Degradation of RPV steel structure after 45 years of operation in the VVER-440 reactor

Abstract This paper presents the results of the complex structural studies of RPV steels (weld and base metal from both beltline and nozzle regions) from the decommissioned in 2017 VVER-440 reactor after 45 years of operation. During its operation, the RPV beltline weld #4 had been annealed twice: by mode 430 °C/150 h in 1987 and after three fuel campaigns by mode 475 °C/100 h. APT and TEM studies were used for characterization of the radiation-induced precipitates and radiation defects (dislocation loops), and AES and SEM fractography – for assessment of the grain boundary segregation level. Complex structural studies of RPV steel after such a long-term operation (45 years) are carried out for the first time. It is shown that the thermal exposure at 290 °C for 45 years leads to the formation of the Cu-rich precipitates with a low number density. Recovery annealing results in a high level of P grain boundary segregation together with incomplete Cu return to the matrix. Re-irradiation leads to further Cu and P matrix depletion through the formation of Cu–P radiation-induced precipitates on the one hand, and the P grain boundary segregation on the other. Eventually, the saturation of secondary precipitate formation and deceleration of the P grain boundary segregation occurs at high fluence.