The temperature difference of 1500 K or more between centre and sur­face of the fuel induces substantial thermal stresses that cause cracks. Figure 2.7 shows a cracking pattern typical of those seen in irradiated fuel. It is important to realise, however, that the pattern of cracks seen when the fuel is cool is quite different from that present during irradi­ation. The conjectured development of cracks in pellet fuel is shown
in Figure 2.10. When the fuel first generates power and experiences a temperature difference stresses are set up as indicated in Figure 2.10a. The tensile hoop stress at the outside exceeds the rupture stress and radial cracks are formed. In the centre, however, cracks are very quickly healed by the recrystallisation process.

After a certain amount of irradiation, usually about 0.1% burnup, the fuel has swollen to touch the cladding. Subsequent swelling is resisted by the cladding and the fuel is subject to a compressive stress. Under the influence of this it creeps rapidly near the centre and very slowly at the periphery. As a result the cracks close at the centre but not at the outside. When they have closed sintering or recrystallisation heals them. Thus after prolonged irradiation the centre of the fuel is free of cracks while the periphery retains cracks that tend to taper inwards (Figure 2.10b).

When the reactor is shut down the fuel cools, the centre con­tracts more than the periphery, and new radial cracks are formed that are wide at the centre and taper towards the outside (Figure 2.10c). These cracks are prominent in pictures of fuel cross-sections, such as Figure 2.7, but it should be remembered that they were not present when the reactor was operating.