Studies on the structural materials in nuclear power plants create the experimental data and background information necessary for the structural integrity assessments of mechanical components. The research is carried out by developing experimental fracture mechanics methods including statistical analysis methods of materials property data, and by studying material ageing and, in particular, mechanisms of material deterioration due to neutron irradiation, corrosion and water chemistry. Besides material studies, new testing methods and sensors for measurement of loading and water chemistry parameters have been developed. The monitoring data obtained in real power plants has been used to simulate more precisely the real environment during laboratory tests. The research on structural analysis has focused on extending and verifying the analysis capabilities for structural assessments of nuclear power plants. A widely applicable system including various computational fracture assessment methods has been created with which different structural problems can be solved reliably and effectively. Research on reliability assessment of maintenance in nuclear power plants is directed to practical case studies on components and structures of safety importance, and to the development of models for maintenance related decision support. A systematic analysis of motor-operated valve has been performed.