[en] The target of this project has been to study and model the changing properties of ageing pressure boundary materials used in nuclear power plants. Models to estimate and methods to measure the behaviour of these materials under various operational conditions in nuclear power plants have been developed and applied. In nuclear reactors stainless steels are mainly used as a material facing the primary circuit environment. Thus, the main focus has been to study the role of the primary circuit environment and surface oxide films containing Fe, Ni and Cr on changes occurring in the mechanical properties and degradation susceptibility of ageing pressure boundary materials. (orig.)