Precipitation kinetics of radiation-induced Ni-Mn-Si phases in VVER-1000 reactor pressure vessel steels under low and high flux irradiation

Highlights: • VVER-1000 RPV steels after low and high flux irradiation are studied by APT, TEM. • Number density of NMS phases increases with irradiation dose and Ni+Mn content. • omposition of NMS phases does not depend on the irradiation conditions. • High flux irradiation leads to the lower size and volume fraction of NMS phases. One of the key embrittlement mechanisms in reactor pressure vessel (RPV) steels is the hardening produced by nanometer features. In this paper low Cu high Ni VVER-1000 RPV steels with a wide range of Ni (1.1–1.94wt.%) and Mn (0.38–1.1wt.%) contents irradiated at 290°C to (6–101) 10²² n/m²² (E>0.5MeV) at both low (surveillance specimens) and high (test reactor) fluxes were analyzed using atom probe tomography and transmission electron microscopy. Formation of high number density of NMS precipitates with the average composition close to at.%: 45Ni-33Si-22Mn regardless of the irradiation conditions was observed. Its number density increases with the increase of the total Ni and Mn concentration in steel. High flux irradiation demonstrates lower size and volume fraction of NMS precipitates compared to the low flux irradiation in case of welds. For the base metal specimens with low Ni (1.1–1.3 wt.%) and Mn (0.38–0.51) content no flux effect was observed. The contribution of NMS phases and dislocation loops into radiation hardening of RPV steels was established by the dispersed barrier strength model (Orowan equation).