Diffusion processes in VVER-1000 reactor pressure vessel steels in view of formation of radiation-induced phases

Reactor pressure vessel (RPV) steels are subjected to the influence of both operating temperature exposure (~300°C for VVER-1000-type RPV) and fast neutron (E>0.5 MeV) irradiation, which can result in degradation of mechanical properties. At this the shift of ductile-to-brittle transition temperature (ΔT_K) is due to two embrittlement mechanisms: the hardening mechanism (formation of radiation defects and radiation-induced precipitates) and the non-hardening mechanism (grain boundary impurities segregation).

This paper considers formation of radiation-induced precipitates under irradiation in VVER-type RPV steels. Fast neutron irradiation, on the one hand, causes formation of radiation defects being the centers for precipitates formation; on the other hand, irradiation accelerates diffusion of precipitates enriching elements providing their effective transport to the clustering places. Radiation temperature should also influence the formation of precipitates by changing both atoms diffusivity and radiation defects formation.

In this regard phase-structural analysis is carried out for VVER-1000-type RPV steel irradiated at 300°C and 400°C. Formation of radiation-induced precipitates enriched with Ni, Mn and Si as well dislocation loops is revealed after irradiation at 300°C, while irradiation at a higher temperature caused formation of neither radiation-induced precipitates nor dislocation loops. GB segregation of phosphorus took place for both irradiation temperatures, implying accelerated phosphorus diffusion under irradiation. The role of diffusion processes in the changes of structural-phase state of RPV steels under irradiation at different temperatures is discussed.