ICONE15-10335 RPV MATERIAL INVESTIGATIONS OF THE FORMER VVER-440 GREIFSWALD NPP

抄録

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 through the investigation of material from the former Greifswald NPP (VVER-440/230). In the first part the paper deals with the retrospective dosimetry based on Niobium, which is a trace element of the RPV material. Unfortunately, a second neutron reaction besides ^<93>Nb(n,n') leading to ^<93m>Nb-activity is the reaction ^<92>Mo(n,γ)^<93>Mo. Therefore, the ^<93m>Nb-activity generated through the Mo path has to be determined separately. Based on the found Nb and Mo contents, it turns out that the ^<93m>Nb generation on the Mo path dominates in the most cases over the fast neutron induced generation from Nb. The comparison between the calculated and the measured ^<93m>Nb activities typically resulted in deviations of 50%. Possible reasons for the observed differences are discussed. In the second part the first results of fracture mechanic investigations are reported. In a first step 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 (T_0) were evaluated. The T_0 measured on the inner surface of this RPV does not show the highest value and, thus, reflect the conservative condition. The T_0 of disc B located between the surface and 1/4 thickness is about 40K higher compared with the surface. The K_<Jc> values adjusted to a specimen thickness of 1T are enveloped by the WWER specific lower bound fracture toughness curve suggested in the VERLIFE procedure. Finally can be concluded, that for a successful application of Nb dosimetry a minimum Nb content of 20 ppm is needed under the given conditions (i.e. Mo content and measuring date). The measured K_<Jc> values are not enveloped by the 5% fractile indexed with T_0 + 2σ according to the Master Curve concept. However, the 5% fractile indexed with the VERLIFE reference temperature RT_<To> 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.