Transformation of Fine Microstructure of WWER-1000 Materials After High Dose Irradiation and Annealing

High nickel WWER-1000 RPV materials were irradiated at high dose rate up to doses close to design values and exceeding these values. The complex investigation of the materials was carried out using LEAP, SEM, AES and Charpy tests. It has been shown that the DBTT shift was conditioned by formation of late-blooming phases (LBP) precipitates, but at a certain fluence this phenomenon looks to saturate. Nevertheless, even with a non-increasing number density of LBP, the DBTT shift continues to increase with fluence. This increase can be induced by intergranular embrittlement. SEM has shown the presence of intergranular fracture on Charpy fracture surfaces and AES revealed a phosphorus content in the grain boundaries (GB) about 20% at. A model of irradiation-induced GB segregation of phosphorus at high nickel content has been used to calculate the phosphorus content in the GBs. The calculations confirm the value obtained by AES and show a significant increase in the GB content at fluxes typical of normal WWER-1000 RPV operation. The same model shows further increase of phosphorus content in high Ni steels during post-irradiation annealing. Thus, in spite of LBP dissolution during early stages of annealing at 450C, full recovery of the DBTT could notmore » be obtained due to intergranular embrittlement.« less