Abstract
Defect energies for halogen impurity atoms (Cl, Br and I) in thoria are calculated using the generalized gradient approximation and projector augmented plane wave potentials under the framework of density functional theory. The energy to place a halogen atom at a pre-existing lattice site is the incorporation energy. Seven sites are considered: octahedral interstitial, O vacancy, Th vacancy, Th-O di-vacancy cluster (DV) and the three O-Th-O tri-vacancy cluster (NTV) configurations. For point defects and vacancy clusters, neutral and all possible defect charge states up to full formal charge are considered. The most favourable incorporation site for Cl is the singly charged positive oxygen vacancy while for Br and I it is the NTV1 cluster. By considering the energy to form the defect sites, solution energies are generated. These show that in both ThO2-x and ThO2 the most favourable solution equilibrium site for halides is the single positively charged oxygen vacancy (although in ThO2, I demonstrates the same solubility in the NTV1 and DV clusters). Solution energies are much lower in ThO2-x than in ThO2 indicating that stoichiometry is a significant factor in determining solubility. In ThO2, all three halogens are highly insoluble and in ThO2-x Br and I remain insoluble. Although ½Cl2 is soluble in ThO2-x alternative phases such as ZrCl4 exist which are of lower energy.
Highlights
The incorporation of impurity atoms in a ceramic can have a strong influence on the chemical, mechanical, optical and diffusional properties of the material [1,2]
Where E1⁄2Xi ThnO2n is the energy of a halogen atom incorporated into a ThO2 supercell, E1⁄2ThnO2n is the total energy of the defect-free ThO2 supercell and E1⁄2X is the energy of a single halogen atom
Calculations were first performed on a halogen atom occupying an octahedral interstitial site in defect-free ThO2
Summary
The incorporation of impurity atoms in a ceramic can have a strong influence on the chemical, mechanical, optical and diffusional properties of the material [1,2]. Volatile halogens are produced in smaller quantities (compared to gaseous fission products) and are present in fuel as impurity atoms but their high electro-negativity means they are especially reactive, leading to detrimental chemical interaction with the fuel. They cause the eventual degradation of mechanical properties through stress corrosion cracking and brittle fracture of the clad [8,10]. Some atomic scale modelling studies on trapping and diffusion of the volatile fission products (Xe, Br, Rb and Cs) [18,20] have been reported but these only considered the incorporation of impurity atoms in neutral point defects and neutral clusters of defects, leaving aside the possibility of incorporation in charged defect configurations. The effect of van der Waals (vdW) interactions is considered and included for all possible defect configurations
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