[en] One of the most remarkable achievements in physical metallurgy occurred right at the start of the Civil Nuclear Reactor Programme in the United Kingdom when Cottrell first predicted, then demonstrated, 'irradiation' or 'Cottrell' creep of uranium. This had immediate practical consequences because a Magnox reactor with uranium fuel was already operating at Calder Hall and it was found that the fuel elements were indeed deforming to an unacceptable extent owing to Cottrell creep. This deformation is a type of Newtonian flow, and other examples were found to occur in the fuel elements. In particular, the magnesium can components deformed by Nabarro-Herring creep; this was the first identification of this fundamental deformation mode occurring in an engineering component. Newtonian flow is also induced if a small load is applied to uranium while it is thermally cycled through its α-β phase change. This is a consequence of the volume changes that take place during the allotropic transformation. Volume changes also occur during nuclear and chemical changes (particularly corrosion reactions in this context) leading to the generation of stress and these processes are now known to be of great importance in the development and performance of the fuel element and other reactor components. (author)