A model for the dynamic intragranular fission gas swelling and release

A Dynamic Intragranular Gas Release and Swelling Computer code (DIGRAS) has been developed to study the behavior of intragranular fission gas in mixed oxides during steady irradiations and in transients. The code models the interactions of non-equilibrium gas bubbles and the dynamic behavior of point defects. A variety of rate processes have been included in the model, such as bubble nucleation, re-solution, coalescence and gas migration to grain surfaces. The describing equations have been simplified using grouping and partitioning techniques. Steady irradiation simulation results agree reasonably well with experimentally measured gas release. On the other hand, simulating recent thermal transient experiments indicated that the final intragranular swelling is strongly dependent on fuel stoichiometry. Bubble diffusion rates are found to strongly affect the bubble distribution and potential for fuel motion. However, the final swelling was only weakly dependent upon bubble diffusion rates. (orig.)