Theoretical Studies Of Electronic Structure And Bonding Nature For Actinide Carbide And Fluoride

As the increasing energy demand of the industrial manufacture and social activities, nuclear energy has attracted more and more attentions for its advantages. As one kind of the clean energy, the nuclear energy has the following advantages with respect to the fossil fuel(coal, petroleum):(1) the nuclear fuel has a small bulk, but can release a large amount of energy,(2) the nuclear reactor’s security index is very high,(3) the nuclear reactor can produce our demanding energy without releasing the toxic gas(sulfur dioxide, nitrogen oxides et. al.). The primary fissile elements in nuclear fuels are 235 U,232Th(the fissile 233 U can be created from the transmutation of 232Th)and 239Pu。Thorim-based molten salt reactor(TMSR) has been studied in our institute. The nuclear fuel has two kinds of mode: the tristructural isotropic(TRISO) particles for the solid fuel and the actinide fluorides dissolved in molten salt for the liquid fuel. We have studied the fuel compounds by using the density functional theory. Our researches include three aspects: the defect stability in thorium monocarbide, the structural and electronic properties of thorim dicarbide and the bonding nature of the actinide tetrafluorides AnF4(An = Th-Cm).The elastic properties and point defects of thorium monocarbide(ThC) have been studied by means of density functional theory based on projector-augmented-wave method. The calculated electronic and elastic properties of ThC are in good agreement with experimental data and previous theoretical results. Five types of point defects have been considered in our study including vacancy defect, interstitial defect, antisite defect, schottky defect and composition-conserving defect. Among these defects, the carbon vacancy defect has the lowest formation energy of 0.29 eV. The second most stable defect(0.49 eV) is one of composition-conserving defects in which one carbon is removed to another carbon site forming a C2 dimer. In addition, we also discuss several kinds of carbon interstitial defects, and predict that the carbon trimer configuration may be a transition state for the carbon dimer diffusion in ThC.Thorium dicarbide also belongs to the main components of the thorium carbides nueclar fuel. ThC2 occurs in three different modifications. At room temperature, a monoclinic unit cell is found. Between 1430 and 1480℃, a rotation of the C2 dumbbells starts, leading to a tetragonal structure which changes to cubic above 1480℃ with complete rotational disorder of the C2 units. In our study about thorium dicarbide, we calculate the geometry structure, electronic properties and phonon diversion curves based on the density functional theory. Our calculated lattice constants and interatomic distances are in excellent with the experimental measurement. Among all the already known low temperature modifications of actinide dicarbides(ThC2, PaC2, UC2, NpC2 and PuC2), only ThC2 has an anomalous monoclinic crystal structure while the rest of actinide dicarbides have a tetrahedral CaC2-type structure. From our calculated Electron Localization Function(ELF) of ThC2, PaC2, UC2 and NpC2, we can see that the anomalous monoclinic geometry of α ThC2 contributes to the electron localization in Th-C bond perpendicular C-C dimer. In this chapter, we also discussed the equation of states(EOS) and the phonon dispersion curve of thorium dicarbide.The knowledge of chemical bonding for actinide fluoride compounds is essential to understand and predict the physical and chemical behavior of actinide elements in fluoride molten salt. In this work, the bonding nature of actinide tetrafluorides AnF4(An = Th-Cm) are investigated by using scalar relativistic density functional theory. Bond order analyses show relatively stronger An-F bonds for An = U-Np and weaker ones for An = Th, Am and Cm. Despite the dominant ionic character of An-F bonds, a considerable covalent interaction is indicated by the overlap integral value of F 2p and actinide 5f, 6d orbitals. Both natural population analyses and electron density analyses show that An-F covalency rises initially before reducing in the latter systems with the maximum at Np and Pu and the obviously strong ionic bonding character in An = Th, Am and Cm. Compared to AnCp4(Cp = η5-C5H5) reported in the literature, our study on AnF4 suggests a much more prominent actinide-ligand covalent interaction. And the roles of orbital overlap and near-degeneracy in driving covalency are discussed.