[en] A model for the non-equilibrium behavior of intragranular fission gas in oxide fuel is developed to study the fundamental phenomena that determine fission gas effects in the fuel. The model explicitly represents the dynamic behavior of point defects and includes the effects of variations in stoichiometry. The mathematics of the model use the principle of moments invariance to allow approximations that significantly reduce computational expense without sacrificing accuracy. The model is intended for use as a tool to identify and explore important variables in the behavior of fission gas and determine the regimes of validity of the various modeling assumptions. The Dynamic Intragranular Gas Release and Swelling (DIGRAS) computer code developed in this work is applicable to simulations of both steady-state and transient irradiations. Flexibility is built into the code and a summary of the physical process dynamics is included in the code output