Influence of Zr doping on solubility of Xe in UO2: A first-principle study

As a major fuel of the light-water reactors, UO2 has excellent properties such as high melting point, good radiation resistance, corrosion resistance, compatibility with cladding materials, and strong ability to tolerate fission gas. The Zr atoms are inevitably introduced into UO2 lattice during the operation of a nuclear reactor, which can affect the solubility of Xe in the UO2. In this paper, we calculate the formation energy of vacancy defect and the binding energy of Xe in vacancy of Zr doped UO2. The calculations presented here are based on density functional first-principle and projector augmented-wave method. A plane-wave basis set with a cutoff energy of 400 eV is used. The generalized gradient approximation refined by Perdew, Burke and Ernxerhof is employed for determining the exchange and correlation energy. Hubbard U term is used for considering the f-electron localization. Brillouin zone is set to be within 555 k point mesh generated by the Monkhorst-Pack scheme. The self-consistent convergence of total energy is 110-4 eV/atom. The calculations are performed in a 222 supercell. In order to verify the calculating process, the formation energies of U and O point defects are compared with those in the literature. Then the influence of Zr doping in the UO2 on the solubility of Xe in the UO2 is studied. The results show that the ability to form the vacancy defects is different in the U-rich and O-rich environment of UO2. The vacancy defects in UO2 are more likely to form in O-rich UO2. The Zr doping will lead to the increasing of the formation energies of defects in both cases. The Zr doping will also change the binding energy of Xe in void. For all the systems studied, only the binding energy of Xe adsorbed to the void consisting of four point defects increases, while the rest decrease. The solution energy, equaling the sum of the binding energy of Xe and the vacancy formation energy, will increase after doping Zr, because the decrement in binding energy is generally less than the increment in vacancy formation energy. In summary, the presence of Zr will weaken the solubility of Xe in UO2, which is mainly due to the hindering of vacancy defects from forming. This result has a certain value in studying the dissolution of fission product Xe after a small amount of Zr has entered into the UO2 fuel in nuclear reactor.