A model to predict the release of fission gas in $UO_2$ pellet during the normal operation of a nuclear reactor is developed being based on the movement of fission gas bubbles under thermal gradient. It incorporates the relevant physical parameters: critical bubble size depending on thermal gradient, average bubble size of lenticular type intergranular bubbles, and bubble size distribution of which the equation is obtained by a nonlinear curve fitting method from the Gruber's standard distribution curve.
The results predicted by the present model are compared with the AECL experimental data and with those of the Hasting model. It is found that the computational results of the present model agree with the experimental data better than those of the Hastings model. Improvement is also made on the reliability of the prediction and the response of the release with respect to reactor power and burnup.