (TGA). Each weight variation curve was characterized and discussed in terms of the Oxidation Onset Temperature (OOT), the Maximum Reaction Temperature (MRT), and the Reaction Rate at the maximum reaction temperature (RRMRT). Scanning Electron Microscopy (SEM) was employed to evaluate the surface morphology. All thermally oxidized samples exhibit the presence of oxides with spallation. Surface morphology of starting materials confirms the different nature of the samples, while the oxidized substrates present the typical pop-corn-like particles. In order to accomplish a better description of UO2 oxidation, the results are discussed relative to the nature of different UO2 precursors, surface area and particle size. The nature of the starting materials affects the rate of U3O8 formation, being faster in synthetic powders than in powder coming from milled pellets. By comparing materials with the same origin, the effect of the width of the particle size distribution is proved for the first time, being the oxidation to U3O8 slower in the substrate with a wider distribution (labelled material P3). Since P3 is more similar to a real irradiated fuel pellet, from a mechanistic point of view, this work present valuable input to security studies of dry storage facilities for spent fuel.