RPV weld material investigations of a VVER-440 NPP

The Greifswald NPP was operated between 1973 and 1990. The units are of Russian WWER-440-230 type. In autumn 2005 the first trepans (diameter 120 mm) were taken from the unit 1 of this NPP. The RPV of this unit was annealed after 14 years of operation, followed by two additional years of operation.
The paper describes the results of weld metal investigations. The trepan originates from the RPV core weld exposed with the highest neutron flux. The neutron dosimetry was based on Monte Carlo fluence calculations as well as retrospective Nb dosimetry. The maximum fluence for E>0.5 MeV was determined to be 4*1019 neutron/cm2, and the maximum gamma fluence was found to be 1.7*1020 gamma/cm2 for E>1 MeV. A decrease of 60% through the thickness of the RPV was found.
The key part of the material testing was aimed on the determination of the reference temperature T0 following the ASTM Test Standard E1921-05 to determine the fracture toughness of the RPV steel in different thickness locations. It could be shown that the Master Curve approach as adopted in E1921 is applicable to the investigated original RPV weld metal. The weld metal located in a distance of about 22 mm from the inner surface of the RPV wall yielded a T0 of 50°C which is about 40K higher than T0 close to the inner surface. This outcome is important for the assessment of results retrieved from so called boat samples taken directly from the RPV surface after the recovery annealing. It shows that boat samples do not mandatory represent the material with the lowest toughness.