A computer code to predict fission gas release in $UO_2$ fuel during normal operations was developed. In this model, the fission gas release was analyzed by solving the diffusion equation with the production rate and diffusion coefficient varying with time during reacter operations.
The following two steps were taken for the analysis of the fission gas release in the model:
a) released from fuel grains to grain-boundaries
b) released from grain-boundaries to internal free volume of the fuel.
For numerical calculation of intragranular model, time-dependent governing equations coupled with the behaviors of the gas atoms and the bubbles in the grain matrix were derived by using a suitable grain-boundary loss term. These coupled equations were solved by the successive iteration method.
The flux of gas atoms arriving at the grain-boundary was computed with grain-boundary loss term. The continual growth of grain-boundary bubbles, resulting from the accumulation of gas atoms on the grain-boundary, leads to grain-boundary interlikage. Then, all gas atoms that subsequently reach the grain-boundary were assumed to be released. The model also deals with the mechanism of knock out release which is independent of the fuel temperature.
The results predicted by the present model were then compared with the experimental data of DIDO reacter fuels obtained by Bellamy and Rich. It was found that the computational results well agreed with the experimental data. This agreement suggests that the present model can be used in analyzing fuel element performance for nuclear fuel design and safety analysis.
