Particle fuels such as sphere-pac and vipac have been considered as promising fuel systems for fast reactors because of their inherent potential in remote operation, cost reduction, and incineration of minor actinides or low-decontaminated plutonium. The FUJI test addresses the questions of fabrication of mixed-oxide (MOX) particle fuels with high Pu content (20%) and its irradiation behavior during the start-up phase. Four kinds of fuel, i.e., MOX sphere-pac, MOX vipac, MOX pellet, and Np-MOX sphere-pac, have been and will be simultaneously irradiated under identical conditions in the High Flux Reactor in Petten, Netherlands. First results show that the particle fuel undergoes a substantial structure change already at the very beginning of the irradiation when the maximum power is reached. The changes in microstructure, i.e., the formation of a central void and the densification of fuel, decrease the fuel central temperature. Thus, the fast and strong restructuring helps to prevent central fuel melting at high power levels.