Summary of the investigations on the decommissioned WWER-440 reactor pressure vessel of the NPP Greifswald

The Greifswald WWER-440/V-230 nuclear reactors represent the first generation of this reactor type. The four units of the Greifswald NPP were eternity shut down in 1990 after 11 –15 years of operation and 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 Units 1, 2 and 4 RPV. The key part of the testing is focussed on the determination of the reference temperature T0 following the ASTM standard E1921 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 results represent the material conditions within the multilayer beltline welding seams and base metals aged under real operating conditions.
• The fracture toughness values at cleavage failure, KJc, of the weld metals generally follow the course of the MC though with a large scatter. A strong scatter of the KJc values of the irradiated and recovery annealed base metal of Unit 1 and Unit 4 RPV, respectively, is observed with clearly more than 2% of the values below the fracture toughness curve for 2% fracture probability.
• There is a large variation in the T0 values evaluated across the thickness of the multilayered welding seams from the investigated RPV’s.
• For the beltline welding seam of the Unit 4 RPV it is demonstrated that the ductile-to-brittle transition temperature (TT) shift predicted by the Russian code [PNAE G-7-008-86] for the present content of deleterious elements P and Cu and the accumulated neutron fluences lies within the scatter of the measured T0 values. The expected shift of T0 is not visible because of the strong variation of toughness caused by the intrinsic weld bead structure and the different filling materials used for weld root and the main weld within the multilayer welding seam. Hence, the position of the crack tip of the specimen in the multilayer welding seam is crucial and defines the cleavage fracture toughness.
• The mitigation of the neutron embrittlement of the weld and base metal by recovery annealing could be confirmed.
• For both weld and base metal the highest value of ductile-to-brittle transition temperatures (MC T0 and and Charpy-V TT47J) were not measured directly at the inner surface of the RPV. This points to the fact that the fracture toughness values measured on specimens machined from the “templates” taken directly at the inner RPV wall may not represent the conservative condition.
• The orientation of the specimens from a multilayer RPV welding seam is of essence for the fracture toughness testing according to ASTM E1921. For TS oriented specimens, the crack propagation across the thickness of the welding seam results in a uniform structure along the crack front, whereas for the T-L specimens with crack propagation in the circumferential direction, the structure along the crack front varies. This influences the KJc values and their scatter as also the MC reference temperature T0. Strictly speaking T-L specimens of weld metal do not fulfil the essential pre-assumption of the MC approach, because of the macroscopically non homogenous structure along the crack front length.