Supercritical Co <sub> 2 </ Sub> And Uranium Surface Interaction Research

The corrosion resistance of uranium surface treated with supercritical carbon dioxide (SCCO2) has been studied by weight gain method and electrochemical method. The reaction product and surface element species have been studied by x-ray diffraction (XRD) and Auger electron spectroscopy (AES) methods. The interaction mechanism of supercritical carbon dioxide with uranium surface by thermodynamics method was also studied. The character of carbon dioxide dimer was studied by quantum chemistry calculation method and the PVT relation of supercritical carbon dioxide was also calculated by BWR state equation. The conclusions are reported as following:The oxidation kinetics curves (AW-t) of uranium has been gained at 60 70% RH condition after its surface was treated with SCCO2. The results show that in our research PtT(pressure ,time and temperature) region the corrosion resistant effect of treated uranium depends on the treated time. The passivation film thickness maybe have an effect on the corrosion resistance.The electrochemical corrosion resistance of treated uranium in the solution (50ug/gCr) has been studied by electrochemical measurement. The investigation indicates the electrochemical corrosion resistance characteristic of uranium surface has also been enhanced.The corrosion resistance of uranium with high carbon content may be lower than that of uranium with low carbon content. Therefore we consider the carbon format (carbide or free carbon) may be the mainly reason of improving corrosion resistance. We consider the reaction product of treated uranium surface is mainly composed of UO2,UC (or UCxOy) and free carbon. It may be the mainly reason for surface layer containing UC (or UCxOy) to improve corrosion resistance. At the same time the change of Gibbs free energies of reaction product of supercritical carbon dioxide with uranium has been computed. From the aspect of thermodynamics (A G<-500kJmol-1) the reaction is possible.We study on the possible molecular structures UCO2 (C2v and C-v) of carbon dioxide adsorbed on metallic uranium surface by Density functional theory(DFT). The results show that its adsorbed structure is unstable. We research on the carbon dioxide dimers(C2h and C2v) by DFT too. The results indicate that dissociation energies is kJmol-1 and intermolecular vibration frequencies are small of both carbondioxide dimers (Cah and Civ). Therefore we prove that the carbon dioxide dirners are weakly bound molecule indeed. We also use Benedict-Webb-Rubin (BWR) state equation to correlate the PVT data of supercritical carbon dioxide at temperature 310-600K,pressure 7.5- 30.0 MPa,and obtain a satisfied result.