A computer simulation of a fuel region has been developed based on movement of fission gas bubbles in solids under a thermal gradient. Within this region, fission gas events are followed via a Monte Carlo technique. Individual bubbles are followed through their time history from nucleation to release from the fuel, with interactions at dislocations and grain boundaries. Saturation in gaseous swelling at elevated temperature is predicted. A maximum in swelling is predicted at intermediate temperatures for a given burnup. These swelling and gas release predictions at high temperatures are in good agreement with experimental results. A low temperature modification of dislocation density is required to allow for effects dominant at low temperature which are not included in the current version of the program. With this low temperature modification, swelling predictions are in good agreement with experimental observations over the entire operating temperature range.