First principles study of H₂ dissociation, H atom and O atom diffusion on Mo doped γ-U (100) surface

Abstract

<sec>As an important uranium alloy, U-Mo alloy has excellent mechanical properties, structural stability and thermal conductivity, which is an important nuclear reactor fuel and tank armor. However, there exists a serious of fundamental problems of U-Mo alloy which need solving for practical applications. U-Mo alloy is easily subjected to surface corrosion of small molecules including the H₂, O₂, H₂O, and CO₂. The hydrogen corrosion and oxidation will have significant influence on it. In order to further investigate the reaction mechanism, based on the density functional theory and the transition state algorithm, the first principles calculation of γ-U (100) with Mo atom doping and Mo coating is carried out.</sec><sec>Firstly, the minimum energy path of H₂ molecule is calculated for the dissociation adsorption on Mo-U and 4Mo-U surface. Secondly, the transition states of H and O atoms are studied during surface diffusing between adjacent most stable adsorption sites. Thirdly, the bulk phase diffusion of H and O atoms are investigated and the relationship is analyzed between adsorption energy and adsorption height in the bulk phase diffusion.</sec><sec>The results show that when H₂ molecule is adsorbed at the configuration of top horizontal position, the H atom needs to overcome a barrier to triggering off the H—H bond-broken and then is adsorbed on surface bridge site by the neighboring atoms. The energy barrier for H₂ dissociation on 4Mo-U is higher than that of Mo-U. Meanwhile, the lower energy barrier is required for O atom to diffuse in Mo-U, so that it can be adsorbed, dissociated and diffused quickly, and then forming an oxidation film on the surface. Furthermore, both H and O atoms need to cross the energy barrier to diffuse into the body phase, forming chemical bonds with the atoms and staying in the body phase stably finally.</sec><sec>In this paper, we comprehensively analyze the dissociation and diffusion of the initial stage for hydrogen corrosion and oxidation on uranium-molybdenum alloy by theoretical studies. The results lay a foundation for theoretically exploring the surface corrosion mechanism of U-Mo alloy. Meanwhile, They provide theoretical support for investigating burn-in and corrosion of uranium-molybdenum alloy, predicting material properties under extreme and special environment, and providing a reference for further research on corrosion resistance of uranium-molybdenum alloy.</sec>