Experiments have shown that pellet-clad mechanical interaction failures due to power increments often show up with some delay after the power increase has been applied. Fission-product stress corrosion has generally been accepted as the reason for this delay. It is suggested, however, that these failures may be caused by purely mechanical effects. Local plastic instability occurs during the power increase due to the stress concentrations over a fuel crack and results in the initiation of a crack at the inner wall which propagates rapidly by the local strain energy in the fuel and cladding. This strain energy may, however, not be sufficient to produce a through-going crack, and the crack propagation may stop unless more energy is supplied by further power increase ; however, this energy may also be supplied by extrusion of fuel along the hot center of the rod. A simple cladding stress analysis with special emphasis on the stress concentrations over fuel cracks is included to simplify the explanation of the plastic-instability fuel-extrusion failure mechanism.