A Study on Defect Formation Energies in UO 2 and UN Nuclear Fuels and on Xenon Stability in the Defects

Atomic Xenon one of the fission products of uranium nitride (UN) and uranium dioxide (UO2) in nuclear reactors, diffuses as a gas. We investigated defect formation and xenon mobility in the fuel by using first-principles calculations as implemented in the VASP code. Frenkel defects in uranium were more difficult to find than Frenkel defects in anions and Schottky defects in the UN and the UO2 fuels. Our calculations of the formation energies of extrinsic defects showed that the vacancy formation energies of anions were higher than the vacancy formation energy of the uranium. When xenon diffused in the UN and the UO2 fuels, it preferred sites of vacancy clusters to single vacancies. This means that xenon in Schottky defects site is difficult to remove.