Diffusion role in radiation embrittlement of reactor pressure vessel steels

This work presents the research of VVER-1000-type reactor pressure vessel (RPV) steels. The paper considers the influence of irradiation with different fast neutron fluxes on the structure, yield strength and ductile-to-brittle transition temperature (T_K) changes as well as on changes of the fraction of brittle intergranular fracture and development of grain boundary (GB) segregation processes in the VVER-1000 RPV base metal (BM) and weld metal (WM). The flux effect is shown to be significant for WM with high (>1.35 wt.%) bulk Ni content with the main contribution of non-hardening mechanism (GB phosphorus segregation) in it. Phosphorus bulk diffusion effective coefficients were evaluated for the temperature exposure, accelerated irradiation and irradiation within surveillance specimens (SS) using the improved kinetic model of phosphorus GB accumulation in low-alloyed low-carbon steels taking into account the influence of operational factors.

Radiation-induced structural changes at irradiation temperature of (290 -300°С) typical for VVER-type RPV imply formation of both dislocation loops and precipitates. Formation of radiation-induced precipitates is a diffusion-driven process, depending on both formation of radiation defects and effective transport of precipitates enriching elements. Increasing irradiation temperature should increase diffusivity but might decrease radiation defects formation as well. In this regard phase-structural analysis is carried out for VVER-1000-type RPV steel irradiated at ~300 °C and 400 °C. At higher irradiation temperature no radiation-induced precipitates were formed in contrast to 300 °C irradiation, while GB segregation was revealed for both irradiation temperatures. Number density and size of the carbide phases change after irradiation at 400 °C with respect to initial state unlike the irradiation at 300 °C, leading to ΔR_P0,2 increase.