Current State and Prospects of Modeling Fission Gas Release in Oxide and Nitride Fuels

This study presents a comprehensive review of fission gas release (FGR) mechanisms in oxide and nitride nuclear fuels, along with modern approaches to their modeling. A schematic representation of the current understanding of FGR processes has been developed, encompassing intragranular diffusion, bubble coalescence, growth at grain boundaries, and the formation of interconnected gas tunnels. Special attention is given to atomistic methods (DFT, MD), which enable the assessment of how fuel type, stoichiometry, and lattice defects influence the migration mechanisms of fission gases. A comparative analysis of activation energy values obtained through DFT and MD simulations has been performed against experimental data, leading to the identification of discrepancies and their underlying causes. Preferred vacancy sites for fission gases in nitride and oxide fuels have been determined. The study also examines the diffusion and kinetic models implemented in fuel performance codes: their features and role in predicting FGR behavior. Key unresolved questions requiring further investigation are highlighted, including the primary nucleation sites of fission gas bubbles (intragranular vs. intergranular), the role of grain boundaries in tunnel formation, and the conditions under which burst release occurs at high burnups. The study emphasizes the necessity of an integrated approach combining experimental investigations and advancements in fuel performance modeling through atomistic and CFD methodologies to improve the predictive capabilities of fuel behavior under various reactor operating conditions.