U And The O <sub> 2 </ Sub> And Co <sub> 2 </ Sub> Kinetics And Uranium - Oxygen Adsorption Characteristics Of Study

The kinetics behavior of uranium sample, whose surface is covered with UO2, has been studied using temperature programmed analytical technology (TPAT). In addition, pilot theoretical simulation of properties of oxygen adsorption on U(001) surface is been completed by Full Potential Linearized Augmented Plane Wave (FLAPW) method.The experimental results of the TPAT in O2-U(UO2) system show that there in turn are three stages in the oxidation of U(UO2) with O2 at various partial pressure of O2 in the argon atmosphere: slow-speed (inducement-term) - quick-speed - slow-speed. The ignition temperature (To) is arrives fast with the increasing of the heating rate. Length of the inducement-term decreases with increasing of heating rate. The T0 relate to the O2 concentration: The T0 varies between 581-606K when the O2 concentration is less than 45%, but the T0 is about 585K when the O2 concentration is 45%. The oxidation of U(UO2) with O2 can be explained with interface control mechanism of the nucleation-growth model. The apparent activation energy (Ea) depends intensively on the oxygen concentration and gas flow rate. At the given gas flow rate 10ml/min, the Ea is 231.9 kJ/mol, 184.9 kJ/mol and 120.5 kJ/mol when the O2 concentration is 16%, 30% and 45%, respectively. The Ea is 134.3kJ/mol when the gas flow rate is 20ml/min and the O2 concentration is 10%. X-ray diffraction (XRD) analysis show that the UO2 is major intermediate resultant of reaction and the final product is U3O8.The experiment of CO2 and U(UO2) is done and the result shows that there is a chemical process of dissociation of the CO2 in the reaction. The results at various partial pressure of CO2 in the argon atmosphere show that there in turn are three stages in the oxidation: slow-speed (inducement-term)-quick-speed-slow-speed as well as in the oxidation of O2 and U(UO2). But in the CO2-U(UO2) system T0 is clearly higher than that in the O2-U(UO2) system. The T0 varies between 1025.9-1081.5K , 694.5-827.8 K, and the Ea is 133.5kJ/mol , 30.4 kJ/mol, respectively, when the CO2 concentration was 25% , 50% with the gas flow rate was 10ml/min.In addition, the properties of Oxygen adsorption on the a-U (001) surface are studied using the FLAPW method based on the density-functional theory (DFT) within the generalized gradient approximation (GGA: Perdew-Bourke-Ernzerhof96). Two chemical adsorption states, metastable state and stable state, are found through optimized calculation at the assumed condition of the surface coverage of 1 and the U-0 being vertical to the (001) plane of uranium crystal. Binding energy is 6.535eV/atom; 8.418eV/atom at the adsorptionheight is 2.474A for the metastable state, 1.771 A for the stable state, respectively. Oxygen atom and uranium atom is banded together with covalent bond. The contribution of 5f electrons to the chemical bond at the metastable adsorption state is less than that at stable adsorption state, partial 5f electrons are localized at the lower energy level and mix with 2s and 2p electrons of oxygen atom to form covalent bonds.