Received December 14, 1981 Accepted for Publication March 18, 1982 A model to predict fission gas behavior in irradiated uranium dioxide fuel during the steady-state operation of a nuclear reactor is developed. The basic physical phenomena encountered in analyzing the disposition of fission gas have been retained, but in a simplified form for ease of calculation. The analysis includes treatment of in-tragranular, grain face, and grain edge gas, and release to open spaces. The code is utilized to obtain comparison with experimental data and to perform fuel behavior studies. The sensitivity studies indicate the importance of grain face and grain edge bubble treatments in modeling fission gas. It is found that representation of release in different sections of the fuel pin is possible in a simple way by assuming evenly spaced bubbles on the edge, and that grain edge bubble interlinkage is a necessary condition for release to the open spaces. The sensitivity studies show that fission gas swelling is mainly due to grain edge bubbles. Grain face bubbles, although large in size, are few in number and contribute little to swelling. Intragranular swelling is intermediate between these two values. The code is successfully used to analyze the Westinghouse fission gas release data from the Zorita, Spain, light water reactor and data from the U.K. reactor DIDO. This success in modeling experiments suggests that the present code can be used in predicting fuel element performance, which is necessary in nuclear fuel design, safety analysis, and interpretation of experimental data on fuel element behavior.