The Influence of Vacuum Annealing on the Nucleation and Growth Kinetics of Uranium Hydride
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The Influence of Vacuum Annealing on the Nucleation and Growth Kinetics of Uranium Hydride J.P. Knowles, I.M. Findlay, D.A. Geeson and S.G. Bazley AWE plc, Aldermaston, Reading, Berkshire, RG7 4PR, UK © British Crown Owned Copyright 2012/AWE ABSTRACT The reaction kinetics of depleted uranium under constant hydrogen pressure (1 bar) have been measured as a function of reaction temperatures between 65 and 385 °C for as-polished and vacuum annealed samples. Enhanced hydrogen reactivity was observed on samples that underwent vacuum annealing prior to hydrogen exposure. The enhanced reactivity was found to be the result of enhanced nucleation rates on annealed samples since the specific rate per reacting unit area remained unaffected. X-ray photoelectron spectroscopy demonstrates that the nucleation kinetics were promoted on annealed samples as a result of the dehydration and partial reduction of the UO2+x outer oxide layer and the formation of an oxycarbide (UOxCy) sub-layer. INTRODUCTION The reaction between uranium and hydrogen gives rise to the formation of uranium hydride. The formation of pyrophoric uranium hydride during long term storage has the potential to cause the dispersal of radioactive particles outside suitable containment. An example of such an occurrence was reported by Woods et al. [1], where a steel drum containing depleted uranium and packing material underwent an unexpected release of chemical energy when opened after a storage period of 5.5 years. For the safe and long term storage of uranium, a comprehensive knowledge of uranium hydriding kinetics is desirable. It has been previously reported that the hydriding kinetics of depleted uranium are influenced by the microstructure and chemical impurities [2-4]. For instance, Bloch and Mintz reported the hydriding rates of uranium as a function of thermal pre-treatment temperatures, concluding that micro-structural changes (grain growth) induced by heat treatments over 400 °C resulted in a three fold increase in the rate of hydrogen consumption [3]. Condon reported a similar observation, describing a 30 fold increase in the hydriding rate as a result of a substantial thermal pre-treatment at 600 °C [2]. However, it is hard to justify such large increases of bulk hydridng rates as the result of heat induced modifications to the grain size and orientation alone. Recently, vacuum annealing has been shown to promote the surface reactivity of some rare earth metals towards hydrogen adsorption owing to the desorption of surface contaminants and modifications to the oxide over-layer [5-7], therefore, a similar enhanced surface reactivity with uranium could be expected [8]. In this study the hydriding kinetics of wrought depleted uranium sheet has been evaluated as a function of reaction temperature and sample pre-treatment in terms of the rate of hydrogen consumption and the time to 1 % reaction. In addition, complementary X-ray photoelectron spectroscopy (XPS) has been employed to elucidate chemical changes in the surface as a result of sample pre-treatment.
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