Uranium Vapor Response Experiments To Study

The kinetics of uranium corrosion in moist atmosphere was studied by thermogravimetric method, the resultant structure of uranium-water vapor reaction and surface element species were observed by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), and the possibility of the uranium water vapor reaction was also researched from the aspects of theoretical kinetical and thermodynamical calculations. On the base of experimental results, the oxidation mechanism and kinetics were discussed. Suggestions have been presented for the future-work.The thermogravimetric experiments of metallic uranium corrosion was carried out in the condition of temperature 50℃⁹0℃, RH (relative humidity) 32%⁸7%, and the kinetics curves were obtained respectively. The results show that there are three stages in the oxidation of uranium-water vapor reaction at various temperatures. First, there was an initial reaction stage, at which the corrosion rate is very fast. And it usually obeys the linear or parabolic reaction law. Second, the corrosion rate tended to be slow down after the first corrosion stage. The weight gain of uranium sample didn't seem to have the same rate as before with time going. At the third stage, the corrosion reaction presented to be a linear reaction, with a stable oxidation reaction rate constant. In this thesis, the dynamics curves on the rear part of linear reaction stage were fitted, and the activation energy deduced was about 42.899 kJ-mol^-1, which is between the range of literature value 29.3-50.2 kl/mol. Uranium-water vapor reacts fast at higher temperatures, and it's so fast that the initial oxidation stage couldn't be easily founded, with a smooth turning point observed on the dynamics curves.The chemical state of resultant species were determined by the characteristic peaks of U and O elements in XPS analysis. The results showed that the output of uranium-water vapor reaction is uranium dioxide (UO₂). One peak with binding energy 530.2eV was observed, and its position did not move much as corrosion time increased. And the results agreed with the previous report that one of O1s peaks (530.0eF) may be attributed to uranium dioxide. As a results of spin splitting of orbit, there were 2 U4f peaks in the uranium spectroscopy, U4f_7/2 å'Œ U4f_5/2 . And they appeared at 380.0eV and 390.8eV with symmetric peaks, which resulted from the formation of uranium dioxide. Also the XRD (X-ray diffraction) analysis confirmed the results further.The thermo-chemistry of uranium-water vapor system was theoreticallycalculated by quantum. The research results showed that water vapor was ready to chemically adsorpted onto uranium surfaces with U-O bond, and that low temperature favors uranium water vapor reaction. The reaction could not go on spontaneously for above 898K, and on the contrary, the intermediate UOH₂ of the chemisorptions would dissociate. Based on the potential energy function of OUH, the reaction dynamic processes for the collision U+OH had been studied by using the Monte-Carlo quasiclassical trajectory approach. The results indicate that the reaction of U+OH (0,0) is a reaction without threshold energy, and its main products are UO+H, with some OUH and few UH. These results were of significance, which provide theoretical foundations for the study of uranium oxidation corrosion in moist environment.