A mechanistic model named KFGR-T has been developed to predict transient fission gas release from $UO_2$ fuel by emphasizing the importance of non-equilibrium behavior of fission gas bubbles. It takes into account relevant physical processes generally considered by other workers, and particularly, migration of fission gas bubbles through channels formed by extension of dislocations to grain boundaries during the transient heat-up stage, grain growth/grain-boundary sweeping during the isothermal annealing stage at high temperatures, and gas release through intergranular cracking.
The present model was applied to calculate transient fission gas releases, and the calculated values were compared with the results of out-of pile experiments performed at Harwell, ANL, ORNL and JAERI with $UO_2$ fuel base-irradiated to burnups in the range of 18 to 35 MWd/kg-U. The absolute values and trend of fission gas releases calculated with use of the present model were in good agreement with the experimental data.
A parametric study was also made to investigate the sensitivity of the present model to variables such as initial grain size, heating rate, temperature gradient and initial gas concentration, and these results were compared with the sensitivity of other models.