[en] The purpose of this paper is to describe the effects of fission gas on fuel and fuel motion from the initiation of a transient overpower (TOP) to fuel pin failure. This paper does not deal with fuel motion after fuel pin failure or with the reactivity of the fuel but reports only the thermal response of oxide fuel during a TOP. A fuel pin can be categorized according to its most advanced fuel microstructure. In low-power irradiated fuel pins the fraction of fission gas in the fuel is greater at the top and bottom than at the midplane of the fuel pin. A high-power irradiated fuel pin has a high-power irradiated fuel microstructure at the axial midplane and a low-power irradiated fuel microstructure near the top and bottom of the fuel pin. The axial distribution of fission gas is peaked at the top and bottom of the fuel pin while the midsection contains a minimum amount of fission gas. During a TOP three cladding loading mechanisms may operate, fuel thermal expansion, fuel volumetric expansion and transient fission gas release. Fuel pin failure in a TOP is dependent upon fuel microstructure and concentrations of retained fission gas. After fuel pin failure fission gas can act as a driving force to eject molten fuel from the fuel pin. (Auth.)