Theoretical Study On The Interaction Between Actinides And Rare Gases

With the vigorous development and application of nuclear energy technology,the basic physical and chemical properties of Actinide materials,especially the influence of rare gases on their structure and physical properties,are attracting increasing attentions in the research fields of both nuclear energy and condensed matter physics.In this work,the understanding and prediction of complex chemical bonds between Actinide oxide?U_xO_y?and rare gases?RG?in a strongly correlated electronic system are systematically studied based on the state-of-the-art density functional theory?DFT?calculations.We adopted some advanced calculation methods,including DFT+U,van der Waals correction,first-principles molecular dynamics,etc.,to accurately understand the surface stability,electronic structure,and magnetism of Actinide oxides,as well as their interactions with rare gaese.The chemical bonds and diffusion characteristics of rare gases on surfaces and their effects on surface stability and electronic features under different environments are further analysed,so as to gain insights in the intrinsic relationship between the RG-U_xO_y chemical bonds and U-5f orbitals.Firstly,we discuss the methodologies on describing weak interactions in strong correlation systems.The adsorption of different RG atoms on UO₂?111?surface and the doping in the bulk cell were calculated by using several commonly used GGA functionals?such as PBE,PBEsol,RPBE and revPBE?with the nonlocal van der Waals corrections.The RPBE functional provided good description on the structure and electronic properties of RG/UO₂?111?systems,which are consistent with actual observations.With the RPBE+U+VDW-D3 functional,we studied several major types of Actinide oxide surfaces.The most-stable UO₂?111?shows an antiferromagnetic?AFM?ordering,in which its spin up and spin down atomic layers arrange alternately along the[001]direction.The formation of oxygen vacancies is significantly easier on the surface and subsurface layers than that in the interior part,whereas it's not easy to form uranium vacancy on the surface.Nitrogen atoms are more easily adsorbed on the surface oxygen vacancies than Carbon atoms,which is consistent with the experimental observations that the nitriding treatment of U and UO surfaces is an effective way for the oxidation resistance.The adsorption and diffusion properties between the rare gas and the Actinide oxide surface/subsurface were then studied.It is found that the rare gases He,Ar and Xe are more likely to adsorb to the hollow position of the UO₂?111?surface.The calculation of the adsorption energy shows that it has a weak interaction with the surface,and the diffusion barrier on the surface is also very low.The adsorption energy decreases slightly with the increase of the concentration of the rare gas,with the delocalized charge distribution of the rare gas at high compositions.The adsorption of rare gas atoms into the inner layer of the surface becomes difficult with the increase of depth,which causes obvious deformation of the local structure.Further research and discussion are performed on the rare gas adsorption under moist environments.Taking the coverage of 1/4ML and 1ML water monolayer on UO₂?111?surface as an example,it is found that the adsorption energy of rare gas on the surface is even slightly reduced in the case of low water molecule concentration,The rare gas occupies the hollow position near the water molecules on the surface of UO₂?111?.When the concentration of water molecules on the surface increases,the rare gas tends to be captured by the hollow position of the water molecule layer,but the adsorption becomes poor.We also analyzed the surface of PuO₂ which has the similar properties as UO₂.The ground state of PuO₂?111?surface has the 1 K-AFM structure,on which the adsorption of RG atom prepers the hollow position.With the increasing atomic radius,the adsorption energy gradually increases and the chemical bond between Xe and the surface is even stronger than that on the UO₂?111?surface.