Three-dimensional atom probe study of irradiated, annealed and re-irradiated VVER 440 weld metals

The radiation embrittlement of the welds of old VVER 440 reactor pressure vessels promoted research on technical ways to ensure their safe operation during design service life. One of those is the recovery annealing of the reactor pressure vessel. Today, very few data are available concerning re-irradiated materials. These data are essential in order to understand the mechanism of re-embrittlement and to be able to predict the evolution of the microstructure and the mechanical properties of the steel during inservice re-irradiation. For that purpose a research programme was launched to carry out atom probe studies of the microstructure of weld metals in irradiated, irradiated-annealed and re-irradiated conditions. This work demonstrates that neutron irradiation promotes the formation of ultra-fine copper-enriched clusters and solute segregation to crystal defects in these Cr-Mo-V steels. The study of the annealed materials allows us to explain how the mechanical properties of the irradiated materials can be recovered. Also, study of the re-irradiated material points out the non-detection of copper-enriched clusters and the phosphorus behaviour during re-irradiation. Copyright (C) 2004 John Wiley Sons, Ltd.