The current trend in nuclear industry to extend the burnup limits up to high burnup requires the analysis of the phenomenology related to the nuclear fuel, specifically the diffusion and fission gas release (FGR) related to this phenomenon plays an important role. In this paper, the diffusion of stable fission gases (xenon and krypton) in nuclear fuel UO2 is addressed through the predictions for FGR to rod void volumes obtained with FRAPCON-3 fuel performance code. The theoretical base of diffusion model in FRAPCON-3 code is shown and some modifications are proposed, such as the removing of empirical correlations not related to diffusion phenomenon and fitting parameters included in the model. Besides, the resolution process in the proximities of grain boundaries is considered in a different way, and the grain growth mechanism from a specific temperature threshold is implemented into the code. The database applied for evaluation is presented and the results with the original and modified model are shown.