Analysis of Mechanical Properties and Nanostructure Changes of VVER-440 Reactor Pressure Vessel Metal after Annealing

Analysis of mechanical testing and microstructural investigations of the irradiated VVER-440 reactor pressure vessel (RPV) metal before and after annealing carried out in the frameworks of the international project "PRIMAVERA" is presented. The role of chemical elements (Cu and P) was studied in relation to irradiation embrittlement of the VVER-440 RPV weld metal in the initial state, after recovery annealing and after post-annealing irradiation. It was shown that under primary irradiation, copper atoms formed small clusters (size similar to 1-2 nm) which also include P, Si and Mn atoms. Phosphorus atoms also segregated on copper clusters, dislocations, grain boundaries, and additionally formed phosphorus clusters and atmospheres. Under thermal annealing at 475 degrees C, the irradiation-induced copper-enriched and phosphorus clusters partially dissolved, increasing the copper concentration in the annealed metal, which was significantly lower compared to the unirradiated material. Since most copper atoms after annealing were not contained in the ferritic matrix but in precipitates, the contribution of copper atoms to an increase in the ductile-to-brittle transition temperature under re-irradiation was minimized and so hardening and embrittlement after subsequent post-annealing irradiation were lower than under primary irradiation.