What one learns from reactor pressure vessels of decommissioned nuclear power plants.

Nuclear power plant operators must demonstrate that the structural integrity of a nuclear reactor pressure vessel (RPV) is assured during routine operations and under postulated accident conditions. The aging of the RPV steels is monitored via surveillance programs. Radiation loading, metallurgical and environmental histories, however, can differ between surveillance and RPV materials. Therefore, the investigation of RPV material from decommissioned NPPs offers the unique opportunity to evaluate the real toughness response. Such an opportunity is now available through the investigation of RPV material from the decommissioned Greifswald NPP (WWER-440/V-230).
The Greifswald RPVs of 4 units represent different material conditions as follows:
• Irradiated (Unit 4),
• irradiated and recovery annealed (Units 2 and 3), and
• irradiated, recovery annealed and re-irradiated (Unit1).
The recovery annealing of the RPV was performed at a temperature of 475° for about 152 hours and included a region covering ±0.70 m above and below the core beltline welding seam.
Material samples of a diameter of 119 mm called trepans were extracted from the RPV walls. The research program is focused on the characterisation of the RPV steels (base and weld metal) across the thickness of the RPV wall.
This paper presents test results measured on the trepans of the beltline welding seam and base metal of the above mentioned conditions. The key part of the testing is focussed on the determination of the reference temperature T0 following the ASTM standard E1921-09 to determine the facture toughness, and how it degrades under neutron irradiation and is recovered by thermal annealing. Other than that the mentioned test results include Charpy-V and tensile test results. Following results have been determined:
• The mitigation of the neutron embrittlement of the weld and base metal by recovery annealing could be confirmed.
• KJc values of the weld metals generally follow the course of the MC though with a large scatter.
• There is a large variation in the T0 values evaluated across the thickness of the multilayered welding seams.
• The T0 measured on TS oriented SE(B) specimens from different thickness locations of the welding seams strongly depends on the structure along the crack tip.
• It was demonstrated that T0 evaluated according to the SINTAP MC extension represents the brittle fraction of the data sets.
• A direct correlation between T0 and TT41J of the investigated weld metal is questionable due to the different thickness location of the crack tip and the notch root in pre-cracked SE(B) and the reconstituted Charpy-V specimens, respectively.
• A strong scatter of the fracture toughness KJc values of the recovery annealed base metal of Unit 1 is observed with clearly more than 2% of the values below the fracture toughness curve for 2% fracture probability. The application of the multimodal MC approach describes the temperature dependence of the KJc values in a satisfactory manner.