Grain boundaries (GBs) act as sinks for defects, reducing defect energies in irradiated materials. However, drastic changes to the grain structure can occur in fuel materials. Irradiated materials are driven systems, with defects constantly being generated, and the microstructure evolves to attempt to control this influx of defects. At high temperature, defects are mobile and can segregate to GBs, though the grains grow, increasing the required migration distance for segregation. At lower temperatures, migration is too slow for segregation to counteract defect production, so grain subdivision occurs creating a structure with a two orders of magnitude smaller grain size. At even lower temperatures, grain subdivision is not sufficient to control the defects and amorphization occurs. In this presentation we use simple models of the system free energy and the defect evolution to begin to define the transition conditions between grain growth, grain subdivision, and amorphization in irradiated nuclear fuel.