ABSTRACT
This paper is concerned with presenting an overview of microstructural development in irradiated RPV steels and of the associated understanding of the underlying embrittlement mechanisms. The key to this approach is an agreed framework for the mechanisms controlling RPV embrittlement. There is a consensus that the fundamental degradation mechanisms are due to cluster hardening and non-hardening embrittlement. The key requirement on microstructural techniques is to provide a full characterisation of the irradiation-induced microstructure at or near the atomic scale. The current capability is reviewed and it is demonstrated that small angle neutron scattering (SANS), field emission electron microscopy (FEGSTEM) and atom probe (AP/FIM) now provide a proven means of fully characterising the size, number density and composition of copper rich clusters, although there is the open question of the level of iron in the copper rich clusters. Positron annihilation (PA) is now routinely used, particularly in conjunction with post-irradiation annealing (PIA). Both lifetime and lineshape analysis (PALA) studies have been reported. Overall there has been greatest success in characterising Cu-rich precipitates. In the case of matrix damage no technique has emerged which can directly resolve damage clusters in RPV steels, but advances have been made into the nature of matrix damage and its parametric dependence by ‘indirect observations’ using post-irradiation annealing studies combined with positron annhilation and hardness measurements. In the case of grain boundary segregation, techniques exist to measure levels of grain boundary coverage of important elements.
