Fracture of ferritic reactor pressure vessel steels is determined by temperature and specimen size. In low temperature fracture occurs by cleavage, with elevation of temperature and decrease of specimen size the ratio of ductile tearing prior to cleavage increases and, in the end, fracture will have a ductile character. There is a temperature interval where both fracture mechanisms are present. Hypothesis of the traditional (global) assessment method has been that the shift of the fracture toughness reference curves due to neutron irradiation is equal with the shift of the ductile-to-brittle transition temperature measured by the Charpy impact test. Local assessment approaches based on the „weakest link” statistical theory recommend the direct measurement of fracture mechanics parameters to structural integrity evaluation. The paper describes the main features of both global and local approaches, analyzing the micro structural aspects of the most known micromechanical models of local approach and drawing conclusions concerning their applicability.