The stress and strain distributions produced in nuclear-fuel-element cladding by the expansion of cracked pellets have been calculated both analytically and by numerical methods. As the radial (and transverse) pellet cracks open, the tendency for the cladding to stretch preferentially over them is reduced by frictional sliding at the pelletclad interface. The frictional forces opposing sliding are intensified by a high coolant pressure (which holds the can onto the fuel) while the ability of the clad to resist the frictional forces, without being locally deformed, depends on its strength. The coefficient of friction, the angle between adjacent radial pellet cracks, and the creep properties of the clad have, in theory, strong effects upon the tendency for clad strain to be concentrated over opening pellet cracks; confirmation of the correctness of these deductions has been obtained from laboratory experiments in which cladding has been stretched by cracked pellets on an expanding mandrel.The numerical analysis has enabled a detailed study of the strain-concentrating processes to be made, revealing that swelling of the pellet during a period at reduced-heat rating increases its diameter so that when high rating operation is resumed and the pellet expands, the cladding is stretched by an amount that depends on the magnitude of the prior swelling. During the expansion of the fuel pellet, the radial cracks in it open up and preferentially strain the adjacent cladding so that the clad strain due to fuel swelling, like that due to thermal expansion of the fuel, tends to be concentrated in arcs of cladding adjacent to pellet cracks. This process is repetitive, occurs whatever the magnitude of the coolant pressure, and is accentuated by the presence of a circumferential temperature gradient in the cladding.