[en] A thorough electron microscope study along radial and axial cuts of highly rated fast breeder fuel (>= 200 W/g with burnups between 1 and 21 at.-% and central temperatures of >= 2500⁰C for oxides) was performed. The replica technique was used to study both intra- and intergranular fission gas bubbles as well as pores and voids. In oxide fuel, the intragranular fission gas bubbles were in well defined size range of 200 to 350 A with densities between 10¹⁴ and 10¹⁵ bubbles/cm³ and only slight dependence on local fuel temperature and burnup. The intragranular swelling was therefore small, and it showed a maximum at intermediate temperatures (approximately 2000⁰C). Model calculations on the kinetics of the mobility of fission gas atoms, together with an examination of the porosity distribution in the fuel during operation indicate that in highly rated oxide fuel most of the gas is released via multiple fission induced re-solution from bubbles and subsequent atomic diffusion. In contrast, bubble and pore mobility together with sweeping by lenticular voids contribute with an estimated < 25% only to the overall release. Carbide fuel was predominantly studied following thermal irradiations with less accurately known time-temperature histories. The swelling behaviour in such fuel was similar to that of oxide fuel only at low temperatures, typically <= 1200⁰C. Above this critical temperature, large bubbles at low densities are observed as well as essential swelling. It is argued that these differences can be explained partly by a smaller re-solution probability at high temperatures. Further possible reasons are discussed and suggestions are made for future work enabling a better understanding of these basic differences between oxide and carbide fuel. (author)