Irradiation effects on toughness behaviour and microstructure of VVER-type pressure vessel steels

The irradiation sensitivity and the annealing behaviour were studied on seven different heats from VVER 440 and VVER 1000-type reactor pressure vessel steels. The specimens were irradiated at the Rheinsberg prototype VVER 2 reactor to mean neutron fluences between 43 and 127.6×1018 n/cm2 [E>0.5 MeV]. Toughness and strength properties were determined and the microstructure was analysed using the small angle neutron scattering (SANS) technique. There is an obvious correlation between the irradiation-induced changes of transition temperature, hardness and volume fraction of microstructural features of radii up to 2 nm. The main parameters of influence are the neutron fluence and the nickel content. The nickel-containing VVER 1000-type pressure vessel steel is more sensitive to irradiation than the VVER 440-type steel which has a low nickel content. For the latter, the sensitivity to radiation embrittlement depends on the copper and phosphorus contents. Annealing at 475°C (100 h) reduces the irradiation effect but not completely in every case.