SANS investigation of RPV weld material from the decommissioned NPP Greifswald

Nuclear plant operators must demonstrate that the structural integrity of a nuclear reactor pressure vessel (RPV) is assured during routine operations or under postulated accident conditions. The aging of the RPV steels is monitored with surveillance program results or predicted by trend curves. Embrittlement forecast with trend curves and surveillance specimens may not reflect the reality. Accordingly, the most realistic evaluation of the toughness response of RPV material to irradiation is done directly on RPV wall samples from decommissioned nuclear power plants (NPP). Such a unique opportunity is now offered with material from the decommissioned Greifswald NPP. The four Greifswald NPP units representing the first generation of WWER-440/V-230 reactors were shut down in 1990 after 11–17 years of operation. Material from RPVs in three different conditions is available: Unit 1 is irradiated, annealed and reirradiated. Unit 2 is irradiated and annealed. Unit 4 is in the as-irradiated condition. Small-angle neutron scattering (SANS) was applied in order to characterize the microstructure of weld material taken from the core-belt regions of Units 1, 2 and 4. Furthermore, material from Unit 4 exposed to post-irradiation annealing corresponding to the large-scale annealings of Units 1 and 2 was also investigated. The results of the SANS experiments are reported and compared with mechanical properties as well as results obtained for surveillance material of the same type.