RPV material investigations of the former VVER-440 Greifswald NPP

The real toughness response of RPV material can only be determined after the final shut down of the NPP. Such a chance is given now by investigating material from the former Greifswald NPP (VVER-440/230).In the first part the paper deals with fast neutron fluence calculations and retrospective dosimetry based on Niobium. Unfortunately, a second neutron reaction besides ⁹³Nb(n,n’) leading to ^93mNb-activity is the reaction ⁹²Mo(n,γ)⁹³Mo. Based on the found Nb and Mo contents in the RPV material, it turned out that the ^93mNb generation on the Mo path mostly dominates over the fast neutron induced generation from Nb.The comparison between the calculated and the measured ^93mNb activities typically resulted in deviations of 50%. Possible reasons for the observed differences are discussed.In the second part first results of fracture mechanic investigations are reported. SE(B) specimens from three thickness positions were tested and evaluated according to the test standard ASTM E1921-05. Cleavage fracture toughness values, K_Jc, were determined and Master Curve based reference temperatures (T₀) were evaluated. The T₀ measured at the inner surface of the RPV did not represent the conservative condition. The T₀ of disc 1-1.3 located between the surface and 1/4 thickness is about 40K higher compared with those of the surface.The measured K_Jc values are not enveloped by the 5% fractile indexed with T₀ according to the Master Curve concept. However, the 5% fractile indexed with the VERLIFE reference temperature RTT_o that includes an additional margin envelops the measured K_Jc values. Therefore the VERLIFE lower bound curve conservatively describes the fracture toughness of the investigated weld metal.