Ceramic Fuel Uranium Dioxide And The Primary Principle Of Structure Defects

Uranium element in Uranium dioxides is actinide element. It has electrons occupied on unfilled f shell. The exchange association effect among f electronics in this shell is very strong, so the traditional generalized gradient approximation (GGA) or the local density approximation (LDA) method cannot reflect the nature of the uranium dioxide. Based on this consideration, many research groups considered the Hubbard U term fixed in the process of computation. But so far, no one give the standard concerning how to chose the U, and no one give the influence of U to state density. Most research workers choose U value between4.0ev to5.0ev, but they do not give reasonable reason for this value. In this paper we take U from0-10eV to observe the change of the properties of materials. With the change of the U value, from state density and band graph analysis, we found that the properties of materials present from a conductor change to semiconductor, and then to conductor. In this study, on the basis of the GGA+U method,we have studied the nature of fluorite uranium dioxide.The results show that the uranium dioxide is f-f Mott-Hubbard semiconductor, its band gap is2.09ev, which is very good compliance with the value of2.1ev in the experiment measurement. Besides, we have studied single oxygen vacancy, the oxygen interstitial atoms, uranium vacancy point defects.This paper is mainly divided into four sections, its content as follows:The first part is preface:it mainly introduced the some simple related content to nuclear materials. The purpose and significance of this section.The second part is mainly about the theory method:The basic knowledge of the first principle, three basic approximation, namely the relativity approximation, Bonn-Oppenheimer approximation, the single particle approximation (rail approximate). In addition, this paper introduces the density functional theory, and the processing methods for exchange association of potential including local density approximation (LDA), generalized gradient approximation (GGA).The third part is the fundamental nature of uranium oxide:The research is mainly on the influence to material from different value of Hubbard U. This mainly contains the changes of state density and the band structure. Besides, we also calculate the basic properties of Uranium dioxide with fluorite structure, including optimization of K points as well as the truncated can,and material properties calculation.The fourth part is the research for point defects.These defects contain single oxygen vacancy, interstitial oxygen atom and uranium vacancy in12atoms,24atoms,48atoms super cell.The fifth part is the conclusion and prospect of this paper:mainly introduces the research results, as well as the future work and the prospects of the problem.