Ceramic fuel pellets consist of fuel grains with a range of grain sizes. The initial grain size distribution is determined by the manufacturing process and sintering conditions. If the fuel temperatures are high enough during irradiation — greater than ~1200 °C in UO2 fuel (Ainscough et at., 1973) — the smaller grains shrink while the larger grains grow in a process known as equiaxed grain growth (since there is no preferential direction for the growth). The net result is an increase in mean grain size. The driving force for the equiaxed grain growth process is provided by the stresses induced at the grain boundaries due to their radius of curvature (Olander, 1976). At still higher temperatures — greater than ~1800 °C in oxide fuel (Olander, 1976), which is generally only achieved for CANDU and fast reactor fuel — the fuel pores become mobile and migrate up the radial temperature gradient via an evaporation-condensation mechanism. In doing so, they create grains elongated in the radial direction — the process is thus known as columnar grain growth. The pores that reach the pellet centre form a central void in solid pellets, or enlarge the bore of annular pellets. The migration of pores causes additional densification to that described above. It also causes plutonium redistribution in fast reactor oxide fuel (Bailly et at., 1999). The increase in fuel thermal conductivity due to removal of porosity, together with the central void formation and pellet bore enlargement, cause a significant reduction in fuel centreline temperature — of the order of several hundred degrees centigrade in fast reactor fuel (Olander, 1976).