The ballooning of cladding tubes

Previous descriptions of clad ballooning have only considered radial displacements and have neglected bending moments. In this paper the problem is formulated to include all three components of displacement and bending. It is found that for non-linear creep deformation bending moments may be neglected for most cases, but tangential displacements are more important than radial displacement for the non-uniform components of deformation. This has been found to lead to three important conclusions: (i) the cladding tends to retain its cylindrical cross-section when subjected to circumferential variations in temperature; (ii) higher harmonics of circumferential deformation are significant until limited by bending, typically for harmonics greater than around the twentieth: this is contrary to a strong restriction on the growth of harmonics greater than the first found when only radial displacements are permitted; (iii) cladding tends not to bulge away at hot spots but thins locally: this can lead to reduced ductility in fuel cluster conditions. The effect of anisotropy is briefly discussed. (author)