A Computer Model for Predicting Transient Fission Gas Release from UO₂ Fuel

The KFGR-T computer model has been developed to predict transient fission gas release from UO₂fuel with an emphasis on the nonequilibrium behavior of fission gas bubbles. It takes into account the relevant physical processes generally considered by other workers, as well as migration of fission gas bubbles through channels formed by the extension of dislocations to grain boundaries during the transient heatup stage, grain growth/grain-boundary sweeping during the isothermal annealing stage at high temperatures, and gas release through intergranular cracking. This computer model is applied to calculate transient fission gas releases, and the calculated values are compared with the results of out-of-pile experiments performed with UO₂ fuel base-irradiated to burnups in the range of 18 to 35 MW·d/kg U. The absolute values and the trends of the fission gas releases calculated with the KFGR-T model are in good agreement with the experimental data. A parametric study is also done to investigate the sensitivity of the model to variables such as initial grain size, heating rate, temperature gradient, and initial gas concentration, and these results are compared with the sensitivity of other models.