An experiment sponsored by the Gesellschaft für Kernforschung at Karlsruhe, by Belgonucléaire, and by Interatom was carried out in the French reactor Rapsodie from February 1971 to September 1972, in the frame of a research and development program for the fast breeder prototype reactor SNR. This experiment consisted in the irradiation of two bundles, each containing 34 mixed-oxide pins, with spacer grids and tie-rods, up to a peak burnup of 10.6 at.% and a peak total neutron fluence of 8 × 10 20 mm −2, corresponding to a peak fast fluence (E > 0.111 MeV) of 6.4 × 1020 mm−2. The postirradiation examinations showed that the cold-worked WNr. 1.4970 cladding stainless steel has a lower irradiation-induced swelling than the solution-annealed WNr. 1.4988. Important inelastic deformations of both types of claddings have been observed; the largest part of these deformations can be explained by the in-pile creep. A mechanical interaction between the fuel and the cladding is not excluded in the case of the pins with the cladding steel WNr. 1.4970. The destructive examinations emphasized the influence of the pellets’ initial stoichiometry on the fission product migration, on the plutonium radial distribution, and on the fuel-cladding chemical interaction. An interaction between the fuel and the blanket pellets, due to the cesium migration and to the formation of cesium uranates, leading to local cladding strains, has been noted. These phenomena, as well as the chemical interaction between the fuel and the cladding, have been investigated by extensive microprobe analysis. Finally, the fuel restructuration has been analyzed and interpreted as a function of the fuel characteristics, irradiation conditions, and cladding deformations.