Integrated analysis of WWER-440 RPV weld re-embrittlement after annealing

The analysis of WWER-440 RPV welds mechanical properties and behaviour of nanoscale structural features under primary irradiation, thermal annealing and re-irradiation has been performed in the framework of the international research project PRIMAVERA. The weld material of WWER-440 with three different levels of phosphorus contents, specifically 0.025, 0.03 and 0.04 wt.% were investigated in the framework of the project. It has been demonstrated that the copper atoms create under irradiation small clusters (d similar to 1-2 nm) surrounded by P. Si or Mn atoms. The phosphorus atoms segregate on Cu-clusters, dislocations and form P-clusters and atmospheres. These mechanisms lead to the embrittlement of RPV steel, which is revealed by the increasing of yield stress and transition temperature shift after irradiation. Due to thermal annealing a substantial part of irradiation induced copper and phosphorus clusters has been dissolved. Moreover, dissolution of Cu clusters occurs simultaneously with the growth of the Cu precipitates. Both steel tensile properties and transition temperature recover due to thermal annealing. A substantial phosphorus effect on ductile-to-brittle transition temperature shift occurs under re-irradiation. Because in the annealed steel a substantial part of the Cu atoms is in the precipitates, the material embrittlement under re-irradiation does not depend so much on Cu, and the Tk shift under re-irradiation is less as compared with primary irradiation. Considering the vacancy type defects, the PAS experiments show that basically three different processes play a role in the microstructure evolution: irradiation induced creation of point defects, thermal induced annealing of these defects and thermal/irradiation induced coarsening of defects. Although, the influence of the vacancy type defects on the mechanical properties was found to be little in comparison to the effect of the Cu-P clusters, the presence of these defects in the irradiated and annealed materials were observed in large number-significantly more than in an equilibrium state. (C) 2012 Elsevier B.V. All rights reserved.