Thermal expansion occurs instantaneously as the fuel temperatures increase. Since the fuel pellets/bars are at significantly higher temperatures than the cladding, differential thermal expansion between fuel pellets/bars and cladding leads to dimensional changes, which tend to close any pellet-cladding, or bar­cladding, gap. In the case of ceramic fuel pellets, the hotter central regions of the pellets expand more than the cooler outer regions, giving the pellets a distinctive ‘wheatsheaf’ or ‘hourglass’ shape. The differential thermal expansion within the fuel pellets also imposes shear stresses, which cause cracking of the pellets. The

resulting pellet fragments can relocate into any pellet-cladding gap. An exaggerated schematic of an idealised cracked and wheatsheafed pellet is reproduced in Fig. 14.1 (Gittus, 1972).

Both pellet-cladding, or bar-cladding, gap closure and fuel fragment relocation decrease the maximum fuel temperature at any given power.