The response of fission gas bubbles to the dynamic behavior of point defects

Abstract

A model has been developed to investigate the response of fission gas bubbles to the dynamic behavior of uranium point defects in uranium oxide. Simple thermodynamics is used to calculate the thermal equilibrium concentrations of both anion and cation point defects in UO 2± x and fully dynamic rate theory has been extended to estimate the dynamic concentrations of uranium vacancies and interstitials and the enhanced self-diffusion of uranium during constant temperature and thermal transient irradiations. During low temperature irradiations, fission production of uranium Frenkel pairs is dominant and the slow migration rate of the vacancies leads to long start-up transients (~10 6 s in stoichiometric UO 2 at 1000 K) as the vacancies reach a quasisteady state with the microstructure. Thermal ramp behavior of the point defects and fission gas cavities depends primarily upon the initial concentration of point defects, and the temperature at which the thermal production of point defects becomes dominant. Our results indicate that non-equilibrium fission gas behavior is important for both constant temperature and thermal transient irradiations and that fission gas bubble behavior models must consider fuel stoichiometry. Below about 2200 K the dynamic behavior of the point defects should be incorporated as well.