The fuel swells at a roughly constant rate throughout irradiation and has usually closed the gap and come into contact with the cladding after about 0.1% burnup. The cladding also swells (section 3.3.2) but only slowly at first, more rapidly later, and at a rate that depends strongly on temperature. The result is that for some cladding materials and in some parts of the core the swelling rate of the cladding may eventually exceed that of the fuel, tending to reduce the stress between the two and reopen the gap.

While cladding and fuel are in contact and there is a compressive stress between them as shown in Figure 2.12 both creep. Swelling of the central part of the fuel inside the load-bearing ring is accommod­ated by expansion into the central void. Swelling of the outer part is accommodated partly by the swelling of the cladding.

The cladding strain is not always uniform and in some cases may be concentrated by cracks in the fuel pellets, and this stress concentration, rather than uniform strain, may determine the maximum burnup to which the fuel can be subjected. Sometimes distortion of the pellets can cause non-uniform strain of the cladding. During irradiation a pellet tends to distort into the shape shown, very much exaggerated, in Figure 2.13. The ends tend to be displaced outwards as shown and can

cause ridges round the cladding, giving it the appearance of a bamboo cane. These ridges are not usually permanent because the pellets soon fuse together, but they are sometimes observed in fuel that has been subjected to abnormal conditions.