Due to Fast Breeding Reactors’ (FBRs) higher power density and burn-up, FBR fuels have higher temperature, higher fission gas release rate, significant fuel and cladding deformation, and the formation of central voids. Therefore, modelling of FBR fuels present new challenges for fuel performance code development. The formation of central voids and the migration of nuclides are the most important phenomena in fast reactor MOX fuel, and correctly simulating the nuclide distribution and pore migration within the fuel pellets is the core issue in fast reactor fuel performance simulation. In this study, OFFBEAT was used to simulate the driver rod in the MYRRHA reactor, considering the effects of different Am content on the fuel rod irradiation behavior. By comparing the simulation results with TRANSURNUS in open literature, models in OFFBEAT for fast reactor simulations are verified and analyzed. Based on the verified model, the linear power of the rod was increased to extend the study of fuel rod irradiation behavior in the MYRRHA reactor at high power level, simulating and analyzing the nuclide redistribution and pore migration phenomena in the fuel. The results indicate that the temperature, gap width, and fission gas release rate in the fuel pellets calculated by OFFBEAT for the MYRRHA reactor are acceptable, with effect of different Am content on fuel behavior. Significant nuclide redistribution and void migration occurred at higher power level. The temperature gradient is the primary factor influencing nuclide redistribution and void migration, and both phenomena tend to transport toward the center of the fuel pellets. Therefore, OFFBEAT meets the simulation requirements for FBRs and the analysis demonstrates the suitability and safety of low Am-enriched MOX fuel under irradiation based on the current MYRRHA design.