In the model adopted for analysis, the strength of the pellet is entirely dictated by that of an outer crust — the zone within which irradiation creep is dominant. The inner bound of the outer crust is the isotherm at which irradiation creep and thermal creep become equal. The crust is assumed free from open radial cracks and in it swelling due to fission-gas precipitation is negligible. Solid fission products are the only source of swelling. Irradiation creep entirely dominates the plastic deformation of the cladding: like the pellet, the cladding swells even at zero stress. The stress ( S) in the cladding at a constant rating is calculated as a function of burnup and rises towards an asymptote. Square wave rating variations produce a new equilibrium in which S alternates between levels that depend on the fraction ( ƒ) of burnup accumulated at high rating. At constant rating, the equilibrium value of S (= S ∞) is about 3.5 MPa (500 psi) and the effects of irradiation dominate the deformation-processes in both pellet and cladding. The equilibrium tensile creep rate of the cladding is about 1.25 × 10 −3/atom % burnup. The approach to S ∞ occurs exponentially, with a time constant ( τ) of about 5 hr (during which a burnup of about 0.02 atom % occurs). If a crack is propagating through the clad by the Tomkins mechanism then, theoretically, there will be a critical value of ƒ at which propagation rate ( r) is at a maximum. ƒ- values that significantly increase r over its constant rating value are unlikely to occur in practice. The conditions which will lead to the formation, during square-wave rating variations, of a gap between pellet and clad are defined and the cyclic stress-states for the cases where a gap forms or does not form are calculated. In both cases the value of ƒ is a determining factor, provided that the cycle period is much less than τ.