Transitional creep mechanisms are exhibited by materials used in power systems and a thorough knowledge of these is a prerequisite in reliable extrapolations of short-term data to service conditions mainly due to the dominance of viscous creep mechanisms such as Nabarro-Herring, Coble and more importantly Harper-Dorn creep at low stresses. The paper summarizes the deformation mechanisms pertinent to various classes of materials. It is shown that blind extrapolation of the short-term laboratory test results to long-term service conditions leads to nonconservative predictions of the creep-rates and life times of the structural materials. Application of such transitional creep mechanisms to the prediction of dimensional changes of Zircaloy cladding in light water reactors is summarized.

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